US20090311945A1

US20090311945A1 - Planarization System

Info

Publication number: US20090311945A1
Application number: US12/140,580
Authority: US
Inventors: Roland Strasser; Wolfgang Gfrerer
Original assignee: Infineon Technologies AG
Current assignee: Infineon Technologies AG
Priority date: 2008-06-17
Filing date: 2008-06-17
Publication date: 2009-12-17
Also published as: DE102009014550A1

Abstract

An embodiment of a planarization system for planarizing a substrate includes a planarizing surface and an encircling element formed to at least partially laterally enclose the substrate, wherein the planarizing system is configured to planarize the substrate with the substrate abutting the planarizing surface during a relative lateral movement between the substrate and the planarizing surface. The substrate is at least partially laterally enclosed by the encircling element. The encircling element abuts the planarizing surface.

Description

BACKGROUND

In modern technologies, many electrical circuits, electrical and electronic devices and other mechanical and non-mechanical devices are based on thin film processes and, hence, may be summarized to be thin film devices. The fabrication of such thin film devices, in many cases, comprise a large number of thin film process steps, like depositing thin films, patterning, milling and etching thin films. Such thin film process steps, furthermore, comprise, in many cases, planarization steps as an initial step, as intermediate steps or as a final step at the interface between the fabrication process and the post-fabrication process.
Depending on the application in mind as well as the challenges involved, planarizing layers, films or substrates may be accomplished on the basis of different processes. A more mechanically abrasive planarization is often referred to as lapping, which may, for instance, be employed to remove a comparably thick layer of a film or a substrate. On the contrary, planarizing a layer, film or substrate to obtain a smooth surface and simultaneously limiting the amount of removed material is often referred to as polishing. In the field of fabricating semiconducting devices, a further planarization technique is frequently employed. Chemical-mechanical planarization or chemical-mechanical polishing, which is commonly abbreviated as CMP, is a technique, which is used in semiconductor fabrication for planarizing, for instance, the top layers of wafers, substrates and other intermediate products during the fabrication process.
Planarizing layers, films and substrates represents, for many fabrication processes, a technically and economically important and sensitive process step, which is significantly influenced by a number of parameters and aspects, which can be controlled directly or indirectly. The parameters comprise the rate of removal of material, the uniformity of the removal, the planarity, the number of defects and the consistency, to name but a few parameters and aspects. These parameters and aspects may, themselves, be influenced by other parameters and may be subjected to at least partially contradicting overall goals.
As a consequence, a demand exists to balance these partly contradicting parameters and aspects and to improve the general result of the planarization process step in view of the concrete design goals and circumstances of a potential application in mind.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments according to the present invention will be described in more detail with reference to the appended drawings.

FIG. 1 a shows a top view of a planarization system according to an embodiment of the present invention;

FIG. 1 b shows a cross-sectional view of a planarization system according to an embodiment of the present invention;

FIG. 2 a shows a cross-sectional view of a substrate holder according to an embodiment of the present invention;

FIG. 2 b shows a top view of the substrate holder of FIG. 2 a;

FIG. 3 shows a top view of a substrate holder according to another embodiment of the present invention;

FIGS. 4 a and 4 b show top views of substrate holders for rectangular substrates according to embodiments of the present invention;

FIG. 4 c shows a cross-sectional view of the substrate holders of FIGS. 4 a and 4 b;

FIG. 5 shows a cross-sectional view of a further substrate holder according to an embodiment of the present invention;

FIG. 6 shows four graphs of measurements of the planarity of wafers; and

FIG. 7 shows a flow chart of a method for planarizing a substrate according to an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Before describing, in context with FIGS. 1 a to 6, different embodiments according to the present invention in the form of planarization systems, substrate holders and methods for planarizing a substrate, a brief introduction into the technical field and application of planarizing layers, films and substrates will be given first. Although depending on the specific application in mind, the terms layers, films and substrates usually refer to different structures of a thin film device, in the following, the term substrate is to be understood as a substrate, which may optionally comprise one or more optionally structured or patterned layers or films. Hence, the terms may be used synonymously in many cases here.
As outlined above, thin film devices are employed today in a great number of applications and technical fields, such as electronic and electrical devices, integrated circuits, and also other mechanical or non-mechanical, optical and other devices. The fabrication of these thin film devices comprise, in many cases, a significant number of thin film process steps, such as depositing, patterning, milling and etching the deposited films. An important additional process step in the framework of fabricating thin film devices is planarizing substrates including the initial substrates, intermediate products or the final product by lapping, polishing or chemical-mechanical planarization or chemical-mechanical polishing, usually referred to as CMP.
While lapping is frequently used to describe a more mechanical removal of material from a substrate, polishing is frequently referred to as a planarization of a substrate to obtain a smooth surface and to limit the amount of removed material. Chemical-mechanical planarization or chemical-mechanical polishing is, however, a technique, which is frequently used in semiconductor fabrication for planarizing top layers of wafers and other substrates, for instance, of intermediate products, but also of substrates of other steps during the fabrication process.
The previously described planarization techniques are part of many overall process flows to fabricate thin film devices and are, hence, technologically as well as economically sensitive. Planarizing substrates by any of the previously mentioned techniques is, in some cases, governed by partially contradicting goals, aspects and parameters, such as the rate of removal, determining how quickly material can be removed from the substrate and other parameters. Important parameters may, for instance, be the uniformity of removal determining the uniformity of the removed material across a local area, such as a rectangular area of a wafer (die) or the substrate or wafer as a whole. A further parameter is the planarity determining how planar or flat the surface of the substrate is after the removal process is completed. In addition, the number of defects, their size and the presence of residues left behind is important, as well as the consistency indicating how consistent the performance of a planarization technique is from substrate to substrate.
Uniformity and planarity may be, for instance, important at the edges of a substrate, since the achievable uniformity and planarity of a planarized substrate may determine an area available for fabricating thin film devices on the respective substrate. The uniformity or planarity of lower quality may, for instance, lead to disregarding a larger portion of the substrate at the edge, which limits the available area for thin film devices on the respective substrate. For instance, nose-like shapes may appear at the outer edges of the substrate. In the case of a round substrate, which is usually referred to as a wafer, undesired structures, like the ones previously mentioned, may be formed in a ring-like area, which may be several millimeters large. For instance, in the case of an 8″-wafer (8 inch wafer) between 3 to 10 mm of the radius may have to be disregarded at each side of the substrate due to the previously described problems of uniformity and planarity.
As previously mentioned, the substrate may comprise additional layers or films, which may be optionally patterned. Moreover, the substrates may have a rectangular shape, a square shape, a polygonal shape, an elliptical shape or a round shape. In the case of a round or circular shape, the substrates are usually referred to as wafers.
The substrates may comprise all kinds of materials, such as semiconducting materials (e.g., silicon (Si)), conductors, metals, insulators, alloys and more complex materials. The substrates may furthermore be monocrystalline substrates, epitactical substrates or polycrystalline substrates. Naturally, in the following, the term substrate is also used for thin film products and intermediate thin film products.
Although a number of different planarization techniques are available, in the following, as an example, CMP will be considered in more detail. However, the planarization systems, substrate holders and methods for planarizing a substrate may also be based on the lapping or polishing technique as well as other appropriate planarization techniques. Hence, the term planarization or planarizing at least comprises the alternatives lapping, polishing and CMP.
The CMP technique, but also lapping and polishing, are based on using a slurry, which may optionally be provided during the whole lapping, polishing or CMP process by means of a pumping system. The slurry is a chemically and/or mechanically active polishing medium, which may be provided to the substrate to perform the planarization in combination with a planarizing surface. In other words, in the case of CMP, an abrasive and corrosive chemical slurry, which is commonly a colloid in conjunction with a polishing pad as the planarizing surface, is used. The polishing surface or pad comprises, in many cases, polyurethane foam or a fleece material treated with polyurethane. Due to the presence of the slurry, the process of the material removal is not simply that of abrasive scraping, like sandpaper on wood, but the chemicals in the slurry may also react with and/or weaken the material to be removed from the substrate. The abrasive accelerates the weakening process and the polishing pad may help to wipe the reacted material from the surface.
FIG. 1 a shows a top view of a planarization system 100 for planarizing a substrate 110 shown in the form of a wafer. The planarizing system 100 comprises a planarizing surface 120, wherein a main surface of the substrate 110 to be planarized abuts the planarizing surface 120.
To illustrate the planarization system 100 in more detail, FIG. 1 b shows a cross-sectional view of along the line A-A′ shown in FIG. 1 a. The planarizing surface 120 is that of an optional pad 130, which can, for instance, comprise the previously mentioned polyurethane foam or may comprise a fleece material treated with polyurethane. The pad 130 is arranged on a platen 140, which is adapted to be rotatable around a central axis or rotation axis 150 by a motor 160. As previously outlined, the planarization is achieved during the operation of the planarization system 100 by the motor 160 creating a relative lateral movement of the substrate 110 with respect to the planarizing surface 120 of the pad 130. The planarization system 100 according to an embodiment of the present invention as shown in FIGS. 1 a and 1 b furthermore comprises an encircling element 170 formed to at least partially laterally enclose the substrate 110. To be more precise, the encircling element 170 as shown in FIGS. 1 a and 1 b is a ring-shaped mechanical element, which also abuts the planarizing surface 120 with a bearing area 180.
As mentioned above, the planarization of the substrate 110 is achieved by the motor 160 creating a relative lateral movement of the substrate 110 with respect to the planarizing surface 120. In the case of the embodiment of the present invention shown in FIGS. 1 a and 1 b, this is achieved by the motor 160 rotating the disc-shaped platen 140 along with the pad 130 and the planarizing surface 120 with respect to the substrate 110. To achieve this, the planarizing system 100 may comprise, as additional components, a substrate holder coupled to a dynamic polishing head, which are not shown in FIGS. 1 a and 1 b for the sake of simplicity. The head may, for instance, create an additional rotation of the substrate 110 around a symmetry access of the head and/or may also optionally move the substrate along a radial line across the planarizing surface 120. Both optional movements are illustrated as dashed lines in FIG. 1 a.
Hence, the dynamic polishing head may optionally be rotated with a different axis of rotation, so that the rotation axis of the head (not shown in FIGS. 1 a and 1 b) and the rotation axis 150 of the planarizing surface 120 are not concentric. Since the planarization results may not only depend on the planarity of the planarizing surface 120, since each deviation from a uniform surface or any other deformation may lead to a negative influence on the final results, the additional movement of the substrate 110 tends to even out any irregular topography, increasing the substrate flatness. To utilize the whole surface area of the planarizing surface 120 as good as possible, a radial movement of the substrate 110 may be imposed as a further optional movement of the substrate 110, which may not only lead to a more even wear-out of the planarizing surface 120, but may also increase the planarity of the overall result by further “averaging out” deviations along the radial direction of the planarizing surface 120.
Achieving a certain degree of planarity may, for instance, be advisable in order to set up the substrate for the formation of additional circuit elements in the case of an integrated circuit or, for instance, to bring the entire surface of the substrate 110 within the depth of field of a lithography technology system or to selectively remove material based on its position.
The planarization system 100 shown in FIGS. 1 a and 1 b furthermore comprises, as an optional component, a slurry supply system 190 over which, during the whole planarization, the previously-described slurry may optionally be provided to the pad 130. Due to the rotational movement of the planarizing surface 120 and the optional movements of the substrate 110 and the substrate holder (not shown in FIGS. 1 a and 1 b), the slurry may be distributed evenly over the planarizing surface. As a consequence, between the substrate 110 and the pad 130, a thin film of slurry is generated, which chemically attacks the surface of the substrate to be polished or planarized. The abrasive particles of the slurry may generate the required mechanical alteration of the surface of the substrate 110 to remove the material.
Embodiments according to the present invention are based on the finding that the uniformity of the substrate 110 may be positively influenced by providing the encircling element 170 abutting the planarizing surface 120 during the planarization, which “enlarges” the area of the substrate 110 so that a possible area of a distorted uniformity or planarity of the substrate 110 is moved outwards. By exerting a pressure or a force onto the planarizing surface in an area at least partially surrounding the substrate, the virtually “enlarged” or “extended” area of the substrate 110 is created. As a consequence, the uniformity and planarity of the surface of the substrate 110 may be improved upon, especially in the outer areas of the substrate 110.
As will be described in more detail below, the encircling element 170 comprises a recess in which the substrate 110 is arranged during the planarization. The inner circumference of the recess of the encircling element 170 may, in some embodiments according to the present invention, comprise a distance from an edge of the surface 110, which is less than one inch (1″) or below. Depending on a great variety of technical parameters, it may also be possible to implement an encircling element 170 with a recess, so that during the operation of the planarization, a distance between the inner circumference of the recess and the substrate 110 in the lateral direction parallel to the planarizing surface 120 is less than about 0.5″, less than about 12 mm, less than about 10 mm, 8 mm, 5 mm or 2 mm.
The encircling element 170 may optionally comprise radially arranged grooves or other structural elements to improve the supply of a slurry to the substrate 110. Hence, by far, it is not required that the encircling element 170 completely laterally encloses the substrate 110 as a whole. According to embodiments of the present invention, a partially laterally enclosing encircling element 170 is possible. Depending on many implementation-specific details, it may, for instance, be enough for the encircling element 170 to enclose 50% of the circumference of the substrate 110, more than 66%, more than 75% or more than 80% of the circumference of the substrate 110. However, also higher ratios of the encircling element 170 enclosing the circumference of the substrate 110 may be implemented. For instance, it may be advisable, under some circumstances, to enclose 90% or more, 95% or more or even 99% or more of the circumference of the substrate 110.
This may have the effects on the outcome of the planarization that, as previously indicated, the uniformity of the substrate at the outer areas of the substrate 110 as well as in the center area of the substrate may be improved or even significantly improved. Moreover, the area of the substrate 110 may eventually be used more efficiently leading to a larger “range” usable for dies and products. In other words, by employing embodiments according to the present invention, a higher chip-related yield by reducing the non-technologically usable area at the outer border or edge of the substrate 110 may be achievable.
As will be outlined in more detail below, embodiments according to the present invention may be used more flexibly, since embodiments according to the present invention may be used for different thicknesses of substrates. Under some circumstances, due to the increased flexibility, the number of substrates or wafers lost may be reduced or even completely excluded.
The planarization system as shown in FIGS. 1 a and 1 b according to the present invention illustrates only a possible implementation. Instead of using a disc-shaped planarizing surface 120, in principle, also a planarizing tape or a sliding table optionally equipped with a pad 130 may be used. In the case of a sliding table, the table may, for instance, be configured to carry out one- or two-dimensional movements along one or more directions to generate the previously described lateral relative movement of the substrate 110 with respect to the planarizing surface 120. As a motor 160, an electromagnetic motor, but also a pneumatic or hydraulic motor may be implemented. Furthermore, linear motors as well as rotational motors may be utilized to generate rotational movements and linear movements, respectively. The motor 160 may further comprise a transmission system for changing rotational movements onto linear movements or vice-versa. Naturally, more than one motor may be comprised in a planarization system according to an embodiment of the present invention to realize a combination of the previously mentioned movements, for instance, by combining two one-dimensional movements to obtain a two dimensional movement or to combine a rotational movement with a one or two dimensional linear movement.
Embodiments according to the present invention may be used for very different applications in mind. For instance, a rate of removal of about 4 nm/s may be achievable in the case of a silicon-related fabrication. Naturally, smaller, but also larger rates of removal may be achievable in the case of using CMP systems. As a consequence, by subjecting a substrate 110 to a planarization for approximately 100 s, a removal of material with a thickness between about 10 nm and about 1000 nm (=1 μm) may be achievable. Depending on different applications in mind, a uniformity or planarity of a substrate 110 of less than about 1 nm may be achievable, which may, for instance, be required due to a typical depth-of-field for the 65 nm-lithography technology. However, also different, less critical requirements, but also stricter requirements concerning uniformity and planarity may, in principle, be achievable by employing embodiments according to the present invention. The precise achievable as well as desirable requirements depend, however, on a great number of implementational and other details.
Before describing further embodiments according to the present invention, it should be noted that similar or equal reference signs will be used for structures, objects and elements with similar or identical structure or functional properties. Moreover, summarizing reference signs will be used for objects, structures and elements appearing more than once in an embodiment according to the present invention. Unless a specific structure, object or element is referred to, summarizing reference signs will be used to describe and discuss more general properties and features of the respective structures. Unless noted otherwise, corresponding portions of the description may also relate to different embodiments and/or different structures, objects and elements in one embodiment. If, however, a specific object is described, the corresponding individual reference sign will be used. Using similar or identical reference signs as well as summarizing reference signs, hence, enables a clearer and more concise discussion of the features and properties of different embodiments according to the present invention.
FIG. 2 a shows a cross-sectional view of a substrate holder 200 for holding a substrate 110 during a lateral relative movement of the substrate 110 and the planarizing surface 120. The substrate holder 200 comprises a carrier 210, which is configured to urge the substrate 110 against the planarizing surface 120 and the previously described encircling element 170 formed to at least partially laterally enclose the substrate 110. The encircling element 170 is, with respect to the planarizing surface 120, vertically slidably attached to the carrier 210 so as to abut the planarizing surface 120 during urging the substrate 110 against the planarizing surface 120.
To achieve this, the carrier 210 shown in FIG. 2 a is arranged inside a recess of the encircling element 170, which is, once again, formed as a ring-shaped structure. To illustrate the geometrical shape of the substrate holder according to an embodiment of the present invention in more detail, FIG. 2 b shows a top view of the substrate holder 200 of FIG. 2 a.
The carrier 210 and the encircling element 170 are slidably in contact with each other at an outer circumference 220 of the carrier 210. The encircling element 170 furthermore comprises a lid-like structure with a central bearing 230, which has a smaller diameter than the outer circumference 220 of the carrier 210. As a consequence, by lifting the carrier 210 from the planarizing surface 120 by a sufficient distance, also the encircling element 170 is lifted from the planarizing surface 120. To allow such a lifting of the carrier 210, the carrier 210 comprises a rod-shaped portion 240 extending through the bearing 230 of the encircling element 170. Hence, the encircling element 170 once again comprises a recess, in which both the substrate 110 and at least part of the carrier 210 are arranged when the carrier 210 urges the substrate 110 against the planarizing surface 120.
FIG. 3 shows a top view of a further substrate holder 200 according to an embodiment of the present invention, which differs from the substrate holder 200 shown in FIGS. 2 a and 2 b only with respect to the encircling element 170. While the encircling element 170 of the substrate holder 200 shown in FIGS. 2 a and 2 b completely laterally encloses the substrate 110, the encircling element 170 of the substrate holder 200 of FIG. 3 only partially laterally encloses the substrate 110. The encircling element 170 is divided into four segments 170-1, . . . , 170-4 being interconnected by interconnections 250-1, . . . , 250-4. The four interconnections 250-1 to 250-4 “close” the ring-like structure of the encircling elements 170 as a whole. As a consequence, the encircling element 170 of the substrate holder 200 can be lifted and lowered along with the carrier 210 as described in context with the substrate holder 200 of FIGS. 2 a and 2 b.
However, the encircling element 170 only laterally encloses a smaller fraction of the circumference of the substrate 110. In the case shown in FIG. 3, the ratio is approximately 50%. Naturally, also a higher ratio can easily be obtained by enlarging the four fragments of the encircling element 170. Furthermore, an asymmetric enclosing may also be implemented.
FIG. 4 a shows a further top view of a substrate holder 200 according to an embodiment of the present invention. While the substrate holder, so far, has been described in context with wafers as examples for circular or elliptical substrates, the subject holder 200 shown in FIG. 4 a is a substrate holder for a square-shaped or rectangular substrate. Apart from the square-shape or rectangular shape of the substrate holder and the corresponding substrate, the embodiment shown in FIG. 4 a does not significantly differ from that shown in FIGS. 2 a and 2 b. To illustrate this further, FIG. 4 c shows a cross-sectional view of the substrate holder 200 shown in FIG. 4 a along the line A-A′.
FIG. 4 b shows a further substrate holder 200 according to an embodiment of the present invention for a square-shaped or rectangular substrate 110. While the substrate holder 200 of FIG. 4 a comprises an encircling element 170 completely laterally enclosing the substrate 110 (not shown in FIG. 4 a), the substrate holder 200 shown in FIG. 4 b, once again, comprises four portions 170-1, . . . , 170-4 interconnected by four interconnections 250-1, . . . , 250-4. The ratio of the encircling element 170 of the substrate holder 200 of FIG. 4 b laterally enclosing the circumference of a possible substrate 110 is larger than the approximately 50% of the embodiment shown in FIG. 3. However, FIG. 4 b also shows that according to embodiments of the present invention, the encircling element 170 is not required to completely laterally enclose the substrate, but only to at least partially laterally enclose the substrate 110 (not shown in FIG. 4 b).
To illustrate the similarity of the embodiments of FIG. 4 b and that of FIG. 3, FIG. 4 c also illustrates the cross section along the line A-A′ as indicated in FIG. 4 b. The substrate holders 200 as shown in FIGS. 2 a to 4 c may be operated by an appropriate planarization system 100 according to an embodiment of the present invention in several modes of operation. As will be outlined in more detail in the context of FIG. 7, the substrate 110 may be arranged on the planarizing surface 120 before the relative movement of the substrate with respect to the planarizing surface 120 is initiated by the motor 160. After disposing the substrate 110, the substrate holder 200 may simply be put over or arranged over the substrate 110 as illustrated in FIGS. 1 b and 2 b.
However, as an alternative, which may, for instance, be used on small-scale laboratory experimental set-ups, the substrate 110 may be coupled to the carrier 210 by an adhesive, such as glue. In such a case, the encircling element 170 is not required to be operated as a retaining ring to limit the movement of the substrate 110 with respect to the substrate holder 200. In some cases, an easily heatable wax may be used as glue to fix the substrate 110 to the carrier 210. Naturally, the glue or wax should not react chemically with the slurry.
As outlined above, the final result of the planarization may depend on a plurality of parameters, which may, for instance, include the planarity of a substrate holder 200, which is sometimes also referred to as the carrier or the chuck as well as the planarity and uniformity of the planarizing surface 120.
To additionally balance uniformity deficiencies of the substrate holder or rather the carrier 210 as well as potential deficiencies of the planarity of a planarizing surface 120, the carrier 210 may optionally be provided with the so-called baking film comprising soft fabrics, so that the substrate holder 200 is capable of balancing the roughness of either the substrate holder 200 or the planarizing surface 120. Moreover, the baking film may also be capable of improving the rotation of the substrate 110 by exerting adhesive forces imposed by the carrier onto the backside of the substrate 110. However, under some circumstances, employing a baking film may not be advisable, since a chemical reaction with the slurry or a potential contamination caused by the fabrics may result.
FIG. 5 shows a cross-sectional view of a substrate holder 200 according to a further embodiment of the present invention. The substrate holder 200 shown in FIG. 5 is intended for circular substrates or wafers 110 and, hence, comprises a rotational symmetry, which is illustrated by limiting the illustration of FIG. 5 to a range beginning with a symmetry axis 260 in the center of the substrate holder 200 to an outer area. However, as a comparison of the FIGS. 2 a and 4 c clearly demonstrate the subject holder 200 as shown in FIG. 5 can easily be adapted or employed for square-shaped or rectangular substrates 110 as well as further more irregular shaped substrates (e.g., polygonal substrates).
The substrate holder 200 comprises a carrier 210 that comprises an upper portion 270, which may, for instance, be connected to a head of the planarization system. Moreover, the carrier 210 comprises a central portion 280, which is sometimes also referred to as the carrier itself and a lower portion 290, which is sometimes also referred to as a plate. The central portion 280 and the lower portion 290 are interconnected or at least to some degree hindered from twisting with respect to each other by one or more screws 300 and an optional bearing in the central portion 280 to accommodate the screw 300.
The encircling element 170 also comprises three portions. The lower portion 310 is sometimes referred to as a retaining ring 310, which is intended to limit the movement of the wafer or substrate 110 with respect to the substrate holder 200 during the planarization. A central portion 320, which is sometimes also referred to as the retaining ring holder, is interconnected with the retaining ring 310 and an upper portion 330 of the encircling element 170 via a screw 340. The upper part 330 of the encircling element 170 represents the overhanging portion of the encircling element 170, which may be considered as a “lid” of the recess formed by the encircling element 170. Apart from the described screws, also other connections may be employed, such as threads, rivets or adhesives. Naturally also combinations may be used.
The upper part 330 comprises a bearing 350 through which the screw 300 optionally interconnecting the central portion and the lower portion 280, 290 of the carrier 210 extends. Between the upper portion 330 and the head of the screw 300, a spring element 360 is arranged, which may, for instance, be implemented as a coil spring. Naturally, more than one spring element 360 can be implemented.
Due to the presence of the spring element 360, the substrate holder 200 is also referred to as a spring carrier or spring chuck. In other words, the retaining ring 310 is coupled to the carrier 210 by the spring element 360. The direction of the force imposed by the spring element 360 is such that the retaining ring 310 will be pressed down onto the planarizing surface 120 when the carrier or substrate holder 200 is lowered and pressed against the polished pad 130. Due to this, a pressure is exerted onto the planarizing surface 120 at least partially surrounding the substrate 110. The position of the retaining ring 310 automatically adapts to the thickness of the wafer 110 and to the planarizing surface 120 of the pad 130.
In other words, the retaining ring 310 being the component of the encircling element 170 to be in contact with the planarizing surface 120 is mechanically pressed onto the polish pad 130 by the spring element 360. As a consequence, the substrate holder 200 according to an embodiment of the present invention is capable of automatically adapting to any wafer thickness. The retaining ring 310 is screwed to the retaining ring holder 320 so that no shims are required to be used during a possibly required pre-assembly of the substrate holder 200 to pre-adjust an extension as it is required in the case of conventional substrate holders for CMP systems.
A conventional standard substrate holder or standard carrier often comprises also a retaining ring, which is screwed to a carrier of the CMP system. However, to adjust the extension of the wafer 110 compared to the lower position of the retaining ring, shims are inserted in between the retaining ring and the carrier. Apart from the costs for the shims, the pre-assembly of the conventional standard carrier consumes a significant time, since it has to be adjusted in principle for every new wafer. The extension, which may, for instance, be in the range of approximately 180 μm, may also require a substantial time to adjust.
By employing the substrate holder 200 according to an embodiment of the present invention, the costs for material for building the substrate holder can be reduced, since the shims are not required. Moreover, the so-called in-house effort to reconstruct the carrier or substrate holder 200 according to an embodiment of the present invention is reduced, since the number of spare parts may be reduced due to the flexibility offered by the encircling element 170 being slidably connected to the carrier 210.
Instead of coupling the retaining ring in a fixed way with a predefined extension, the substrate holder 200 according to an embodiment of the present invention utilizes the slidable encircling element 170. The time for assembling the standard carrier can, therefore, be significantly reduced.
Moreover, it might even be possible to reduce the operational costs, since in the case of a standard carrier, the retaining ring may only be used until a minimum height is acquired. Afterwards, the retaining ring may no longer be used and has to be rejected. In the case of the substrate holder according to an embodiment of the present invention, the retaining ring may be used further after the retaining ring height falling below the previously mentioned minimum height. Moreover, by using an adjusted polishing recipe, which may, for instance comprise the mixture of the slurry, the required process time as well as the slurry consumption may eventually be reduced. Since, as previously described, the actual “size” of the wafer is extended by the bearing area 180 of the encircling element 170 or the retaining ring 310, the uniformity of the planarized substrate 110 may be improved, for instance, in the outer border areas of the substrate 110.
Using the spring elements 360 offers, furthermore, the possibility of adjusting the pressure or force exerted by the retaining ring 310 onto the polishing pad 130 of the planarizing surface 120 by adjusting the strength of the spring elements 360 or by altering a bias tension of the spring elements 360 or the available distance by introducing, for instance, lower portions 290 of the carrier 210 with different heights.
Apart from using coil springs as the spring elements 360, also other spring elements may be employed in embodiments according to the present invention. For instance, flat springs, pneumatic springs or rubber elements may be incorporated as spring elements 360.
By adapting the strength of the springs or by adjusting the bias tension of the spring elements 360, a “redistribution” of the force with which the carrier 210 urges the substrate 110 onto the planarizing surface 120 may, in principle, be freely adjusted. In many cases, the carrier 210 presses the substrate 110 purely due to its weight onto the planarizing surface. In such a case, the weight of the carrier 210 and/or the exerted force may have to be adjusted to the pad 130 and the composition of the slurry to be used. Due to employing the spring elements 360, an additional degree of freedom is introduced, since the weight and/or the exerted force of the carrier 210 may be “redistributed” by the forces exerted by the spring elements 360 onto a larger area also comprising not only the surface of the substrate, but also the bearing area 180 of the retaining ring 310. In such a case, the weight of the retaining ring is typically lower than that of the carrier 210.
However, in principle, the situation may also be different. If, for instance, the retaining ring 310 or, to be more general, the encircling element 170 comprises a higher weight than the carrier 210, the gravitational force may be redistributed by oppositely biased spring elements 360 towards the carrier 210 and the substrate 110. In such a case, the spring elements 360 would press the carrier 210 instead of the encircling element 170 onto the planarizing surface.
A further possibility introduced by embodiments according to the present invention is that an original carrier of an existing CMP system may be adapted and reconstructed to become a substrate holder 200 of a planarization system 100 according to an embodiment of the present invention. This can be achieved in such a way that available retaining rings 310 may be further used. As outlined above, this may even be true for retaining rings, which would have to be rejected due to the height being too small, which may result in an additional cost-saving potential.
For instance, in the case of a carrier for a CMP system Auriga for 8″-wafers, the original or standard carrier may be adapted to become a spring carrier according to an embodiment of the present invention by routing off parts of a stainless steel ring to which the retaining ring was originally intended to be mounted. As a consequence, the center portion 280 of the carrier 210 will have the same diameter as the lower portion or plate 290. Next, the retaining ring holder 320 as well as the upper portion 330 are manufactured, which serve as a mechanical element to accommodate the retaining ring 310. The encircling element 170 (retaining ring 310, retaining ring holder 320 and upper portion 330) may then, for instance, be connected to, through the existing holes in the center portion 280 of the carrier 210 and the existing threads in the lower portion 290 (plate) of the carrier 210, by the newly manufactured screws 300. The screws 300, as previously explained, also serve to accommodate the spring element 360.
By employing a retaining ring 310 coupled to the carrier 210 by the spring element 360 according to an embodiment of the present invention, an improvement concerning the uniformity and planarity of planarized substrates 110 may be achievable in the border area of the substrates. As previously outlined, an existing planarization system as well as an existing substrate holder may be reconstructed according to an embodiment of the present invention, so that in principle a great number of existing CMP systems may be modified. Naturally, as a substrate, an intermediate product or a final product or products, the fabrication of which involves one or more CMP steps, may be fabricated using embodiments according to the present invention.
To illustrate possible improvements concerning the planarity of wafers 110, FIG. 6 shows a comparison of four graphs 400, 410, 420, 430 of four measurements of the planarity of four different 8″-wafers having a diameter of approximately 200 mm each. Each of the 4 measurements of the four graphs 400, . . . , 430 comprises 199 data points measured at wafers comprising final products. The measurements indicate the thickness t, measured in nanometers (nm), of the respective wafers 110 along its diameter x in millimeters (mm) in a range between 100 mm and +100 mm, which are sometimes also referred to as “anko-measurements”. In other words, the resulting graphs are not taken after CMP process steps, but after a plurality of process steps, some of which create significant topological height differences.
The first of the graphs 400 visualizes a result of a process based on the previously outlined standard carrier based on a rotational speed of the polishing pad of 12 rpm (revolutions per minute) and a rotational speed of the substrate holder and the carrier of 33 rpm. The polishing time is 150 s. The measurement shows at outer edges of the wafer 110 two distinct nose-like shapes at locations of approximately x=+95 mm and x=−95 mm having a height of about 100 nm above the surface of the rest of the wafer. This nose-like shape above the surface of the wafer 110, indicating a higher thickness at the outer edges of the wafer 110, is sometimes also referred to as a “speedfam nose”. Due to the nose-like shape the available area for devices, chips and dies may be significantly reduced compared to the overall area of the wafer. In the example shown, a loss of more than 1 mm at each edge of the wafer may be lost for devices. Such a loss may not only be a result of the height differences, but also of the slopes encountered, since rapid changes of the thickness may not be favorable.
By employing a planarization system 100 including a substrate holder 200 according to an embodiment of the present invention, this effect may even be reversed by varying a plurality of possible parameters, such as the (rotational) speed of the pad, the rotational speed of the substrate holder 200, the polishing time, the forces exerted by the spring elements 360 and the composition of the slurry. To illustrate this further, FIG. 6 also shows three additional graphs 410, 420, 430 of the thicknesses of wafers 110 being processed based on embodiments according to the present invention. Graph 410 is based on a pad speed of 15 rpm, a carrier speed of 75 rpm and a polishing time of 100 s.
FIG. 6 also shows results of processes based on a pad speed of 13 rpm and a carrier speed of 75 rpm (graphs 420, 430) for a polishing time of 85 s (graph 420) and 133 s (graph 430). Since only graph 430 shows an indication of the presence of a nose-like shape at the outer edges, FIG. 6 illustrates that the effect of appearing nose-like structures may be minimized, suppressed or even reversed by adjusting different parameters leading to lesser thicknesses at the outer edges of the wafer 110. An at least partially optimized result may be achieved by adjusting the contact pressure caused by the spring elements 360.
Naturally, also other parameters may be employed. For instance, a pad speed of 11 rpm with a carrier speed of 50 rpm or a pad speed of 25 rpm with a carrier speed of 100 rpm may be used, leading to comparably flat wafer surfaces. An adaptation of the slurry composition or other parameters may also be advisable to obtain excellent results.
FIG. 7 shows a flowchart of a method for planarizing a substrate according to an embodiment of the present invention. After a start (step S100) in a first optional step S110, a substrate may be provided to the planarizing surface 120 of a planarizing system 100 according to an embodiment of the present invention. Afterwards, in an optional step S120, the substrate holder 200 may be provided over the substrate 110 during step S120. In a third optional step of providing the slurry onto the planarizing surface 120 during step S130, a relative lateral movement between the substrate 110 and the planarizing surface 120 is generated in step S140 such that the encircling element 170 at least partially laterally encloses the substrate 110. Moreover, the relative lateral movement is created such that the encircling element 170 abuts the planarizing surface 120. In other words, as previously described, the bearing area 180 will contact the planarizing surface 120 or the pad 130. Afterwards, in step S150, the method for planarizing the substrate may end after a certain amount of time has expired or another limiting or termination condition is fulfilled.
In the case of a computer-implemented planarization system or in the case of a program, providing a substrate may, for instance, be achieved by controlling valves, motors, step motors or other actuators to position an arm carrying the substrate 110 to be planarized to the planarizing surface 120. This may, for instance, be achieved by a transportation system employing a low pressure or vacuum-lifting system for a wafer or substrate.
Similarly, by controlling actuators, the polishing head comprising a substrate holder according to an embodiment of the present invention may be provided and positioned over the substrate 110 to be planarized. Then, by switching on a pump system or another supply system, the slurry may be provided to the planarizing surface 120. Naturally, also the generation of the relative movement may be initiated by controlling the motor 160.
All the previously described controlling steps may be implemented in a concrete implementation by writing status values or other control values to registers or other addresses. However, implementational details may differ from system to system according to embodiments of the present invention, since different hardware architectures may be involved.
Depending on certain implementation requirements of methods according to embodiments of the present invention, embodiments of the inventive methods can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, in particular, a disc, a CD or a DVD having electronically-readable control signals stored thereon, which co-operate with a programmable computer or processor such that an embodiment of the inventive method is performed. Generally, an embodiment of the present invention is, therefore, a computer program product where the program code is stored on a machine-readable carrier, the program code being operative for performing an embodiment of the inventive method when the computer program product runs on the computer or processor. In other words, embodiments of the inventive methods are, therefore, a computer program having a program code for performing at least one of the embodiments of the inventive methods when the computer program runs on the computer or processor. The processor can be formed by a computer, a chip card, a smart card, a system-on-chip (SOC), an application-specific integrated circuit (ASIC) or an integrated circuit (IC). The processor may also be part of a control system of a production system, a production line or another fabrication-related or experiment-related machine. An embodiment according to the present invention may, therefore, be also implemented as a computer program for performing, when running on a processor, a method for planarizing a substrate.
While the foregoing has been particularly shown and described with reference to particular embodiments above, it will be understood by those skilled in the art that various other changes in the forms and details may be made without departing from the spirit and scope thereof. It is to be understood that various changes may be made in adapting to different embodiments without department from the broader concept disclosed herein and comprehended by the claims that follow.

Claims

1. A planarization system for planarizing a substrate, comprising:

a planarizing surface; and

an encircling element formed to only partially laterally enclose the substrate,

wherein the planarization system is configured to planarize the substrate with the substrate abutting the planarizing surface during a relative lateral movement between the substrate and the planarizing surface, the substrate being only partially laterally enclosed by the encircling element, and wherein the encircling element abuts the planarizing surface.

2. The planarization system according to claim 1, wherein the encircling element comprises a recess such that a distance between the substrate and a circumference of the recess is smaller than or equal to one inch.

3. The planarization system according to claim 1, wherein the encircling element laterally encloses 50% or more of the circumference of the substrate.

4. The planarization system according to claim 1, comprising a substrate holder, wherein the substrate holder comprises the encircling element.

5. The planarization system according to claim 4, wherein the encircling element comprises a retaining ring such that a free movement of the substrate is limited with respect to the substrate holder.

6. The planarization system according to claim 4, wherein the substrate holder comprises a carrier to urge the substrate against the planarizing surface.

7. The planarization system according to claim 6, wherein the carrier comprises a weight such that the carrier is adapted to press the substrate against the planarizing surface due to its weight.

8. The planarization system according to claim 6, wherein the substrate holder comprises a spring element, wherein the spring element couples the carrier to the encircling element.

9. The planarization system according to claim 8, wherein the spring element is adapted to press the encircling element onto the planarizing surface.

10. The planarization system according to claim 1, wherein the planarizing surface is formed by a pad.

11. The planarization system according to claim 10, wherein the pad is configured to rotate about a rotation axis.

12. The planarization system according to claim 1, wherein the planarization system is at least one of a lapping system, a polishing system and/or a CMP system.

13. The planarization system according to claim 1, further comprising a motor configured to create the relative lateral movement between the substrate and the planarizing surface.

14. A substrate holder for holding a substrate during a lateral relative movement between the substrate and a planarizing surface, comprising:

a carrier configured to urge the substrate against the planarizing surface; and

an encircling element formed to only partially laterally enclose the substrate,

wherein the encircling element is vertically movably attached to the carrier so as to abut the planarizing surface during urging the substrate against the planarizing surface.

15. The substrate holder according to claim 14, wherein the encircling element comprises a recess such that a distance between the substrate and a circumference of the recess is less than or equal to one inch.

16. The substrate holder according to claim 14, wherein the encircling element laterally encloses 50% or more of the circumference of the substrate.

17. The substrate holder according to claim 14, wherein the encircling element comprises a retaining ring, wherein the retaining ring is adapted to limit a free movement of the substrate with respect to the substrate holder.

18. The substrate holder according to claim 14, wherein the carrier comprises a weight such that the carrier is adapted to press the substrate against the planarizing surface.

19. The substrate holder according to claim 14, comprising a spring element coupling the carrier and the encircling element.

20. The substrate holder according to claim 19, wherein the spring element is adapted to press the encircling element onto the planarizing surface.

21. The substrate holder according to claim 14, wherein the substrate holder is a substrate holder for at least one of a lapping system, a polishing system and/or a CMP system.

22. A method for planarizing a substrate abutting a planarizing surface, comprising:

creating a relative lateral movement between the substrate and the planarizing surface such that an encircling element formed to only partially laterally enclose the substrate abuts the planarizing surface.

23. The method according to claim 22, further comprising providing the substrate abutting the planarizing surface.

24. The method according to claim 22, further comprising providing a substrate holder comprising the encircling element.

25. A method for planarizing a substrate, comprising:

creating a relative lateral movement between the substrate and a planarizing surface;

exerting a pressure onto the planarizing surface in an area only partially laterally surrounding the substrate.

26. The method according to claim 25, wherein exerting the pressure comprises providing an encircling element abutting the planarizing surface.

27. A planarization system for planarizing a substrate, comprising:

a planarizing surface; and

a substrate holder, the substrate holder comprising an encircling element formed to at least partially laterally enclose the substrate, a carrier formed to urge the substrate against the planarizing surface, and a spring element, wherein the spring element couples the carrier to the encircling element, and the spring element comprises a coil spring,

wherein the planarization system is configured to planarize the substrate with the substrate abutting the planarizing surface during a relative lateral movement between the substrate and the planarizing surface, the substrate being at least partially laterally enclosed by the encircling element, and wherein the encircling element abuts the planarizing surface.

28. A substrate holder for holding a substrate during a lateral relative movement between the substrate and a planarizing surface, comprising:

a carrier configured to urge the substrate against the planarizing surface;

an encircling element formed to at least partially laterally enclose the substrate; and

a spring element coupling the carrier and the encircling element, wherein the spring element comprises a coil spring, and