WO2002082491A2

WO2002082491A2 - Plasma source having supplemental energizer for ion enhancement

Info

Publication number: WO2002082491A2
Application number: PCT/US2002/010335
Authority: WO
Inventors: Aseem Srivastava; Daniel Richardson
Original assignee: Axcelis Technologies Inc.
Priority date: 2001-04-06
Filing date: 2002-04-04
Publication date: 2002-10-17
Also published as: TW533752B; WO2002082491A3; AU2002256040A1

Abstract

A supplemental ion source (74) for a plasma processing system (10) having a plasma processing chamber (16) is provided. The ion source (74) comprises: a signal generator (82, 96) for generating an output signal; and an antenna assembly (76, 90) located proximate the process chamber (16), whereby energization of the antenna assembly by the signal generator ionizes plasma confined within the processing chamber (16) to create plasma having a substantial ionized content. The antenna assembly (76, 90) is generally planar in shape and may take the form of a plate or coil antenna. The signal generator preferably generates an output signal in the radio frequency (RF) range. The supplemental energizer operates independently of the first plasma source such that either, or both, may be switched on or off at any time.

Description

PLASMA SOURCE HAVING SUPPLEMENTAL ENERGIZER FOR ION ENHANCEMENT

Field of the Invention

[0001] The present invention relates generally to the field of semiconductor processing systems such as photoresist ashers, and more specifically to a plasma source, for use in such systems, having a supplemental energizer for ion enhancement.

Background of the Invention

[0002] In the manufacture of integrated circuits, photolithography techniques are used to form integrated circuit patterns on a substrate. Typically, the substrate is coated with a photoresist, portions of which are exposed to ultraviolet (UV) radiation through a mask to image a desired circuit pattern on the photoresist. The portions of the photoresist left unexposed to the UV radiation are removed by a processing solution, leaving only the exposed portions on the substrate. Sometimes, but not always, these remaining exposed portions are baked during a photostabilization process to enable the photoresist to withstand subsequent processing.

[0003] After such processing, in which the integrated circuit components are formed, it is generally necessary to remove the baked photoresist from the wafer. In addition, residue that has been introduced on the substrate surface through processes such as etching must be removed. Typically, the photoresist is "ashed" or "burned" in the presence of atomic oxygen and other gases, and the ashed or burned photoresist, along with the residue, is "stripped" or "cleaned" from the surface of the substrate.

[0004] One manner of removing photoresist and residues is by directing an

RF-energized or microwave-energized plasma at the substrate surface. In the case of a microwave-energized plasma, the plasma is formed by a gas mixture that is transported through a plasma tube that passes through a microwave cavity. Microwave energy within the cavity is introduced into the plasma tube to excite the gas mixture therein and form a plasma. The plasma passes from the tube into a process chamber in which resides a photoresist-coated semiconductor substrate to be ashed. This type of asher is known as a "downstream asher", where the substrate is removed from the plasma generator, which is known as an "upstream" plasma source.

[0005] Photoresist and residue removal processes are typically accomplished with reactive radical species created in a plasma generator and transported to the wafer process chamber. For an oxygen-based plasma, a quartz plasma tube is suitable and provides an efficient means for creating and transporting the plasma to the process chamber] In such a "bulk" asher, the plasma generator and tube have in the past have been designed to reduce the content of ions in the afterglow plasma impinging upon the wafer in order to reduce the effect of wafer charging, which could otherwise damage integrated circuits residing on the wafer. [0006] For certain ashing and other plasma-related processes (e.g., residue removal), it has been found that a source of fluorine may be added to the process gas mixture constituency to provide for more effective or efficient processing (e.g., enhanced ash rates). Such improved ash rates (and residue removal capabilities of fluorine), however, are achieved at the expense of degradation of the quartz plasma tube. The degradation is caused by the fluorine in the process gas mixture that etches the inner surface of the quartz tube. The use of a sapphire tube for a fluorine-based plasma will prevent fluorine etching of the inner surface of the tube, while providing enhanced photoresist stripping capabilities. Like the "bulk" asher, the "enhanced strip" has heretofore been designed to reduce the content of ions in the afterglow plasma impinging upon the wafer in order to reduce the effect of wafer charging.

[0007] Some ashing and residue removal applications, however, ^vare improved when ions are present in the afterglow plasma impinging uniformly upon the wafer. For example, the time spent removing or breaking through the carbonized crust of ion-implanted photoresist is greatly reduced when ions in the afterglow plasma bombard the crusted surface. Other processes that are improved when ions are present include soft etch processes and anisotropic residue removal processes. By creating a plasma with a significant ion content, the system throughput can be correspondingly increased when performing these processes. [0008] Accordingly, it is an object of the present invention to provide an ashing and residue removal system incorporating a plasma generator and transport mechanism that will (i) increase the throughput of wafers that are stripped and cleaned by the system while (ii) preventing the adverse effects of wafer charging. It is a further object of the invention to provide plasma generator and transport mechanism that provides for selective control of the ion content of the plasma generated and transported to the wafer. It is yet a further object of the invention to provide such a mechanism while minimizing metallic or particulate contamination of the wafer.

Summary of the Invention

[0009] A plasma source is provided for a plasma processing system, comprising a first plasma source and a supplemental energizer for enhancing the ion content of the plasma generated by the first plasma source. The supplemental energizer comprises a signal generator for generating an output signal; and an antenna assembly located proximate the process chamber. Energization of the antenna assembly by the signal generator ionizes plasma confined within the processing chamber to create plasma having a substantial ionized content. The antenna assembly is generally planar in shape and may take the form of a plate or coil antenna. The signal generator preferably generates an output signal in the radio frequency (RF) range. The supplemental energizer operates independently of the first plasma source such that either, or both, may be switched on or off at any time.

Brief Description of the Drawings

[0010] Figure 1a is a cross sectional view of a prior art photoresist asher

(having a quartz plasma tube) into which may be incorporated the plasma source and supplemental energizer of the present invention; I

[0011] Figure 1 b is a cross sectional view of a prior art photoresist asher

(having a sapphire plasma tube) into which may be incorporated the plasma source and supplemental energizer of the present invention;

[0012] Figure 2 is a cross-sectional view of a first embodiment of a plasma source and supplemental energizer of the present invention which can be coupled to the process chamber of either of the photoresist ashers of Figures 1a or 1 b; [0013] Figure 3 is a cross-sectional view of the supplemental energizer of

Figure 2, taken along the line 3-3;

[0014] Figure 4 is a second embodiment of a plasma source and supplemental energizer of the present invention which can be coupled to the process chamber of either of the photoresist ashers of Figures 1a or 1b; and [0015] Figure 5 is a is a cross-sectional view of the supplemental energizer of Figure 4, taken along the line 5-5.

Detailed Description of a Preferred Embodiment

[0016] Referring now to the drawings, Figures 1 a and 1 b discloses a prior art photoresist asher 10a and 10b, respectively, each comprising a gas box 12; a microwave power generator assembly 14; a process chamber 16 in which is heated a semiconductor substrate or workpiece such as a wafer 18; and a radiant heater assembly 20 for heating the wafer 18, situated at the bottom of the process chamber. A temperature probe 24, such as a thermocouple, is used to monitor the temperature of the wafer 18. A vacuum pump 26 is used to evacuate the process chamber 16 for processes requiring vacuum conditions. A monochromator 28 is used to monitor the optical emission characteristics of gases within the chamber to aid in process endpoint determination.

[0017] In operation, a desired mixture of gases is introduced into a plasma tube 32 from gas box 12 through an inlet conduit 34. In Figure 1a, the plasma tube 32a is made of quartz and in Figure 1 b, the plasma tube 32b is made of sapphire. The gases forming the desired mixture are stored in separate supplies (not shown) and mixed in the gas box 12 by means of valves 36 and piping 38. One example of a desired gas mixture is forming gas (primarily nitrogen with a small percentage of hydrogen), and oxygen. Optionally, in the asher of Figure 1 b, a fluorine containing gas such as carbon tetrafluoride may be added to the gas mixture to improve ashing rates for certain processes.

[0018] The desired gas mixture is energized by the microwave power generator assembly 14 to form a reactive plasma that will ash photoresist on the wafer 18 in the process chamber 16 when heated by the radiant heater assembly 20. A magnetron 40 generates microwave energy (at 2.45 gigahertz (GHz)) that is coupled to a waveguide 42. Microwave energy is fed from the waveguide through apertures (not shown) in microwave enclosure 44, which surrounds the plasma tube 32. Although not shown in Figures 1a, 1b, 2 and 4, the desired gas mixture may be energized by a radio frequency (RF) power source in place of magnetron 40, as is known in the art.

[0019] As shown in Figure 1 b, the plasma tube 32b is made of alumina

(AI₂O₃) or single crystal sapphire to accommodate fluorine plasma chemistries. An outer quartz cooling tube 46 surrounds the sapphire plasma tube 32b, slightly separated therefrom. Pressurized air is fed into the gap between the tubes 32b and 46 to effectively cool the sapphire tube 32b during operation. [0020] In Figure 1b, the microwave enclosure 44 is segmented into sections shown by phantom lines 45. Segmentation of the enclosure 44 allows uniform microwave power distribution across the length of the sapphire tube, and protects it from overheating by preventing an unacceptably large thermal gradient from developing along its axial length when suitable input power is provided. Unlike quartz, sapphire is inclined to crack when heated unevenly. Each segment of the enclosure 44 is separately fed with microwave energy that passes through the quartz tube 46 and the sapphire tube 32 passing therethrough. [0021] In either case (Figures 1 a or 1 b), the gas mixture within the plasma tube 32 (quartz or sapphire) is thereby energized to create a plasma. Microwave traps 48 and 50 are provided at the ends of the microwave enclosure 44 to prevent microwave leakage. Energized plasma enters the process chamber 16 through an opening 51 in the top wall of lid 52 thereof. [0022] The reactive plasma ashes the photoresist on the wafer 18. An apertured, dual layered baffle plate assembly 54, comprised of upper baffle plate 54a 'and lower baffle plate 54b, evenly distributes the reactive plasma across the surface of the wafer 18 being processed. The radiant heater assembly 20 comprises a plurality of tungsten halogen lamps 58 residing in a reflector that reflects and redirects the heat generated by the lamps toward the backside of the wafer 18 positioned within the process chamber 16 on pins 68. One or more temperature sensors 72, such as thermocouples, are mounted on the exterior of process chamber lid 52 to provide an indication of chamber temperature. [0023] The present invention is embodied in a novel improved plasma source that comprises a conventional microwave or radio-frequency (RF) plasma source coupled with an optionally operable supplemental RF ion source 74 positioned near the surface of the wafer. The conventional plasma source generates a plasma that is rich in reactive atomic species but having a poor concentration of ions. The supplemental ion source 74 generates a plasma having a substantial ionized content near the surface of the wafer. The conventional plasma source and the supplemental ion source are independently operable to accommodate different plasma characteristic requirements (e.g., low ion content or high ion content) in a single process.

[0024] Although the improved plasma source of the present invention may be incorporated into either of the ashers shown in Figures 1a or 1 b, it is shown in Figures 2 and 4 as being incorporated into an asher of the type of Figure 1 b. Components in Figures 1 b that are common with Figures 2 and 4 are provided with identical reference numbers. Additionally, the supplemental ion source may be incorporated into any type of upstream plasma based asher. [0025] First referring to Figure 2, which shows an asher 10c, the supplemental ion source 74 is provided in the form of a coil antenna assembly 76 located between the chamber lid 52 and the plasma tube 32b. The chamber lid 52 in asher 10c is made of quartz or a similar dielectric material so that the signal emitted by coil antenna assembly 76 may pass therethrough and into the process chamber 16. The coil antenna assembly 76 comprises a metallic (e.g., copper) coil antenna 78 embedded within a base 80. The base 80 in the preferred embodiment is made of a fluoropolymer resin such as polytetrafluoroethylene (PFTE), more commonly referred to as Teflon®. Teflon® is a registered trademark of E. I. du Pont de Nemours and Company.

[0026] The coil antenna 78 should preferably reside outside of the process chamber so that there is no contact with the energetic plasma, and no resulting particulate or sputtered metal contamination problem. The coil antenna, which is generally planar in shape, is sufficiently large to cover the entirety, or substantially the entirety, of wafer 18.

[0027] The antenna 78 is energized by an RF signal output by RF signal generator 82. In the preferred embodiment, the RF signal operates at 13.56 megahertz (MHz). However, the operating frequency may be any radio frequency that is permitted within the ISM bands. Between the RF signal generator 82 and the antenna 78 is a matching network 84 that minimizes reflected power from the antenna back into the RF generator. The connections between the RF generator 82 and the matching network 84, and the matching network and antenna 78, are typically made using coaxial cables, or waveguides. The antenna 78 energizes and ionizes the plasma within the chamber 16 so as to create ions useful in some plasma ashing processes.

[0028] In a typical operation, the ashing of ion-implanted photoresist was performed using the asher 10c of Figure 2. The 200 mm test wafer was coated with a 2.4 micron layer of photoresist implanted with a 30 keV phosphorous implant (dose of 5E15). The wafer temperature was ramped to 120 degrees C with the plasma on during the ramp to minimize any temperature overshoot that could have caused blistering or popping, that might otherwise occur if the wafer temperature exceeds 120 degrees C before the carbonized crust of the implanted photoresist is completely removed.

[0029] By using the supplemental ion source 74, the process time required to break through the thin carbonized crust formed atop the blanket implanted photoresist on the wafer was significantly reduced, as compared to processes that did not utilize the supplemental ion source. Increasing RF power to the supplemental power source and operating pressure provided even better results. In addition, there appears to be no wafer temperature excursion during crust breakthrough with simultaneous operation of the primary microwave power source and the supplemental RF power source.

[0030] Referring now to Figure 4, which shows an asher 10d, the supplemental ion source is provided in the form of a plate antenna assembly 90 located between the chamber lid 52 and the plasma tube 32b. Again, in Figure 4, the chamber lid 52 is made of quartz or a similar dielectric material so that the signal emitted by plate antenna assembly 90 may pass therethrough and into the process chamber 16. The plate antenna assembly 90 comprises a metallic (e.g., copper or aluminum) plate antenna 92.

[0031] The plate antenna 92 is generally circular in shape (see also Figure

5), and is provided with a central aperture 93 that aligns with the plasma tube 32b. The plate antenna, which is generally planar in shape, is sufficiently large to cover the entirety, or substantially the entirety, of wafer 18.

[0032] The antenna 92 is energized by an RF signal output by RF signal generator 96. In the preferred embodiment, the RF signal operates at 13.56 megahertz (MHz). However, the operating frequency may be any radio frequency that is permitted within the ISM bands. Between the RF signal generator 96 and the antenna 92 is a matching network 98 that minimizes reflected power from the antenna back into the RF generator. The connections between the RF generator 96 and the matching network 98, and the matching network and antenna 92, are typically made using coaxial cables, or waveguides. The antenna 92 energizes and ionizes the plasma within the chamber 16 so as to create ions useful in some plasma ashing processes.

[0033] Although both Figures 2 and 4 show the use of an RF source for the supplemental energizer, the use of other energizing (e.g., microwave) sources is contemplated for the supplemental energizer. In addition, although the test data presented above relates to a 200-mm wafer, the invention has applications to 300- mm wafers as well.

[0034] Accordingly, preferred embodiments of a plasma processing system, having a primary source and a supplemental energizer for ion enhancement, have been described. With the foregoing description in mind, however, it is understood that this description is made only by way of example, that the invention is not limited to the particular embodiments described herein, and that various rearrangements, modifications, and substitutions may be implemented with respect to the foregoing description without departing from the scope of the invention as defined by the following claims and their equivalents.

Claims

What we claim is:

1. A supplemental ion source (74) for a plasma processing system (10) having a plasma processing chamber (16), the ion source (74) comprising: a signal generator (82, 96) for generating an output signal; and an antenna assembly (76, 90) located proximate the process chamber (16), whereby energization of the antenna assembly by the signal generator ionizes plasma confined within the processing chamber (16) to create plasma having a substantial ionized content.

2. The supplemental ion source (74) of claim 1 , wherein said antenna assembly (76, 90) is generally planar in shape.

3. The supplemental ion source (74) of claim 2, wherein said signal generator generates an output signal in the radio frequency (RF) range.

4. The supplemental ion source (74) of claim 3, wherein said output signal is roughly 13.56 megahertz (MHz).

5. The supplemental ion source (74) of claim 2, wherein said generally planar antenna assembly includes a coil antenna (78).

6. The supplemental ion source (74) of claim 2, wherein said generally planar antenna assembly includes a plate antenna (92).

7. The supplemental ion source (74) of claim 2, wherein said generally planar antenna assembly is embedded in a fluoropolymer resin.

8. A plasma processing system (10) comprising: first power generator assembly (14) including a plasma tube (32) and a generator (40) for ionizing gas introduced into the plasma tube; a process chamber (16) for processing a workpiece connected to the first power generator assembly (14); and a supplemental ion source (74) comprising (i) a signal generator (82, 96) for generating an output signal, and (ii) an antenna assembly (76, 90) located proximate the process chamber (16), whereby energization of the antenna assembly by the signal generator ionizes plasma confined within the processing chamber (16) to create plasma having a substantial ionized content.

9. The plasma processing system (10) of claim 8, wherein said antenna assembly (76, 90) is generally planar in shape.

10. The plasma processing system (10) of claim 9, wherein said signal generator generates an output signal in the radio frequency (RF) range.

11. The plasma processing system (10) of claim 10, wherein said output signal is roughly 13.56 megahertz (MHz).

12. The plasma processing system (10) of claim 9, wherein said generally planar antenna assembly includes a coil antenna (78).

13. The plasma processing system (10) of claim 9, wherein said generally planar antenna assembly includes a plate antenna (92).

14. The plasma processing system (10) of claim 9, wherein said generally planar antenna assembly is embedded in a fluoropolymer resin.

15. The plasma processing system (10) of claim 9, wherein said generator (40) is a magnetron.

16. The plasma processing system (10) of claim 9, wherein said generator (40) is a radio frequency (RF) power source.

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