The present invention relates to a method for patterning a photosensitive tape comprising the steps of translating the photosensitive tape between a tape-feeding source and a tape-receiving source; and optically coupling an optical system between a predetermined pattern to be projected and the tape. The step of translating the tape between the source and receiver, and the step of optically coupling are both described in U.S. Patent 3,562,005..
Up to a few years ago, wire-bonding was the most commonly used method for making external connections to an integrated circuit (IC) chip. An alternative to wire-bonding consists of using a tape carrier, similar to a movie film, having lead frames formed along its surface. In this tape carrier approach, usually a polyimide film carries a copper lead pattern that repeats itself along the length of the film. The finger-like leads of an individual site on the film are bonded simultaneously to the pads of an IC chip, as for example, described in U.S. Patents No. 3,689,991, and 3,968,563.
These processes make use of a thin strip of a continuous electrically insulating tape having a plurality of prepunched apertures at regularly spaced intervals. A thin foil-like strip or layer of electrically conducting material is secured to the tape. By photolithographic masking and etching, portions of the layer are removed to form a plurality of sets of metallic finger-like leads. This subtractive technique, used to form the finger-like leads, is described in an article by S. E. Grossman entitled "Film-carrier Technique Automates the Packaging of IC Chips" in Electronicics, May 16, 1974, pages 89-95. According to this article, the technique consists in first bonding a 1-ounce copper foil to a polyimide film by means of an adhesive. Photoresist techniques form the image of the desired lead frame in a step-and-repeat fashion along the film-mounted copper laminate. This step-and-repeat projection requires indexing, settling and alignment, all of which are time-consuming and expensive operations. Moreover, such a projection technique is difficult if lens lengths of tapes are needed since accelerations associated with high speed indexing are damaging to the fragile tape. Furthermore, the high cost of the polyimide carrier increases the cost per site of such a tape especially when small quantities of custom tapes are needed.
According to the present invention there is provided a method for patterning a photosensitive tape (1) comprising the steps of: translating the photosensitive tape between a tape-feeding source (2) and a tape receiver (3); and coupling an optical system (11) between a predetermined pattern (8) to be projected and the tape (1); characterized in that the method further comprises the steps of continuously rotating in synchronism with the translation of the tape (1) a cylindrical body (6) having on a cylindrical surface (7) thereof the predetermined pattern (8); and illuminating the pattern (8) for continuously projecting a sequence of images representative of the pattern onto the tape via the optical system (11). Other equivalent forms of radiation are included within the word optical.
In accordance with an illustrative embodiment of the present invention, a method for continuously patterning photosensitive tape comprises the steps of translating the photosensitive tape at a predetermined speed between a
In a particular illustrative embodiment of the present invention, the rotating step comprises the step of synchronously locking the cylindrical body and the tape to each other to within the lens resolution of the optical system. Also , the translating step comprises the step of engaging the tape on a cylindrical drum having its axis parallel to the axis of the cylindrical transparent body.
An illustrative embodiment of an apparatus for continuously patterning a photosensitive tape according to the present indention comprises an arrangement for translating the photosensitive tape in a predetermined direction at a predetermined speed between a tape-feeding source and a tape-receiving source, a cylindrical transparent body having its longitudinal axis perpendicular -to the predetermined direction and its cylindrical surface adapted to receive a predetermined pattern to be projected, circuitry for synchronously coupling the translating arrangement and the cylindrical body such that the pattern rotates at the predetermined speed, and an optical system . for projecting an image of the pattern onto the tape, whereby the movement of a projected image is in the predetermined direction.
One advantage of the embodiments is to achieve a highly accurate method and apparatus for continuously patterning a photosensitive tape or foil. Another advantage is to achieve a high-speed continuous patterning process of a tape or foil by means of projection exposure techniques.
Another advantage is to realise a projection exposure system having a very long mask life.
Undue tensions on the tape er foil are prevented during the patterning process.
The embodiments provide a simple, flexible and high-speed projection exposure system for photosensitive tapes or foils.
A photosensitive tape or foil is continuously patterned and the tape or foil is prevented from breaking, thereby achieving an economically attractive projecting process.
Reference is now made to the accompanying drawings in which
- FIG. 1 shows apparatus made according to an embodiment of the present invention;
- FIG. 2 is an enlarged view of a portion of the apparatus shown in FIG. 1 including its optical system;
- FIG. 3 shows a circuit diagram of the phase-locking system of the apparatus shown in FIG. 1;
- FIGS. 4 and 5, respectively, illustrate a front and side view of another illustrative embodiment apparatus;
- FIG. 6 shows a further illustrative embodiment apparatus;
- FIG. 7 is an enlarged view of a portion of the apparatus shown in FIG. 6 including its optical system; and
- FIG. 8 shows a still further illustrative embodiment apparatus.
In the illustrative embodiment of the invention, shown in FIG. 1, an apparatus for continuously patterning a photosensitive tape 1 comprises a tape-feeding reel 2, a tape-receiving reel 3, and a drum 4 for translating the tape 1 at a predetermined speed between reels 2 and 3. The tape 1 can be of any form as described in the art, and the photosensitive region is applied to the tape in accordance with tape processing requirements. The drum 4 is mechanically coupled to a driving mechanism 5 comprising, for example, a motor having its shaft directly coupled to the drum 4
. However, other translating mechanisms may be substituted for drum 4, as will be explained in connection with another illustrative
1, the axes of body 6 and drum A are parallel. Transparent cylindrical body 6 cerries on a surface 7 thereof a predetermined pattern or mask 8 to be projected on the photosensitive tape 1.
The pattern 8 may be directly on the outer surface 7 and may be formed by first coating the surface with a thin metal film and then selectively removing portions thereof by thermal machining of the film. Another alternative for depositing pattern 8 onto the cylindrical surface 7 consists in first producing a predetermined pattern on a 16 mm or 35 mm filmstrip by means of conventional techniques. The filmstrip comprising a plurality of individual frames or patterns could be wrapped around the cylindrical body 6 and held by vacuum against the surface 7. Both ends of the filmstrip would be butted to produce a contiguous set of patterns on the cylindrical surface 7. The cylindrical body 6 is mechanically coupled to a driving mechanism 9 comprising, for example, a motor having its shaft directly coupled to the body 6.
The cylindrical transparent body 6 and the drum 4 are synchronously coupled by means of a coupling circuit 10 responsive to a reference frequency signal fo. The coupling circuit 10 may comprise a pair of phase-locked loops arranged such that the cylindrical transparent body 6 is the "slave". Thus, both cylinders 4 and 6 rotate at precisely the same rate but in opposite directions as shown by the arrows in FIG. 1. In effect, an electrical link exists between drum 4 and cylinder 6 resulting in locking of both cylinders to each other. The apparatus further comprises an optical system 11 positioned between cylindrical body 6 and drum 4 for projecting an image of the pattern 8 onto the tape 1. The optical system 11 may be positioned as shoMn in FIG. 1 between body 6 and drum 4, or may be rotated by a 90-degree angle such that its axis is parallel to the axes of the body and the drum. In the latter, object and image have the same orientation in the direction of the width of tape 1, but have opposite orientation in the longitudinal direction of tape 1. Also, object and image would be offset in the latter direction.
Shown in FIG. 2 is an enlarged portion of the illustrative embodiment of FIG. 1 including the structural details of the optical system 11. By way of background, a known one-to-one imaging optical system is described in an article by J. Dyson entitled "Unit Magnification Optical System without Seidel Aberrations," published in journal of the Optical Society of America, Volume 49, No. 7, July 1959, pages 713-716. This known Dyson system consists of two components, namely, a concave spherical mirror of radius R, and a thick plano-convex lens of radius r, index n and thickness equal to r. The centers of curvature of both spherical surfaces are substantially coincident, and r is chosen so that parallel rays incident on the piano surface are focused on the mirror surface, i.e.,
In this known system, object and image surfaces lie on or close to the plane face of the lens, and object and image are of opposite directions.
In this embodiment, the optical system 1
1 of FIG. 2 is a modified Dyson-type system comprising a plano-convex lens 111 of radius r and index n and a spherical concave mirror 112 of radius R having substantially coincident centers of curvature. The plane face of the plano-convex lens 111 is cemented to two right-angle prisms 113 and 114 in order to bring object and image to usable positions. The pattern 8 to be imaged on the tape 1 is preferably placed or formed on the outer surface of the cylindrical transparent body 6 which is made, for example, of quartz. A narrow strip of this pattern 8 is imaged by the system onto the photosensitive resist coated tape which is held
fused silica with its attendant high transmission in the ultraviolet range. The optical system 11 is telecentrie and hence insensitive to first-order distortions due to focal plane shifts. Since the design is completely symmetric, distortions, coma, and lateral color are zero. Resolution is nearly oiffraction limited over a 2 mm x 16 mm field at F/2.5 and still has acceptable resolution at a 2 x 22 nm field at F/4. Resolution is all cases is better than 5 Ám which is adequate for lead patterns whose narrowest feature would be larger than 50 Ám. Over the of 3000-4400 Angstroms the opticel system is nearly achromatic.
Illumination is provided, for exaisple, by a 1 kW water-cooled mercury capillary arc 12. However, alternative light sources may be used. Water cooling filters out most of the infrared radiation beyond 1 Ám and assures cool operation. A combination of lenses and mirrors schematically shown in FIG. 2, is coupled to the arc for. directing the arc"s rays onto the cylindrical surface 7. The operation of the optical system is such that an object 13 that is part of the pattern 8, when illuminated by light source 12, is projected onto an image plane corresponding to the tapa 1. The incoming object . radiations 15 are first reflected by right-angleprism 113 and directed to lens 111 and mirror 112. The rays from mirror 112, after reflection by right-angle prism 114, are directed to the tape 1 to form the image 14 thareon. As shown in FIG. 2, object 13 and image 14 Lave the same orientation in the direction of translation of width of the taps. i.e., in a plane perpendicular to the page in FIG.2. there is an inversion between object and image. Furthermore, as the pattern 8 rotates,the movement of the image 14 is in the same direction as the movement of tha tape 1. thus enabling a continuous projection patterning of the tape.
Since in the present case, scanning of the pattern takes place in the direction of movement of the tape 1, there is no inversion in the scaming direction and no need for image reversion.
As described above;, the tape-carrying drum 4 and the cylindrical transparent body 6 are synchronously coupled by means of coupling circuit 10. The latter is schematically illustrated is FIG. 3 whersin the drum 4 and the cylindrical body 6 are mechanically driven by electrie motors 5 and 9, respectively. The drum 4 and the body 6, rotate at precisely the same rate., namely synchronously, but in opposite rotational directions. Moreover, the drum 4 and the body 6 are locked to each other within the lens resolution of the optical system on the circumference, i.e., within less than 3 Ám. This corresponds to a rotational tolerance of approximately 20 arc seconds. By using precision optical encoders 31 and 32 and nhase- lacked loop techniques, the body 6 can more with respect to the tape-carrying drum 4 with a speed accuracy of 0.001%. Zn this illustrative embodiments the drum 4 is locked to a predetermined speed by scans of a reference frequency signal fo
couple to one input terminal of phase detector 33. The other isput tsrsdnal of the phase encoder 31. A low-oass filter 35 has its input terminal coupled to the phase detector 33 output terminal, and its operational amplifier 37 is coupled to the output terminal of optical encoder 31 via a froquency-to-amplitude converter 39. The cutput terminal of amplifier 37 is coupled to the driving sotcr 5 of the tape-carrying drum 4.
In this illustrative suhodiment of the ocupling circuit, the motion of tape-carrying drum 4 serves as the aster". The output of the optical encoder 31 serves as the reference frequency to which the cylindrioal transparent body 6 is the "slaye". Thus. lew frequency torque disturbances on the drum 4 are tracked by the body 6, and high frequencies are damped hy the inertia of the loop and motors. The system comprising the drum 4, the body 6, the motors 5 and 9, end the coupling circuit 10 is stiff enough so that torque disturbances in the tape disturb the tape position by less than the image resolution. In the illustrative embodiment of the coupling circuit 10, the reference frequency signal fo is, for example, a 1000 Hz signal and the optical encoders 31 and 32 are 16-bit encoders generating 21D or 65,536 pulses/revolution. The phase detectors 33 and 34, the filters 35 and 36, the amplifiers 37 and 38, and the converters 39 and 40 may be selected from conventional and commercially ayailable components.
The tape-carrying drum 4 and the cylindrical transparent body 6 can both be the "slaves" of the reference frequency signal fO. This is achieved by coupling the reference signal fO to phase detectors 33 and 34, and by connecting the optical encoder 31 output signal only to the other input terminal of phase detector 33. Thus, instead of having a "master-slave" arrangement as shown in FIG. 3, the drum and the body would be "slaves" and locked to fO.
Another illustrative embodiment of the present invention for patterning both sides of a photoresist coated tape is shown in FIGS. 4 and 5. The apparatus for projecting an image onto the continuous tape 1 comprises
embodiment of FIG .1, the patterns 8 and 8' may be formed, for akample, directlv on zurfaces 7 and 7' by thermal machining. alternativaly patterns 8 and 8' may be formed on a filmstrip erapped around the drums 6 and 6' and held by vacuum against surfaces 7 and 7' .However, other means of forming a pattern eonto a cylindrical surfacecan alternatively be used.
Referring now to FIGS. 6 and 7 , wherein double-sided patterning of photosensitive tape 1 is shown, identical numerals corresponding to the numerals of the previous figures are utilized to illustrate the similarities of the illustrative embodiments. In this illustrative embodiment, topside exposure of photosensitive tape 1 is obtained by projection printing from the drum 6 of pattern 6 as explained in connection with the previously described embodiments. The back or other side of tape 1 is patterned by means of contact printing of a pattern 60 onto the tape. The pattern 60 on drum 6' and pattern 8 on drum 6 may be identical. However, different patterns may be used when it is desirable to project on both sides of the tape a different beam lead pattern. Contact printing consists of first forming a mask 60 according to oonventional nask producing tachniques, and wrapping the mask around the eylindrioal surface 7' of drum 6' In this illustrative embodiment. as in the embodiment of FIG. 1, both drums 6 and 6' are synchronously coupled and looked to each other by means of coupling cirouit 10.
are implemented. As deseribed in connection with the other embediments, the pattern 8 may be formed accorording to the above-described methods.
In all of the above illustrative embodiments of the present invention, the photosensitive tape 1 may be a photoresist coated copper tape or a photoresist coated continuous metal-composite tape. Either negative photoresists or positive photoresists may be employed. After patterning the photosensitive copper tape by using any of the above-described method and apparatus, the copper is etched where exposed (if positive resist is used) leaving a set of thin copper leads suitable for "gang" or sumultaneous bonding to a chip.