Brim and gas escape for non-contact wafer holder

The present invention comprises a brim surrounding a wafer or wafer-like object during plasma etching in a non-contact wafer holder, such brim facilitating uniform flow of the plasma discharge around the edge of the wafer during plasma etching. The brim of the present invention avoids plasma instability and non-uniform flow typical of conventional plasma etching near the edges of the wafer being etched. The brim of the present invention, by facilitating uniform and stable plasma flows, decreases non-uniform etching. One embodiment of the present invention permits the brim to move in the axial direction from a position substantially. This permits the etching process to be controlled for more uniform and precise wafer etching as lowering the brim tends to shadow the edge region of the wafer from the plasma, reducing etching in the edge region while not significantly affecting etching in the central regions of the wafer. Another embodiment of the wafer includes a barrier on the upper side of the brim directed upward from the brim at an oblique angle away from the wafer. This barrier contacts the upper surface of the brim so as to leave a protrusion or debris-collecting shelf on the upper interior portion of the brim. This shelf in combination with the upward oblique barrier deflects the plasma and debris from plasma etching away from the wafer. Another embodiment of the invention includes a gas-controlling baffle in which gas flow around the edge of the wafer may be controlled to compensate for mechanical imprecision in the gap between the brim and the wafer and/or to provide an additional means of controlling etching in the vicinity of the edge of the wafer.

BACKGROUND OF THE INVENTION
 1. Technical Field
 The present invention relates to the field of plasma etching of wafer-like
 objects held in a non-contact wafer holder and, more particularly, to a
 brim and gas escape structure around the circumference of the wafer
 reducing unstable and non-uniform plasma traverse near the wafer's edge,
 while deflecting plasma and etching debris from the wafer's edge and
 unetched face and providing additional means of controlling wafer etching
 in the edge region.
 2. Description of Related Art
 Numerous areas of technology require that a workpiece be held in position
 while processes are performed thereon. The processing of semiconductor
 wafers into integrated circuits requires the wafer to be held by a
 suitable holder without impeding the processing steps directed to the
 exposed flat surface of the wafer. Following production of the integrated
 circuits, wafer processing is generally not finished. In particular, many
 areas of technology (cell phones, smart cards and the like) require that
 the integrated circuit providing the internal intelligence for the device
 be very thin. It is typically inconvenient to fabricate the integrated
 circuits directly on a thin wafer as distortion, non-uniform processing,
 perhaps even rupture, may occur during the several processing steps
 typically required for integrated circuit fabrication. Therefore, it is
 common practice for a wafer containing numerous fully fabricated
 integrated circuits thereon to be thinned by means of etching from the
 reverse side of the wafer. Plasma etching is the typical method for wafer
 thinning. For economy of language we will refer to the "face" of the wafer
 and intend thereby the face undergoing (or destined to undergo) etching,
 resulting in thinning of the wafer. The "opposite face" of the wafer
 denotes the face of the wafer not undergoing etching, typically having the
 integrated circuits thereon and lying in close proximity to the wafer
 holder.
 Several challenges must be met for successful wafer thinning by plasma
 etching. The wafer is (or soon becomes) very thin, rendering it
 susceptible to distortion. Distortion would generally lead to non-uniform
 etching, non-uniform heating of the wafer and potential damage to the
 integrated circuits lying on the reverse face of the wafer from that being
 etched. Thus, an important challenge to be met by a wafer holder is the
 ability to hold thin, easily distorted wafers in a flat position during
 etch. Of course, the wafer holder must not contact the exposed integrated
 circuits lying on the opposite face. Thus, non-contact support for a
 flexible wafer must be provided.
 To be definite in our description of the invention, we will describe the
 common instance of the processing of semiconductor wafers as may typically
 occur in the production of integrated circuits or in wafer post-processing
 for thinning, etc. However, the processing of any wafer-like object in a
 non-contact holder is also a potential area of application for the present
 invention. Flat panel displays and other rectangular, circular,
 star-shaped or irregularly shaped planar objects may require processing by
 means of a non-contact holder. For economy of language we will refer to
 all such wafer-like objects herein as "wafers" recognizing that such
 objects may be large and need not be rectangular, circular or regular in
 shape. Since semiconductor wafers are expected to be an important area of
 application for the present invention, we will describe the primary
 features of the present invention in terms of semiconductor processing,
 not intending to limit the invention to this particular choice or
 particular example. Semiconductor wafers aptly illustrate the features of
 the present invention and permit obvious modification for use in
 processing other wafer-like objects.
 The floating of a wafer above a layer of compressed gas is described in the
 work of Pirker (U.S. Pat. No. 5,896,877). The wafer is held in position by
 gravity while the air cushion prevents contact with the wafer holder.
 Work of Siniaguine and Steinberg (PCT International Publication No. WO
 97/45862) describes a non-contact holder for wafer-like objects in which a
 vortex of rotating air provides both the vacuum support for the wafer and
 the air cushion.
 One challenge to be met by a non-contact wafer holder relates to preventing
 debris from the plasma etch from contaminating the integrated circuits on
 the opposite face of the wafer. More stringently, the wafer holder should
 also prevent debris from impacting the thin edge of the wafer. Non-contact
 support invariably implies a gap between the wafer and the holder. Etching
 debris clearly need to be kept out of this gap. Preferably, the etching
 debris should also be kept from the wafer edge. While not as serious a
 problem as debris contacting the opposite face of the wafer, edge
 contamination may lead to rejection of the particular integrated circuits
 lying on the contaminated regions of the edge thus, reducing yield.
 Plasma etching of a wafer typically occurs by means of the wafer passing
 through a largely stationary plasma discharge. The plasma discharge tends
 to preferentially dwell on the leading and trailing edges of the wafer as
 the wafer passes through the discharge rather than uniformly traversing
 onto and off of the wafer. This can lead to non-uniform etching. The brim
 structure of the present invention is intended to provide for a smooth
 transition of the plasma discharge onto and off of the wafer, facilitating
 thereby uniform etching of the entire wafer including the edge portions
 thereof.
 BRIEF SUMMARY OF THE INVENTION
 The present invention comprises a brim surrounding a wafer or wafer-like
 object during plasma etching in a non-contact wafer holder. This brim is
 preferably 20 mm to 25 mm in width, approximately 1.3 mm thick and the gap
 is substantially constant throughout the circumference of the wafer. The
 gap is preferably approximately 0.5 mm for etching of integrated circuit
 wafers. A brim so dimensioned and configured will facilitate the plasma
 discharge in flowing smoothly onto and off of the wafer, avoiding thereby
 the plasma instability and non-uniform flow typically of conventional
 plasma etching near the edges of the wafer being etched. The brim of the
 present invention, by facilitating uniform and stable plasma flows,
 decreases non-uniform etching.
 One embodiment of the present invention permits the brim to move in the
 axial direction from a position substantially aligned with the lower face
 of the wafer to a position typically 1 mm to 3 mm below the plane of the
 wafer. This permits the etching process to be controlled for more uniform
 and precise wafer etching as lowering the brim tends to shadow the edge
 region of the wafer from the plasma, reducing etching in the edge region
 while not significantly affecting etching in the central regions of the
 wafer.
 Another embodiment of the wafer includes a barrier on the upper side of the
 brim, that is the side opposite the plasma. This barrier is directed
 upward from the brim at an oblique angle away from the wafer. This barrier
 contacts the upper surface of the brim so as to leave a protrusion or
 debris-collecting shelf on the upper interior portion of the brim. This
 shelf in combination with the upward oblique barrier deflects the plasma
 and debris from plasma etching away from the wafer. Debris is typically
 deposited on the shelf portion of the brim from which it is readily
 cleaned following the etching process.
 Yet another embodiment of the present invention includes a baffle for
 controlling the vertical flow of gas exiting from the non-contact wafer
 holder. Additional control of the etching process (especially near the
 edge of the wafer) is obtained by adjusting the baffle to direct the gas
 flow to a greater or lesser degree in the direction towards the wafer
 holder versus escaping around the wafer's edge.

DETAILED DESCRIPTION OF THE INVENTION
 In the following description and figures, similar reference numbers are
 used to identify similar elements.
 FIG. 1 depicts schematically and in cross section (but not to scale) a
 wafer-like object ("wafer") undergoing plasma etching. The wafer, 1, may
 optionally have integrated circuits or other structures, 2, fabricated on
 the face of the wafer nearer the wafer holder, 3. A common non-contact
 wafer holder, 3, could hold wafer, 1, (for example) by means of
 vortex-created partial vacuum attraction while providing an air (or
 similar gas) cushion, 4, preventing direct contact between wafer and
 holder. Wafer, 1, may be supported from above by vortex-created partial
 vacuum generated by numerous vortex "chucks" on the face of the wafer
 holder. These individual vortex chucks are not depicted in the figures.
 The air may exit from each vortex chuck preferentially in certain
 directions, merging in the region between wafer and holder into the
 overall partial vacuum support and air cushion in a complex flow pattern.
 The net flow of air around the wafer denoted as 4 in FIG. 1 may not exit
 from the gap between the wafer and holder completely uniformly in all
 radial directions. However, even in such cases, significant air exits from
 all regions of the circumference of the wafer holder to provide adequate
 separation of wafer and wafer holder, 7, typically 0.10 mm to 0.40 mm. The
 present invention is not limited to use with a particular form of
 non-contact wafer holder but may be used with advantage to stabilize the
 plasma and deflect etching debris in many types of non-contact holders.
 Wafer, 1, would typically be etched by causing a plasma discharge to pass
 across the face of the wafer removing material from the wafer while so
 doing. In practice, the wafer would typically be in motion with respect to
 the laboratory and pass through a stationary plasma. However, it is more
 convenient and economical of language to describe the present invention as
 if the wafer and wafer holder were stationary and the etching plasma
 passed over the wafer's face. Descriptions given in terms of a stationary
 wafer are simply translated into a frame with stationary plasma by
 reversing directions of motion.
 FIG. 1 denotes as 6 the direction of travel of the plasma across the face
 of the wafer, 1. Plasma discharge would first encounter the wafer as 5a,
 move across the face of the wafer, typically as 5b, and move off the wafer
 as 5c. The preferred shape of the plasma discharge is 5b for uniformity of
 etching. However, in typical plasma etching processes, the plasma
 discharge tends preferentially to dwell on the face of the wafer when
 entering or leaving the wafer surface, depicted as 5a and 5c. This
 preferential attraction typically leads to distortion of the shape of the
 discharge as schematically depicted by 5a and 5c. Thus, plasma flow is
 unstable near the edges of the wafer. This distortion of the plasma
 discharge, instability of the plasma and preferential attraction of the
 discharge for the wafer may result in uneven etching. Thus, one problem to
 be addressed by the brim and gas escape structure of the present invention
 is to reduce or eliminate the plasma non-uniformities depicted as 5a and
 5c. More uniform etching is achieved as the plasma discharge is rendered
 more stable near the edges of the wafer.
 FIGS. 2 and 3 depict a brim and gas escape structure of the present
 invention (not to scale) as 9. The structure, 9, depicted in FIGS. 2 and 3
 is generally annular in shape having the same central axis as that of the
 wafer holder, 3. One part of the cross section is depicted in FIGS. 2 and
 3, the full structure being obtained by rotation of 9 about the vertical
 central axis of the wafer holder, 3.
 Brim, 9, is typically located in proximity to the edge of the wafer leaving
 a gap of about 0.5 mm, 10. This gap is sufficiently small that edge
 effects of the plasma discharge are substantially reduced or eliminated.
 Direct contact between wafer, 1, and brim, 9, is contraindicated for
 several reasons. The radial force resulting from direct contact between
 brim and wafer may lead to distortion or buckling of the wafer, especially
 for very thin wafers. Direct contact would typically affect etching at the
 very edge of the wafer as well. Therefore, a small gap, 10, is preferred
 in the practice of the present invention.
 The presence of gap, 10 causes plasma discharge on the edge region of the
 wafer to take on a form, 5d, more characteristic of the desired plasma
 discharge shape, 5b. Therefore, plasma distortion near the edge of the
 wafer is substantially reduced or eliminated, resulting in more uniform
 etching of the entire wafer.
 Brim, 9, should not be etched by the plasma, 5d in order to maintain its
 dimensional integrity and structure for long periods of etching. Typical
 plasma discharges etch by means of fluorine while brim, 9, is typically
 made of aluminum, not etched by fluorine.
 Gas dynamic effects are also used to improve the plasma etching according
 to the present invention. Plasma gas typically has very fast and vigorous
 flow, 8, which will penetrate gap, 10 and deflect air, 4, upward. Yet
 another advantage of the present invention is the structure keeping plasma
 and the debris carried by the plasma from contaminating both the upper
 face of the wafer and the edge. This is accomplished according to the
 present invention by providing a debris collector, 13. Collector, 13, is
 typically about 0.5 mm to about 1.3 mm thick (14 in FIG. 3) while
 providing a platform surface, 13 about 1 mm to about 3 mm in length. This
 platform would typically be adjacent to upward directed wall, 12.
 Typically wall 12 would have an angle of about 0 deg. to 60 deg (20 in
 FIG. 3) from vertical.
 The structure described herein induces plasma passing through gap, 10 to
 collide with air stream, 4, and typically execute turbulent, swirling
 motion, 8. This swirling motion, 8, tends to deposit debris carried by the
 plasma onto shelf 13. Thus, the structure of the present invention keeps
 both the opposite face of the wafer and the edge free from plasma etching
 and debris carried by plasma etching.
 An additional embodiment of the present invention is depicted in FIG. 2A.
 An adjustable baffle, 17, may be included with the brim structure of the
 present invention. The purpose of baffle 17 is to provide additional
 control of the gas flow, 4, both in the direction towards the wafer
 holder, 4a, and in the opposite direction, 4b. Adjustment of baffle 17
 changes the resistance to gas flow in the direction of 4a. Thus, baffle 17
 can cause more or less gas to travel in direction 4a, and simultaneously
 less or more gas to travel in direction 4b. Baffle 17 may adjust the
 resistance to gas flow by being physically movable in direction 19,
 changing thereby the gap 18 for gas flow. Preferably, baffle 17 would be
 fixed in position leaving a small (or no) gap, 18. In this embodiment,
 baffle 17 would typically be provided with several holes through which gas
 could flow in direction 4a. Some fraction of the holes in baffle, 17,
 could be plugged prior to processing, leading to adjustment of flow 4a on
 a case-by-case basis depending on the number and location of holes
 plugged.
 In practical wafer processing systems, the gap between the wafer and brim,
 10, may not be precisely machined. Thus, the adjustable features of
 baffle, 17, could be used to compensate for variations in the gas escape
 4b arising from normal variation in manufacturing tolerance.
 Additionally, the adjustment of baffle 17 can be used to adjust the plasma
 etching in the region of the edge of wafer, 1. That is, increased flow in
 the direction 4b (decreased flow in direction 4a) reduces the etching of
 the edge region of wafer, 1. For very thin wafers, it is frequently
 advantageous to leave a thicker region around the circumference of the
 wafer to facilitate handling of the wafer in later processing steps.
 Control of gas flow 4a, 4b, by the baffle of the present invention is a
 means for accomplishing this result.
 During processing, the wafer holder is typically one of several in a
 carousel rotating around a central axis. This results in a centrifugal
 force tending to push the wafer, 1, against the circumference of the
 carousel. Limiting pins are typically provided against which the wafer is
 centrifugally pressed to maintain a constant location within the holder
 during processing. One such limiting pin is depicted as 21 in FIG. 1. The
 limiting pins occupy only a very small fraction of the circumference of
 the wafer holder, having no significant effect on the gas dynamics
 described elsewhere herein. However, we depict in FIG. 4 one such limiting
 pin, 21, to illustrate that the limiting pin, not brim, 9, keeps the wafer
 in the holder, permitting brim, 9, to be displaced in the direction 16.
 FIG. 4 depicts brim, 9, in a lower position than in FIGS. 2, 2a and 3.
 Thus, brim location may be adjusted to affect gas flow as may be desired
 by the process engineer. The width of the brim, 15 in FIG. 2, is typically
 20 to 25 mm. In an embodiment of the present invention the brim can be
 moved to various axial positions with respect to the wafer. That is, the
 brim can be moved in direction 16. Lowering the brim (in the direction of
 the plasma) causes the brim partially to shield ("shadow") the edge of the
 wafer from the plasma, thereby lowering the etch rate in the edge region.
 Typically, in this embodiment of the present invention, the brim may be
 lowered to a position of approximately 1-3 mm. below the position of the
 wafer. Etch rate near the edge of the wafer can thus be controlled by
 suitable vertical positioning of the brim structure of the present
 invention.
 Having described the invention in detail, those skilled in the art will
 appreciate that, given the present disclosure, modifications may be made
 to the invention without departing from the spirit of the inventive
 concept described herein. Therefore, it is not intended that the scope of
 the invention be limited to the specific and preferred embodiments
 illustrated and described. Rather, it is intended that the scope of the
 invention be determined by the appended claims.