WO2019226525A1

WO2019226525A1 - Apparatus for supporting thinned semiconductor wafers

Info

Publication number: WO2019226525A1
Application number: PCT/US2019/033072
Authority: WO
Inventors: Raven Persaud; John W. Stayt; George E. Harris
Original assignee: Ii-Vi Delaware, Inc.
Priority date: 2018-05-23
Filing date: 2019-05-20
Publication date: 2019-11-28

Abstract

A vacuum chuck is configured so that a wafer handling fixture (including the supported wafer) may be loaded into a central recessed area of the chuck. The depth of the central recessed area with respect to the surrounding portion of the wafer chuck is controlled such that the exposed surface of the thinned wafer slightly protrudes above the vacuum chuck. The vacuum force of the chuck functions to hold the wafer handling fixture securely in place. Thus, a thinned wafer remains completely supported by the handling fixture during a film application process, and will not bow and allow no gaps to form between the wafer and the film during the application process.

Description

APPARATUS FOR SUPPORTING THINNED SEMICONDUCTOR WAFERS

Cross-Reference to Related Applications

This application claims the benefit of U.S. Provisional Application No. 62/675,234, filed May 23, 2018 and herein incorporated by reference.

Technical Field

The present invention relates to semiconductor wafer processing and, more particularly, to an apparatus for supporting thinned semiconductor wafers during post- fabrication processes.

Background of the Invention

While there exists various techniques and arrangements for handling thinned wafers, concerns remain when there is a need to apply any type of pressure across the surface of the wafer. For example, there are situations where it is desired to apply a film across a surface of a thinned wafer (e.g., a silicon film across a backside surface for bow compensation, an anti static film across one or both surfaces to minimize possibility for ESD damage during transport), including the critical post-fabrication step of applying a tape across the backside of the wafer prior to dicing into separate components. During these film application processes, the wafer may bow and the film may not properly adhere to the entire wafer surface.

Summary of the Invention

The present invention addresses these concerns and relates to an apparatus for supporting thinned semiconductor wafers during post-fabrication processes associated with apply a film across the wafer surface.

The apparatus utilizes a combination of a specific wafer handling fixture (described in detail in our co-pending PCT Application No. PCT/US19/32608 and hereby incorporated by reference) and a vacuum chuck including a recessed central area for supporting the wafer handling fixture. In accordance with the principles of the present invention, the thinned wafer remains adhered to the handling fixture during a process of applying a film across the exposed wafer surface, with the handling fixture itself being loaded into a vacuum chuck to maintain the arrangement motionless during the film application.

In accordance with the principles of the present invention, a wafer is first adhered to the wafer holding fixture in an orientation where a backside surface of the wafer may be exposed (with the“worked”, active surface of the wafer thus in contact with the surface stiction layer of the wafer handling fixture). The recessed central area of the vacuum chuck is configured so that as the handling fixture (including the adhered wafer) is positioned in place, the exposed surface of the thinned wafer slightly protrudes above the vacuum chuck. The vacuum force of the chuck functions to hold the handling fixture securely in place. Thus, in accordance with the principles of the present invention, the thinned wafer remains completely supported by the handling fixture such that during a film application process, the wafer will not bow and no gaps will form between the wafer and the film.

An exemplary embodiment of the present invention takes the form of a vacuum chuck suitable for use with thinned semiconductor wafers. The vacuum chuck comprises a central recessed area sized to accommodate a wafer handling fixture supporting a thinned

semiconductor wafer, where the central recessed area is positioned a depth y below a top surface of the vacuum chuck (such that an exposed wafer surface slightly protrudes above the vacuum chuck top surface). A plurality of vacuum channels is formed within the central recessed area and disposed in a spaced-apart pattern so as to hold the wafer handling fixture in place upon the application of a first vacuum force. A separate vacuum port is formed through the central recessed area of the vacuum chuck (isolated from the plurality of vacuum channels) and controlled by a second vacuum force to release the thinned semiconductor wafer from the wafer handling fixture at the appropriate point in time.

Another embodiment of the present invention is defined as a method of applying a film across an exposed surface of a thinned semiconductor wafer, including the steps of: (1) adhering a thinned semiconductor wafer to a wafer handling fixture such that the exposed surface of the thinned semiconductor wafer remains visible; (2) loading the wafer handling fixture into a central recessed area of a vacuum chuck such that the exposed surface of the thinned semiconductor wafer slightly protrudes above a top surface of the vacuum chuck; (3) activating a first vacuum force to hold the wafer handling fixture in place within the central recessed area; and (4) applying a film across the exposed surface of the thinned semiconductor wafer, wherein the presence of the wafer handling fixture prevents bowing of the thinned semiconductor wafer as the film is applied.

Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

Brief Description of the Drawings

Referring now to the drawings, where like numerals represent like parts in several views:

FIG. 1 illustrates an exemplary wafer handling fixture suitable for use with the vacuum chuck of the present invention;

FIG. 2 is a cut-away side view of the wafer handling fixture of FIG. 1;

FIG. 3 is a simplified diagram of a portion of a vacuum chuck formed in accordance with the principles of the present invention;

FIG. 4 illustrates the vacuum chuck of FIG. 3, with a wafer handling fixture (and adhered wafer) positioned in place within the vacuum chuck;

FIG. 5 shows the application of a film across an exposed surface of a wafer supported by a wafer handling fixture loaded in a vacuum chuck;

FIG. 6 illustrates an alternative embodiment of the present invention, here including a depth-adjustable central recessed area;

FIG. 7 shows a particular positioning of the adjustable central recessed area with respect to the surrounding portion of the vacuum chuck;

FIG. 8 is a prior art tape-on-wafer mounting apparatus;

FIG. 9 shows the prior art apparatus of FIG. 8, showing a bowing of a thinned semiconductor wafer during a conventional taping process;

FIG. 10 illustrates an exemplary vacuum chuck with a central recessed area formed to support a wafer handling fixture, and also illustrating a taping frame; FIG. 11 shows a first step in releasing a taped wafer from the associated handling fixture;

FIG. 12 illustrates the release of the taped wafer from the handling fixture;

FIG. 13 illustrates the removal of the empty handling fixture from the vacuum chuck; and

FIG. 14 shows the taped wafer as removed from the handling fixture.

Detailed Description

As will be discussed in detail below, a wafer vacuum chuck is configured to include a central recessed area sized to accept a wafer handling fixture (as opposed to prior art arrangements where a“bare” wafer is loaded onto a chuck). By supporting the wafer on the handling fixture during the process of applying a film, the possibility of wafer bow being introduced as the film is being applied is essentially eliminated. A vacuum source is used to retain the handling fixture in place within the vacuum chuck during the film application process. At the completion of a final process of applying a dicing tape across the exposed wafer surface, a wafer release force mechanism of the handling fixture is activated so that the “final” thinned and taped wafer may be removed from the handling fixture.

An exemplary wafer handling fixture 10 suitable for use with the inventive vacuum chuck is shown in an isometric view in FIG. 1, with a cut-away side view in FIG. 2 (it is to be noted that FIG. 2 is not drawn to scale). Wafer handling fixture 10 comprises a bottom support plate 12 of a suitable material, such as a high impact strength plastic (for example, a polycarbonate resin thermoplastic). A thin mesh structure 14 is disposed on support plate 12, with a surface layer 16 of a somewhat“tacky” material disposed over mesh structure 14, as best shown in the cut-away side view of FIG. 2. Surface layer 16 may comprise a material such as, but not limited to, acrylics, plastics, silicon resins, cellulose acetate sheets, polyethylene, and polymer materials of the like.

Also shown in both FIGs. 1 and 2 is an aperture 18 formed through bottom support plate 12 of wafer handling fixture 10. As fully described in our co-pending application, aperture 18 is one type of release mechanism that is utilized to control the separation of the wafer from surface layer 16 of wafer handling fixture 10. In the particular embodiment of these illustrations, a single aperture is used and located in essentially the center of support plate 12. Other configurations and number of specific apertures may be utilized, as preferred for some applications.

A wafer 30 is shown in FIG. 2 as positioned on surface layer 16 of wafer handling fixture 10. As discussed in detail in our co-pending application, a static friction (“stiction”) force is created between surface layer 16 and wafer 30, where this force is sufficient to hold wafer 30 in place as it is transported during post-fabrication processes, including being loaded into a wafer vacuum chuck. For the purposes of discussing various aspects of the present invention, it is presumed that wafer 30 is oriented with its“active surface” A adhered to surface layer 16 and its backside surface B exposed, since at this point in the post-fabrication process most films are applied across the backside of the wafer. However, it is to be understood that the apparatus of the present invention is not limited to this arrangement, and a wafer handling fixture including a wafer oriented with its“active” side exposed may be positioned within the vacuum chuck so that an anti-static film may be applied across active side A of wafer 30.

In accordance with an aspect of the present invention, a specific wafer chuck apparatus is proposed for use with wafer handling fixture 10 as described such that the thinned wafer is completely supported during various film application processes. FIG. 3 is a simplified cut-away side view of a portion of a vacuum chuck 50 particularly configured to allow for the wafer handling fixture itself to be loaded into the chuck and positioned such that the exposed surface of the wafer is disposed just above the surface of the vacuum chuck. In particular and as shown in FIG. 3, vacuum chuck 50 includes a central recessed area 52 that is sized to accept wafer handling fixture 10 (handling fixture not shown in FIG. 3). Central area 52 is formed to be recessed a depth y with respect to a top surface 51 of vacuum chuck 50 such that when wafer handling fixture 10 is seated within central area 52, only backside B of wafer 30 extends above surface 51. At times, vacuum chuck 50 is referred to as a“pocket chuck”, with central recessed area 52 being the“pocket”.

Vacuum chuck 50 is shown as further comprising a plurality of vacuum channels 54, used in a conventional fashion to hold in place the component (in this case, wafer handling fixture 10) within central recessed area 52. The plurality of vacuum channels 54 is shown as exiting at a top surface 53 of central recessed area 52. The individual apertures of the plurality of channels 54 are disposed in a spaced-apart pattern across top surface 53. Thus, upon the activation of a first vacuum source (not shown) coupled to channels 54, the vacuum force will pull a loaded wafer handling fixture 10 against top surface 53, securing it in place within vacuum chuck 50.

A separate vacuum port 56 is also shown in the configuration of FIG. 3, and is disposed through the thickness of central recessed area 52 at a central location CL of the apparatus. In particular, vacuum port 56 is positioned so that it will align with aperture 18 of a loaded wafer handling fixture 10 (see FIG. 2), where the application of a vacuum through port 56 and aperture 18 will function to release wafer 30 from handling fixture 10 at the appropriate point in the manufacturing process (as discussed in our co-pending application). Vacuum port 56 is controlled by a separate vacuum source and is isolated from vacuum channels 54. It is to be understood that during the film application process(es), no vacuum is applied through vacuum port 56 and wafer 30 remains securely adhered to surface layer 16 of wafer handling fixture 10. Only when the final taping process is completed is a vacuum drawn through port 56 so that the“taped” wafer can be released from the fixture.

FIG. 4 illustrates wafer handling fixture 10 (with adhered wafer 30) loaded in place within central recessed area 52 of vacuum chuck 50. Vacuum channels 54 function to pull the adjacent surface of bottom support plate 12 downward to hold wafer handling fixture 10 firmly in place within central recessed area 52. As shown, wafer 30 is adhered to surface stiction layer 16 of wafer handling fixture 10, where for the purposes of a backside film application process the active side A of wafer 30 is in contact with surface stiction layer 16. Backside B of wafer 30 remains exposed and extends slightly above top surface 51 of vacuum chuck 50 as shown.

Once wafer handling fixture 10 is loaded and held in place via vacuum channels 54, a conventional film application process may be initiated. As shown in FIG. 5, a film 60 is disposed over and adhered to backside B of wafer 30. Film 60 may comprise, for example, a silicon film that is applied across the backside of the thinned wafer to provide additional support and limit the opportunities for bow. Another example is the application of an anti- static film, which may be applied to either (or both) surface(s) of thinned wafer 30 and used to minimize the possibility of ESD damage to the wafer during transport and shipping operations.

FIG. 6 illustrates an alternative embodiment of the present invention, in this case where the position of a central recessed area 52A is adjustable with respect to top surface 51 of vacuum chuck 50. Inasmuch as not all wafer handling fixtures are necessarily of the same dimensions, it is preferable for central recessed area 52A to be able to accommodate fixtures of different sizes while still allowing for only the exposed surface of the wafer loaded on the fixture to protrude above top surface 51 of vacuum chuck 50. In the illustration of FIG. 6, central recessed area 52A is attached to a positioning table 65, which is controlled by personnel to move central recessed area 52A in the ± y direction with respect to the surrounding portion of vacuum chuck 50 until proper positioning is obtained. That is, the depth of central recessed area 52A is controlled by the“up” and“down” movement (i.e., ± y- direction, as shown) of positioning table 65. FIG. 7 illustrates a particular configuration where central recessed area 52A has been lowered by an additional depth D (with respect to the illustration in FIG. 6), to accommodate a taller wafer handling fixture.

Besides the application of bow compensation films and ESD protection films, the apparatus of the present invention is particularly well-suited for use in applying the backside tape used as part of the final process of dicing the wafer into separate components.

FIG. 8 shows an example of a conventional prior art dicing tape-on-wafer mounting apparatus and method. In particular, FIG. 8 is a schematic sectional view of a dicing tape-on- wafer mounting apparatus 2, taken along a line passing the center of a wafer 1. FIG. 9 is a sectional view of the prior art dicing tape-on-wafer mounting apparatus 2, with a dicing tape 3 applied to wafer 1.

In FIG. 8, the dicing tape-on-wafer mounting apparatus 2 consists mainly of a wafer stage 4, a tape stage 5 and a roller 6. Tape stage 5, which has a flat and ring-shaped dicing frame 5A on its top surface for mounting dicing tape 3, is coupled to a chamber (not shown) and moves up and down with respect to wafer stage 4 in a fashion that it is lowered over wafer stage 4 to begin the taping process. Roller 6 moves in the direction of the arrow show in FIG. 2, while rotating across the top surface of tape stage 5. Wafer stage 4 typically includes a truncated cone-shaped recess (or hole) 4 A in its top surface for mounting wafer 1 therein. Recess 4A is typically constructed such that the diameters of its upper and lower bases are larger and smaller than that of wafer 1, respectively and, when wafer 1 is loaded into recess 4 A, the upper surface of wafer 1 stands higher than the top surface of wafer stage 4. Then, when roller 6 is rolled along in the direction of the arrow, dicing tape 3 is pressed down to adhere to the surface of wafer 1 (for example, to backside B of wafer 1).

This conventional method is useful in sticking a tape onto a wafer that is relatively thick and will not bow as roller 6 is pressed into its surface. However, when used with today’s thinned wafers, it is likely that at least the center of the wafer will bow under the application of pressure. Since wafer 1 is only supported in wafer stage 4 around its circumferential edge, it will tend to cave and hang down at its center portion as it becomes larger and thinner.

Therefore, when roller 6 is pressed onto dicing tape 3 with a thinned wafer 1 in this bowed state, as shown in FIG. 9, dicing tape 3 may fail to completely contact wafer 1 and develop a gap at the center portion, collecting air there and preventing the dicing tape 3 from sticking uniformly and firmly on the backside surface B of wafer 1.

FIG. 10 illustrates an exemplary vacuum chuck with a central recessed area formed in accordance with the principles of the present invention to accommodate wafer handling fixture 10, as well as the apparatus utilized for the application of dicing tape. In particular, FIG. 10 shows wafer handling fixture 10 (and adhered wafer 30) in place within central recessed area 52, with handling fixture 10 held in place via a vacuum pulled through vacuum channels 54. Also shown is a dicing tape 70 that is disposed over and adhered to backside B of wafer 30. A film frame 72, which is a supporting structure for dicing tape 70, is also shown in FIG. 10. The dicing tape itself is applied as in the conventional prior art process discussed above in association with FIGs. 8 and 9, using a roller to press tape 70 onto backside B of wafer 30.

Once the tape has been applied, wafer 30 is a“finished product” that can be removed from wafer handling fixture 10 (and vacuum chuck 50) and diced into separate devices. In accordance with this aspect of the present invention, the removal is a two-step process. The first step is associated with the release of wafer 30 from handling fixture 10. The next step is the actual removal of the empty wafer handling fixture 10 from vacuum chuck 50.

FIG. 11 shows this first step, where a vacuum force is applied through port 56 of vacuum chuck 50. As described above, separately-controlled vacuum port 56 is located so as to align with aperture 18 of wafer handling fixture 10 so as to apply a vacuum force sufficient to overcome the static friction force holding wafer 30 to handling fixture 10. That is, the vacuum pulling through port 56 will also pull through aperture 18 in bottom support plate 12 of wafer handling fixture 10. The application of a vacuum (or positive pressure) through aperture 18 is sufficient to overcome the static friction force holding wafer 30 in place against surface stiction layer 16, releasing wafer 30 from layer 16. This release of wafer 30 from handling fixture 10 is illustrated in FIG. 12 (with the movement of surface stiction layer 16 exaggerated for the purposes of illustration).

Once wafer 30 (and its associated dicing tape 70) is released from wafer handling fixture 10, the vacuum supplied through vacuum support 56 is turned off. Thereafter, the separate vacuum supplied to vacuum channels 54 is also turned off, allowing the now-empty wafer handling fixture 10 to be removed from vacuum chuck 50, as shown by the illustration of FIG. 13. Advantageously, wafer handling fixture 10 is then ready for use with another wafer.

As shown in FIG. 14, the wafer and dicing tape combination released from fixture 10 is then ready to be diced into separate components, using any common technique well-known in the art.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present invention, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present invention be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.

Claims

What is claimed:

1. A vacuum chuck suitable for use with thinned semiconductor wafers, the vacuum chuck comprising

a central recessed area sized to accommodate a wafer handling fixture supporting a thinned semiconductor wafer, the central recessed area positioned a depth below a top surface of the vacuum chuck such that an exposed wafer surface slightly protrudes above the vacuum chuck top surface;

a plurality of vacuum channels formed within the central recessed area and disposed in a spaced-apart pattern so as to hold the wafer handling fixture in place upon the application of a first vacuum force; and

a separate vacuum port formed through the central recessed area and disposed at a central location so as to align with a wafer release aperture of the wafer handling fixture, the separate vacuum port isolated from the plurality of vacuum channels and controlled by a second vacuum force, wherein upon application of a vacuum through the vacuum port and the wafer release aperture, the thinned semiconductor wafer is released from the wafer handling fixture.

2. The vacuum chuck as defined in claim 1 wherein the central recessed area comprises a separate element that is adjustable in depth with respect to the vacuum chuck top surface so as to accommodate wafer handling fixtures of different dimensions.

3. The vacuum chuck as defined in claim 2 wherein a positioning table is coupled to the central recessed area and used to adjust the depth of the central recessed area.

4. A method of applying a film across an exposed surface of a thinned semiconductor wafer, including the steps of:

adhering a thinned semiconductor wafer to a wafer handling fixture such that the exposed surface of the thinned semiconductor wafer remains visible;

loading the wafer handling fixture into a central recessed area of a vacuum chuck such that the exposed surface of the thinned semiconductor wafer slightly protrudes above a top surface of the vacuum chuck; activating a first vacuum force to hold the wafer handling fixture in place within the central recessed area; and

applying a film across the exposed surface of the thinned semiconductor wafer, wherein the presence of the wafer handling fixture prevents bowing of the thinned semiconductor wafer as the film is applied.

5. The method as defined in claim 4 wherein a backside of the thinned semiconductor wafer is the exposed surface and the film comprises a silicon film applied to provide bow compensation for the thinned semiconductor wafer.

6. The method as defined in claim 4 wherein the film comprises an anti-static film applied as a covering to protect from electro- static discharge (ESD) damage.

7. The method as defined in claim 6 wherein a backside of the thinned semiconductor is the exposed surface, with the wafer thereafter turned over and the front side of the thinned wafer is the exposed surface such that an anti-static film is applied as a covering to both surfaces of the thinned semiconductor wafer.

8. The method as defined in claim 4 wherein a backside of the thinned semiconductor wafer is the exposed surface and the film comprises a dicing tape, the method further comprising the steps of:

activating a second vacuum source to pull a vacuum through a vacuum port of the central recessed area aligned with a wafer release aperture of the wafer handling fixture, the vacuum releasing the thinned semiconductor wafer from the wafer handling fixture;

removing the taped, thinned semiconductor wafer from the wafer handling fixture and the vacuum chuck;

de-activating the second vacuum source;

de-activating the first vacuum source; and

removing the wafer handling fixture from the central recessed area of the vacuum chuck.

9. The method as defined in claim 9, further comprising the step of:

dicing through an active surface of the thinned semiconductor wafer without also dicing through the thickness of the tape applied across the backside surface.