These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiment of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein:
According to an embodiment of the present invention, disclosed herein is a method for reducing breakage of semiconductor wafers that presently occurs during the wafer thinning process associated with fabricating BSI CMOS devices. Specifically, a first example discloses trimming away a perimeter edge of the wafer prior to thinning the wafer. This trimming process may be implemented either before or after the wafer is bonded to a carrier substrate.
Referring to
The wafer 2 may have what are termed “front” 3 and “back” 5 sides. The front side 3 of the wafer 2 will typically comprise the active surface 6 (i.e., it will contain device wiring and other circuitry for the CMOS BSI device), while the back side 5 will function as the window through which photons will enter the pixel(s) of the imaging device. The wafer 2 may have an original thickness “T.” This original thickness may be too great to provide the desired degree of transparency for photon transmission to the pixel(s), and thus, the wafer 2 may be thinned (by grinding or other appropriate process) to achieve a reduced thickness “t”, as shown in
After thinning, further processing of the wafer 2 then be performed, such as the addition of one or more encapsulating layers 14 and/or leads 16. The wafer 2 may then be etched along dice lines 10 as shown in
As previously noted, the wafer 2 may be bonded to a carrier substrate 4 (
One appropriate bonding technique is termed “direct bonding,” and is of the type that does not require an intermediate layer between the wafer 2 and carrier substrate 4. The direct bonding process involves pressing the wafer and substrate together and heating the combination to about 1000 degrees Celsius (° C.) for a predetermined time period. Alternative direct bonding techniques include “surface activated bonding,” and “vacuum bonding.” With surface activated bonding, the surfaces of the wafer and carrier substrate are made atomically clean by argon fast atom beam (Ar-FEB) in ultra high vacuum (UHV) and brought into contact. This may be performed at room temperature, or at elevated temperatures of about 200-250° C. With vacuum bonding, the wafer and carrier substrate are pressed together using a vacuum, and heated to about 200-250° C.
Alternatively, “anodic bonding,” techniques may be used, in which the wafer 2 and carrier substrate 4 are clamped together between two metal electrodes, heated to about 300-500° C., and a potential difference of about 1000 Volts is applied between the two. Where a glass carrier substrate 4 is used, sodium ions are displaced from the bonding surface of the glass by the applied electric field. The depletion of sodium ions near the surface of the glass makes the surface highly reactive with the silicon surface of the wafer, thus forming a solid chemical bond between the two.
Examples of bonding techniques that require an intermediate layer include eutectic bonding, adhesive bonding, and glass frit bonding. Eutectic bonding involves coating the wafer 2 and carrier substrate 4 with separate components of a eutectic alloy composition. The two wafers are then heated and brought into contact, and diffusion occurs at the interface and alloys are formed. It is the melted eutectic layer that forms the bond. In one exemplary embodiment, a Si—Au eutectic alloy is used, and the process is performed at about 370° C.
Adhesive bonding, using epoxies, silicones, photoresists, polyimides, etc., can also be used to bond the wafer and carrier substrate. Typically, adhesive bonding requires the application of heat (about 120-140° C.).
Glass Frit bonding is a further alternative technique for bonding the wafer and carrier substrate, and involves the use of a low melting point glass material to form the bond between the pieces. The glass layer may be applied to one or both pieces as a preform, spin-on, screen print, sputtered film, etc. and may be patterned to define sealing areas. The wafer and carrier substrate are then pressed together and heated to about 400-500° C.
Although all of these processes have been used successfully to achieve bonding between wafers and carrier substrates, their effectiveness is limited to the flat regions of those pieces, and still the problem of poor bonding between the pieces at the bevel region 13 remains.
As previously noted, breakage/cracking of the wafer 2 in the bevel region 13 quickly propagates into the device areas of the wafer 2, resulting in the aforementioned waste. Since it may not be practical to achieve complete bonding of the wafer and carrier substrate in the bevel region, a solution to the breakage problem is to remove the offending portion of the bevel region. By eliminating the inception point of the breakage (the bevel region, or region of inadequate bonding) prior to the thinning process, wafer breakage may be effectively reduced or eliminated.
Referring to
The trimming can proceed can be performed by making a single pass of the cutting blade 18 around the wafer perimeter 12, or by maintaining the cutting blade stationary and rotating the wafer 2. Alternatively, multiple cutting passes may be made, to obtain the desired reduced band width dimension “rw”. Multiple cutting passes may be appropriate when a larger band width dimension “rw” is desired. Additionally, when multiple cutting passes are employed, the cutting blade may be moved in a spiral fashion with respect to the wafer, or it may be moved in an oscillating fashion. In one embodiment, this band width dimension “rw” is from about 0.3 mm to about 5.0 mm, and preferably about 2.0 mm. Thus, in one embodiment, the band width dimension “rw” is about 2.0 mm, and the trimming depth is selected to provide a desired final thickness of about 25 μm to about 50 μm.
Once the blade 18 has been used to remove an appropriate portion of the bevel region 13, the wafer 2 may be bonded to the carrier substrate 4 as illustrated in
In an alternative to the edge trimming step shown in
An alternative fabrication process incorporating edge trimming is shown in
Once the bevel region 13 of the wafer 2 has been removed, the wafer 2 may be thinned as shown in
Referring to
As an alternative to the blade trimming technique described above, a wet etching process could be used to remove all or a portion of the bevel region 13 as part of the process steps of
Although wafer cracking could be reduced by providing a carrier substrate 4 having a diameter that is substantially larger than that of the wafer 2, such a process would require modification of the wafer handling tool, among other tools. Thus, edge trimming is viewed as a simpler way of avoiding wafer breakage.
Additionally, although the wafer edge trimming process has been described in relation to the BSI manufacturing process, it may also be used in MEMS or SOI processes which implement wafer bonding and backside grinding steps.
While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope and range of equivalents of the appended claims.