Bonded wafers prepared by bonding a bond wafer to a base wafer and then reducing the thickness of the wafer used for fabricating device (bond wafer), have been used as a wafer for a high-performance device.
An SOI wafer has been known as one of the bonded wafers. This wafer can be manufactured as follows, for instance. That is, two mirror-polished silicon wafers (bond wafer and base wafer) are prepared, and an oxide film is formed on at least one wafer. Then, these wafers are bonded and then subjected to heat treatment at 200 to 1200° C. to increase bonding strength. After that, the wafer used for fabricating device (bond wafer) is ground and polished down to a desired thickness to thereby complete an SOI wafer having an SOI (silicon on insulator) layer formed thereon.
Besides, a bonded wafer is manufactured by directly bonding silicon wafers together not through an oxide film in some cases. Further, an insulator wafer made of quartz, silicon carbide, alumina, or the like may be used as a base wafer.
Referring now to FIGS. 4 and 5, conditions of an outer peripheral portion of a mirror-surface wafer before bonding and after bonding an outer peripheral portion of bonded bond wafer and base wafer are explained.
As shown in FIG. 4, the outer peripheral portion of a mirror-surface wafer 40 before bonding generally includes a chamfered portion 41 and a slightly thin portion called a polishing sag 42. If a bonded wafer is manufactured as above with the mirror-surface wafer 40, the chamfered portion 41 and the polishing sag 42 portion are not bonded and remain unbonded.
FIG. 5 shows this state. In FIG. 5, an oxide film 53 is formed on a bond wafer 51, and then a base wafer 52 is bonded thereto. As shown in FIG. 5, the chamfered portion or polishing sag portion of the mirror-surface wafer is left as an unbonded portion 54 after bonding. This unbonded portion 54 extends about 3 mm or less from the outer edge.
If the thickness of the bond wafer including such unbonded portions is reduced by grinding or the like, the unbonded portions are partially delaminated during the step of reducing the thickness. Thus, the thinned bond wafer might be smaller in diameter than a wafer as a base (base wafer) or an array of small irregularities might be formed in its peripheral portion.
If this bonded wafer is applied to a device manufacturing process, the remaining unbonded portion is delaminated during the process, with the result that particles are generated and the device yield is lowered.
To avoid such a situation, it is necessary to previously remove the unbonded portion left in the outer peripheral portion of the bond wafer before reducing the thickness of the bond wafer through grinding or the like. A portion obtained by removing the unbonded portion to expose the base wafer is referred to as a terrace portion. FIG. 6 shows a terrace portion 55 formed before the step of reducing the thickness of the bond wafer. In FIG. 6, reference numerals other than the terrace portion 55 are identical to those of FIG. 5.
In general, two methods are conceivable for forming such a terrace portion.
To explain the first method, the entire surface of the bond wafer is ground through surface grinding, and a tape is put on the ground surface of the bond wafer except the outer peripheral portion of several mm from the outer edge. As a result, the outer peripheral portion of the bond wafer is exposed and then, the exposed portion of the bond wafer is melted and removed by etching to thereby form a terrace portion.
The second method is explained with reference to FIG. 3. Here, the case of bonding a bond wafer 31 having an oxide film 33 formed thereon to a base wafer 32 is explained by way of example (see FIG. 3(a)). First, the outer peripheral portion of several mm from the outer edge of the bond wafer 31 is ground by several mm into a thickness of 10 to 100 μm (see FIG. 3(b)). Then, the remaining is etched off to form a terrace portion 35 (see FIG. 3(c)).
The latter method has been employed in most cases to keep up with recent trend to automation and mass-production (see Japanese Unexamined Patent Application Publication No. 2000-223452, for example).
Moreover, a demand to increase a quality of the terrace portion recently grows, and it is required to produce smoother terrace portions with uniform quality.
If grinding and etching for forming a terrace portion are compared, etching is more likely to involve unevenness than grinding as a machining process due to temperature, composition, or other such conditions. Hence, in order to produce smooth terrace portions with uniform quality, an etching amount has to be reduced as much as possible. Thus, the etching amount is reduced and grinding removal is increased to meet the demand to increase the quality.
However, the following problem occurs as a result of reducing an etching amount and increasing grinding removal.
That is, if the grinding removal is increased, an outer peripheral portion of the bond wafer becomes too thin (see an encircled portion of FIG. 3(b)). The too-thin outer peripheral portion of the bond wafer is caught in a grind stone and then chips or delaminates in some cases. Moreover, if the chipped or delaminated portion damages an insulator film underlying the bond wafer, corrosion proceeds up to the base wafer in a subsequent etching step, resulting in small recesses (dimples) in the terrace portion. As a result, a product is unusable for practical applications, which causes reduction in production yield. Further, the terrace dimples cause a problem such as dusting during a device manufacturing process. The wafer cannot function as an SOI wafer in some cases.
Due to the unbonded portions corresponding to the shape of the chamfered portion of the outer peripheral portion of the bond wafer and the polishing sag, the outer peripheral portion of the bond wafer becomes too thin when the polishing removal is increased. That is, the outer peripheral portion of several hundreds μm from the outer edge is delaminated from the base wafer as described above. As a result, the outer peripheral portion of the wafer is broken, chips, or delaminates due to vibrations or shock applied upon grinding with the grind stone.
As described above, in the case of removing the outer peripheral portion of the bonded bond wafer to form a terrace portion, dimples formed in the terrace portions cannot be entirely prevented and there is still room for improvement.