JP2011071193A - Lamination soi wafer and manufacturing method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lamination SOI wafer which reduces a crystal defect on a lamination interface which has occurs in the case of lamination heat treatment, and reduces a warp occurring in the wafer, and to provide a manufacturing method of the wafer. <P>SOLUTION: In the manufacturing method of the lamination SOI wafer, the thickness of an active layer wafer is adjusted to become smaller than that of a supporting wafer, and oxide films are formed so that the thickness of the oxide film formed on the whole face of the active layer wafer becomes smaller than that of the oxide film formed on the whole face of the supporting wafer. The active layer wafer is superposed on the supporting wafer via the oxide film. The superposed wafers are heat-treated and laminated. The oxide film interposed between the two wafers is made to be an embedded oxide film layer. The active layer wafer is thinned and the lamination SOI wafer is manufactured. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a bonded SOI (Silicon On Insulater) wafer suitable for a high breakdown voltage device having a large buried oxide film layer thickness and a method for manufacturing the same.

  The SOI wafer has a structure in which a single crystal silicon layer (hereinafter referred to as an SOI layer) serving as an active layer is formed through a buried oxide film (hereinafter referred to as BOX) layer using a silicon single crystal as a supporting substrate. Have Application to high withstand voltage devices has been made by taking advantage of the fact that elements are electrically isolated by dielectric separation of the BOX layer. In particular, in many fields such as automotive electronics, flat panel displays, motor control and power supplies, there is a demand for SOI wafers corresponding to higher voltage devices.

  In general, in order to adapt to a device having a higher breakdown voltage, the electrical resistance between elements may be increased. That is, an SOI wafer corresponding to a high breakdown voltage has been manufactured so far by increasing the thickness of a BOX layer used for electrical isolation between elements.

  As an SOI wafer manufacturing method, an active layer wafer to be thinned and a supporting wafer are bonded to each other, or a region having a predetermined depth from the wafer surface by injecting oxygen ions from the wafer surface. There is a SIMOX (Separation by Implanted Oxygen) method for forming a BOX layer.

  Among these, as shown in FIG. 7, in the manufacturing method of the SOI wafer by the bonding method, first, an active layer wafer 1 and a supporting wafer 2 are prepared, and either one or both wafers are oxidized by thermal oxidation. 1a and 2a are formed. In FIG. 7, oxide films are formed on both wafers. Next, the two wafers 1 and 2 are bonded to each other through the oxide films 1a and 2a by applying pressure by superimposing the two wafers 1 and 2 on each other. Since the laminated wafer 3 is only bonded by hydrogen bonding and has a low bonding strength, a bonding heat treatment is performed at a temperature of 1100 to 1250 ° C. in an oxidizing atmosphere to increase the bonding strength. Thus, the oxide films 1a and 2a interposed between the stacked wafers are used as the BOX layer 5. Furthermore, the active layer wafer of the superposed wafer 4 after the heat treatment is subjected to a thinning process so as to have a desired thickness by grinding and polishing to form an SOI layer 6, and further subjected to terrace polishing to obtain a bonded SOI wafer. 7 is manufactured.

  When manufacturing the above-described SOI wafer for a high voltage device, the above-described thermal oxidation time has been increased to form a thick BOX layer. However, an SOI having a thick BOX layer with a BOX thickness of 2 μm or more. When manufacturing a wafer (hereinafter referred to as a thick BOX-SOI wafer), if the difference in thickness between the oxide film thickness formed on the active layer wafer and the oxide film thickness formed on the support wafer increases, At this time, crystal defects called slips were generated in the peripheral edge regions of the cross-sections of the two wafers that were superposed (see, for example, Patent Document 1). For this reason, in manufacturing a thick BOX-SOI wafer, the thickness difference between the thickness of the oxide film formed on the active layer wafer and the thickness of the oxide film formed on the support wafer is reduced, specifically, 2 μm or less. There was a need.

  In addition, in order to greatly suppress the occurrence of slip that occurs during the bonding heat treatment, two wafers may be overlapped, and the active layer wafer may be thinned to a desired SOI layer thickness, followed by bonding heat treatment. It has been proposed (see, for example, Patent Document 2).

Japanese Patent Laid-Open No. 2001-93788 (claims 1 and 2, paragraphs [0006] to [0008], FIG. 1) JP-A-5-109678 (Claim 1, paragraph [0007], paragraph [0012] to paragraph [0020], FIGS. 1 to 6)

  However, when a thick BOX-SOI wafer is produced by the method described in Patent Document 1, a difference in thickness is inevitably generated between the BOX layer thickness and the oxide film thickness on the back surface of the supporting wafer. The warpage of the wafer increased due to this thickness difference. When this warp is 100 μm or more, it causes exposure failure and adsorption failure in the semiconductor manufacturing process.

  Further, in the method described in Patent Document 2, if the bonding heat treatment is performed after thinning to a desired SOI layer thickness, the strength of the active layer wafer itself is weakened due to the thinning. There was a possibility that the wafer was cracked by thermal stress. Furthermore, since the thinning is performed until the desired SOI layer thickness is obtained by chamfering processing, grinding processing, and polishing processing in a state where the adhesive strength simply by overlapping the two wafers is weak, during processing, It is expected that the two stacked wafers will be peeled off at the bonding surface, and the processing itself cannot be performed.

  An object of the present invention is to provide a bonded SOI wafer and a method for manufacturing the same, which can reduce crystal defects at the bonding interface that have occurred during the bonding heat treatment, and can reduce warpage generated in the wafer.

  A first aspect of the present invention is a bonded SOI wafer in which an SOI layer is formed on a supporting wafer via a BOX layer, and the thickness of the oxide film formed on the back surface of the supporting wafer is 0.45. In the range of ˜6 μm, the thickness difference between the thickness of the BOX layer and the thickness of the support wafer back surface oxide film is 1 μm or less, and no slip dislocation exists in the SOI layer.

  As shown in FIG. 1, the second aspect of the present invention is that an oxide film 11b is formed on the entire surface of the active layer wafer 11, and an oxide film 12a is formed on the entire surface of the supporting wafer 12 for supporting the wafer 11. Then, the active layer wafer 11 is laminated on the supporting wafer 12 via the oxide films 11b and 12a, and the two laminated wafers 13 are heat-treated and bonded together. The intervening oxide films 11b and 12a are formed into a BOX layer 15, and thereafter, an SOI layer 16 is formed on the supporting wafer 12 through the BOX layer 15 by subjecting the active layer wafer 11 to a thinning process. In which the thickness of the active layer wafer 11 is smaller than the thickness of the supporting wafer 12 before the bonding heat treatment, and the active layer wafer is manufactured. Entirely in the thickness of the oxide film 11b to form the 11 being less than the thickness of the oxide film 12a is formed on the entire surface of the supporting wafer 12.

  A third aspect of the present invention is the invention based on the second aspect, wherein the thickness of the oxide film formed on the back surface of the supporting wafer is in the range of 0.45 to 6 μm, and the BOX layer And the thickness of the supporting wafer back surface oxide film is 1 μm or less.

  A fourth aspect of the present invention is the invention based on the second or third aspect, wherein the thickness of the active layer wafer is supported when the thickness of the supporting wafer is in the range of 325 to 725 μm. It is characterized by being in the range of 300 to 700 μm, which is smaller than the thickness of the wafer for use.

  A fifth aspect of the present invention is the invention based on the second aspect, and is characterized in that the BOX layer has a thickness of 0.5 to 7 μm.

  A sixth aspect of the present invention is the invention based on the first aspect, wherein the thickness of the BOX layer is 0.5 to 7 μm μm, and the warp generated in the wafer is 100 μm or less. .

  In the method for manufacturing a bonded SOI wafer according to the present invention, the thickness of the active layer wafer is adjusted so that the thickness of the active layer wafer is smaller than the thickness of the supporting wafer. Since the difference is reduced and the thermal stress is reduced, the crystal defects at the bonding interface that occurred during the bonding heat treatment can be reduced, and the thickness of the oxide film formed on the active layer wafer can be reduced to the supporting wafer. Since the volume expansion difference between the BOX layer and the back surface oxide film of the support side wafer is reduced by making the thickness smaller than the thickness of the oxide film formed on the substrate, the warpage occurring in the obtained SOI wafer can be reduced to 100 μm or less. it can.

  Moreover, since the bonded SOI wafer of the present invention has reduced crystal defects at the bonded interface, it can solve the deterioration of the pressure resistance characteristics, and since the warpage is reduced, exposure in the semiconductor manufacturing process. Defects and adsorption defects can be reduced.

It is a manufacturing-process figure in the manufacturing method of the bonding SOI wafer which concerns on this invention. It is a manufacturing process figure which performed the preliminary thinning process in the manufacturing method of the bonding SOI wafer which concerns on this invention. It is a manufacturing-process figure which shows another form which performed the preliminary thinning process in the manufacturing method of the bonding SOI wafer which concerns on this invention. It is a manufacturing process figure which shows another form which performed the preliminary | backup thinning process in the manufacturing method of the bonding SOI wafer which concerns on this invention. It is a figure which shows the SP1 result in the bonding interface of the wafer for active layers of the laminated wafer after finishing the bonding heat processing of the comparative example 1. FIG. It is a figure which shows the SP1 result in the bonding interface of the wafer for active layers of the lamination wafer after finishing the bonding heat processing of Example 1. FIG. It is a manufacturing-process figure of the conventional bonding SOI wafer.

  Next, an embodiment for carrying out the present invention will be described based on the drawings.

  In the method for manufacturing a bonded SOI wafer of the present invention, first, as shown in FIG. 1, an active layer wafer 11 and a supporting wafer 12 for supporting the wafer are prepared. Both the active layer wafer 11 and the supporting wafer 12 are preferably mirror-finished wafers having a diameter of 200 mm. The thickness of the supporting wafer 12 is preferably in the range of 325 to 725 μm. Further, in order to reduce the temperature difference in the thickness direction of the wafer during the bonding heat treatment, it is effective to make the thickness of the active layer wafer 11 before the bonding heat treatment smaller than the thickness of the supporting wafer 12. . Therefore, in the stage of manufacturing the active layer wafer 11, the thickness adjusting wafer 11 a manufactured in advance to be smaller than the thickness of the supporting wafer may be used as the active layer wafer 11, or the active layer wafer 11 may be used. As shown in FIG. 2 to FIG. 4, the active layer wafer 11 is spared so as to be smaller than the thickness of the support wafer 12. A thinning process may be applied. This preliminary thinning treatment is preferably carried out by grinding using a grindstone or the like for the majority of the desired thinning amount and by mirror finishing using a polishing cloth or the like. Further, as shown in FIG. 2, after the preliminary thinning process, the oxide film is formed, the two wafers may be overlapped, and the bonding heat treatment may be performed in order, or as shown in FIG. 3 and FIG. This preliminary thinning process may be performed before or after the wafers are stacked.

  In addition, when what thinned until it became desired SOI layers, such as 0.3-250 micrometers, is used for the wafer 11 for active layers, there exists a possibility that the wafer 11 for active layers may crack at the time of the bonding heat processing performed after that. Therefore, it is desirable that the thickness of the active layer wafer 11 a be in the range of 300 to 700 μm, which is smaller than the thickness of the support wafer 12. A slip defect that has conventionally occurred is considered to be due to a large temperature difference in the thickness direction of the wafer and an increase in thermal stress during the bonding heat treatment performed after the two wafers are overlaid. On the other hand, by processing the thickness of the active layer wafer 11 before the bonding heat treatment to be smaller than the thickness of the supporting wafer 12, the temperature difference in the thickness direction of the laminated wafer is small during the bonding heat treatment. Since the thermal stress is reduced, crystal defects at the bonding interface that have occurred during the subsequent bonding heat treatment can be reduced.

  Subsequently, the thickness of the active layer wafer 11a and the supporting wafer 12 are subjected to SC-1 cleaning, pure water rinsing, and hydrofluoric acid organic acid cleaning in this order to clean the surface of each wafer.

  Next, an oxide film 11 b is formed on the entire surface of the active layer wafer 11 a whose thickness has been adjusted, and an oxide film 12 a is formed on the entire surface of the support wafer 12. The BOX layer of the SOI wafer is formed by making the thickness of the oxide film 11b formed on the entire surface of the active layer wafer 11a whose thickness is adjusted smaller than the thickness of the oxide film 12a formed on the entire surface of the support wafer 12. And the back surface oxide film of the support side wafer become small, so that the warpage generated in the obtained SOI wafer can be reduced to 100 μm or less. The thicknesses of the oxide films 11b and 12a formed on the two wafers 11a and 12 are adjusted so that the thickness of the BOX layer of the finally obtained SOI wafer becomes 0.5 to 7 μm. . Further, the thickness of the oxide film 12a formed on the back surface of the supporting wafer 12 is in the range of 0.45 to 6 μm, and there is a difference in thickness between the thickness of the BOX layer and the thickness of the supporting wafer back surface oxide film 12a. It is preferable to adjust the thicknesses of the oxide films 11b and 12a formed on the two wafers 11a and 12 so that the thickness is 1 μm or less. For forming an oxide film on each wafer, it is preferable to perform a heat treatment in a hydrogen and oxygen mixed gas atmosphere. In addition, when an oxide film is formed only on one of the wafers and an oxide film is not formed on the other wafer, the occurrence of slip cannot be reduced, and contamination at the bonding interface may occur in the active layer. This causes a problem of spreading.

  Subsequently, SC-1 cleaning, pure water rinsing, and hydrofluoric acid organic acid cleaning are performed in this order on the two wafers 11a and 12 having an oxide film formed on the entire surface, and the surface of each wafer is terminated with hydrogen. After that, the two wafers 11a and 12 whose surfaces are hydrogen-terminated are stacked and a heavy stone is placed on the two wafers 11a and 12 to apply pressure, thereby bonding the two wafers 11a and 12 through the oxide films 11b and 12a.

  Next, the bonding heat treatment increases the bonding strength of the laminated wafer 13 bonded by hydrogen bonding. The bonding heat treatment is performed by holding at a temperature of 1200 ° C. for 60 to 180 minutes in a hydrogen and oxygen mixed gas atmosphere. Note that the oxide films 11b and 12a interposed between the two wafers 11a and 12 that are overlapped become the BOX layer 15.

  Next, a thinning process is performed so that the active layer wafer side of the laminated wafer 14 subjected to the bonding heat treatment becomes an SOI layer having a thickness of 0.3 to 250 μm. The thinning process is performed by surface grinding or mirror polishing. As a result, the SOI layer 16 is formed on the supporting wafer 12 via the BOX layer 15.

  Further, the peripheral edge of the BOX layer 15 and the SOI layer 16 is subjected to terrace polishing so that the SOI layer 16 is formed on the supporting wafer 12 having the oxide film 12a formed on the back surface via the BOX layer 15. A combined SOI wafer 17 is obtained.

  In the bonded SOI wafer of the present invention, the thickness of the oxide film 12a formed on the back surface of the support wafer 12 is in the range of 0.45 to 6 μm, and the thickness of the BOX layer 15 and the back surface of the support wafer 12 are The difference in thickness with respect to the thickness of the oxide film 12a formed in 1 is 1 μm or less, and no slip dislocation exists in the SOI layer. Further, the thickness of the BOX layer 15 is 0.5 to 7 μm, and the warp generated in the wafer is 100 μm or less. Since the crystal defects at the bonding interface are reduced, it is possible to solve the deterioration of the pressure resistance characteristics, and since the warpage is reduced to 100 μm or less, it is possible to reduce exposure defects and adsorption defects in the semiconductor manufacturing process. .

  The thickness of the wafer is measured by capacitance measurement, the thickness of the BOX layer and oxide film is measured by reflectance spectroscopy, and the slip dislocation is measured by irradiating the bonding interface with laser light and analyzing the scattered light. Further, the warpage generated in the wafer is measured by non-contact type capacitance measurement.

  Next, examples of the present invention will be described in detail together with comparative examples.

<Example 1>
First, a mirror-finished wafer having a thickness of 725 μm and a diameter of 200 mm was prepared as an active layer wafer and a supporting wafer. Next, as a pre-thinning process, the active layer wafer is ground using a grindstone to reduce the wafer thickness to 650 μm, and then the ground wafer is ground. Mirror finish processing was performed using, and the wafer thickness was adjusted to 640 μm. The thinly processed active layer wafer and supporting wafer were subjected to SC-1 cleaning, pure water rinsing, and hydrofluoric acid organic acid cleaning in this order to clean the wafer surface. Next, the two wafers were heat-treated in a hydrogen and oxygen mixed gas atmosphere to form an oxide film having a thickness of 1 μm on the active layer wafer and an oxide film having a thickness of 5 μm on the supporting wafer. Then, the two wafers were subjected to SC-1 cleaning, pure water rinsing, and hydrofluoric acid organic acid cleaning in this order, and the surfaces were hydrogen-terminated, and then the two wafers were stacked and bonded. Next, a bonding heat treatment was performed on the laminated wafer at a temperature of 1200 ° C. in a hydrogen and oxygen mixed gas atmosphere. The active layer wafer side of the laminated wafer subjected to the bonding heat treatment is subjected to surface grinding by surface grinding so that the thickness of the active layer wafer becomes an SOI layer of 15 μm, and further subjected to terrace polishing and pasting. A combined SOI wafer was obtained.

<Example 2>
A bonded SOI wafer was obtained in the same manner as in Example 1 except that the thickness of the active layer was adjusted to 700 μm by adjusting the thickness by preliminary thinning treatment.

<Example 3>
Prepare both active layer wafer and support wafer as mirror-finished wafers with a thickness of 325μm and a diameter of 200mm, and adjust the thickness by pre-thinning to make the active layer wafer thickness 300μm. A bonded SOI wafer was obtained in the same manner as in Example 1 except that a 4 μm thick oxide film was formed on the supporting wafer.

<Comparative Example 1>
A bonded SOI wafer was obtained in the same manner as in Example 1 except that the active layer wafer was not subjected to preliminary thinning treatment.

<Comparative Example 2>
A bonded SOI wafer was obtained in the same manner as in Example 1 except that the active layer wafer was not subjected to preliminary thinning treatment and an oxide film having a thickness of 3 μm was formed on each of the active layer wafer and the supporting wafer. .

<Comparative Example 3>
The same as in Example 1 except that the active layer wafer was not subjected to preliminary thinning treatment, an oxide film having a thickness of 2 μm was formed on the wafer for active layer, and an oxide film having a thickness of 4 μm was formed on the supporting wafer. A bonded SOI wafer was obtained.

<Comparative example 4>
A bonded SOI wafer was obtained in the same manner as in Example 1 except that an oxide film having a thickness of 3 μm was formed on each of the active layer wafer and the supporting wafer.

<Comparative Example 5>
As a support wafer, a mirror-finished wafer having a thickness of 640 μm and a diameter of 200 mm is prepared, and the active layer wafer is not subjected to preliminary thinning treatment, and the active layer wafer and the support wafer are respectively thick. A bonded SOI wafer was obtained in the same manner as in Example 1 except that a 3 μm oxide film was formed.

<Comparative Example 6>
A bonded SOI wafer was obtained in the same manner as in Example 1 except that an oxide film having a thickness of 6 μm was formed only on the supporting wafer.

<Comparative Example 7>
A bonded SOI wafer was obtained in the same manner as in Example 1 except that an oxide film having a thickness of 3 μm was formed only on the active layer wafer.

<Comparative Example 8>
A bonded SOI wafer was obtained in the same manner as in Example 1 except that an oxide film having a thickness of 2 μm was formed on the active layer wafer and an oxide film having a thickness of 1 μm was formed on the supporting wafer.

<Comparative Example 9>
As a supporting wafer, a mirror-finished wafer having a thickness of 640 μm and a diameter of 200 mm is prepared, and the thickness of the active layer is adjusted to 700 μm by a preliminary thinning process. A bonded SOI wafer was obtained in the same manner as in Example 1 except that an oxide film having a thickness of 2 μm was formed on a supporting wafer and an oxide film having a thickness of 4 μm was formed.

<Comparison test and evaluation 1>
An active layer in which a laminated wafer after finishing the bonding heat treatment in Examples 1 to 3 and Comparative Examples 1 to 9 was prepared, the laminated wafer was peeled off at the BOX layer, and the oxide film was removed by cleaning with hydrofluoric acid. Slip observation was performed on the bonding interface of the wafers using a surface inspection apparatus (SP1). The presence or absence of slip occurrence is shown in Table 1 below. In addition, about the thing which slip generate | occur | produced, the value of the total length of the slip generation length was shown in the table | surface.

  5 shows the SP1 image at the bonding interface of the wafer for active layer not subjected to the preliminary thinning process of Comparative Example 1, and FIG. 6 shows the thickness after the preliminary thinning process of Example 1 is applied. SP1 images at the bonding interface of the wafer for active layer processed to 640 μm are shown.

<Comparison test and evaluation 2>
For the bonded SOI wafers obtained in Examples 1 to 3 and Comparative Examples 1 to 9, warp indicating warpage was measured by a flatness measuring device (ADE9500). The results are shown in Table 1 below.

In the active layer wafer of Comparative Example 1 that was not subjected to the preliminary thinning process of FIG. 5, the slip along the crystal orientation was confirmed, whereas the preliminary thinning process of FIG. 6 was performed. It was confirmed that no slip occurred in the active layer wafer of Example 1. From this result, by processing the thickness of the active layer wafer to be smaller than the thickness of the supporting wafer by preliminary thinning treatment, by reducing the temperature difference in the thickness direction of the wafer during the bonding heat treatment, It can be seen that crystal defects at the bonding interface are reduced because the thermal stress is reduced.

  Further, as apparent from Table 1, in Comparative Examples 2 to 5 and 7 to 9 in which the thickness difference between the BOX layer thickness and the supporting wafer backside oxide film thickness is large, the warpage generated in the obtained SOI wafer was increased. . Furthermore, even in Comparative Examples 6 and 7 in which an oxide film was formed only on one wafer, slip occurred at the bonding interface. On the other hand, in Examples 1 to 3, there was no slip at the bonding interface, and the warpage generated in the obtained SOI wafer was reduced to 100 μm or less.

  In each example and each comparative example, the active layer wafer side after the bonding heat treatment is thinned (surface grinding) to form an SOI layer having a predetermined thickness, and then terrace polishing is performed. However, after the active layer wafer subjected to the bonding heat treatment is subjected to terrace polishing, the active layer wafer side may be subjected to thinning treatment (surface grinding) to form an SOI layer having a predetermined thickness.

DESCRIPTION OF SYMBOLS 11 Active layer wafer 11a Thickness-adjusted active layer wafer 11b Oxide film 12 Support wafer 12a Oxide film 13 Overlapping wafer 14 Overlapped wafer after heat treatment 15 Embedded oxide film layer 16 SOI layer 17 SOI wafer

Claims (6)

In a bonded SOI wafer in which an SOI layer is formed on a supporting wafer via a buried oxide film layer,
The thickness of the oxide film formed on the back surface of the supporting wafer is in the range of 0.45 to 6 μm, and the thickness difference between the thickness of the buried oxide layer and the thickness of the back surface oxide film of the supporting wafer is A bonded SOI wafer having a thickness of 1 μm or less and having no slip dislocation in the SOI layer. An oxide film is formed on the entire surface of the active layer wafer, an oxide film is formed on the entire surface of the support wafer for supporting the wafer, and the active layer wafer is stacked on the support wafer via the oxide film. Then, the two laminated wafers are heat-treated and bonded to each other so that an oxide film interposed between the two laminated wafers is embedded to form an oxide film layer, and then the active layer wafer is thinned. A method of manufacturing a bonded SOI wafer in which an SOI layer is formed on the supporting wafer via the buried oxide film layer by performing a treatment,
Before the bonding heat treatment, the thickness of the active layer wafer is smaller than the thickness of the supporting wafer, and the thickness of the oxide film formed on the entire surface of the active layer wafer is formed on the entire surface of the supporting wafer. A method for producing a bonded SOI wafer, wherein the thickness is smaller than a thickness of an oxide film to be bonded.   The thickness of the oxide film formed on the back surface of the supporting wafer is in the range of 0.45 to 6 μm, and the thickness difference between the thickness of the buried oxide film layer and the thickness of the supporting wafer back surface oxide film is 1 μm or less. The method for producing a bonded SOI wafer according to claim 2.   4. The bonding according to claim 2, wherein when the thickness of the supporting wafer is in the range of 325 to 725 [mu] m, the thickness of the active layer wafer is in the range of 300 to 700 [mu] m smaller than the thickness of the supporting wafer. Manufacturing method of SOI wafer.   The method for producing a bonded SOI wafer according to claim 2, wherein the buried oxide film layer has a thickness of 0.5 to 7 μm.   2. The bonded SOI wafer according to claim 1, wherein the buried oxide film layer has a thickness of 0.5 to 7 [mu] m and a warp generated in the wafer is 100 [mu] m or less.