Wafer fixing method and semiconductor manufacturing apparatus

Abstract

The invention provides a wafer fixing method and a semiconductor manufacturing apparatus, which can reduce adverse conditions caused by stress release generated when a wafer is separated. When the processing temperature of a wafer (W) is greater than or equal to a first reference temperature (T1) higher than room temperature, the wafer (W) is fixed by a wafer fixing tool (F) which is provided with a mechanical jig (M) and an electrostatic chuck (E) and is used in a semiconductor manufacturing apparatus, only the mechanical jig (M) is used for fixing the wafer (W). In addition, the semiconductor manufacturing apparatus (IM) is provided with: a wafer fixing tool (F) for fixing the wafer (W) by means of a mechanical jig (M) and an electrostatic chuck (E); and a control device (C) for controlling the fixing of the wafer (W), the control device (C) fixing the wafer (W) using only the mechanical jig (M) when the processing temperature of the wafer (W) is greater than or equal to a first reference temperature (T1) higher than the room temperature.

Description

Technical Field

[0001] The present invention relates to a wafer fixing method using a mechanical clamp and an electrostatic chuck, and a semiconductor manufacturing device using the fixing method. Background Art

[0002] The ion implantation process is performed on the wafer heated to a high temperature using an ion implantation device as part of a semiconductor manufacturing device. For example, it is difficult to recover the crystal defects of SiC wafers in the annealing process after ion implantation. In order to avoid adverse effects on device characteristics, the ion implantation process is performed while the wafer is heated to a high temperature.

[0003] During ion implantation, the wafer is fixed by a mechanical clamp or an electrostatic chuck. For example, Patent Document 1 discloses that a mechanical clamp is used together with an electrostatic chuck during high temperature implantation at a temperature above a predetermined temperature in order to assist in reducing the attraction force caused by degradation of the electrostatic chuck's characteristics.

[0004] In addition, Patent Document 2 discloses the use of a mechanical clamp without a power mechanism and an electrostatic chuck in combination to fix a chip. Prior art literature

[0005] Patent Document 1: Japanese Patent Publication No. 2013-531374 Patent Document 2: Japanese Patent Publication No. 2015-203140

[0006] The performance of the electrostatic chuck deteriorates in a high temperature region. However, since the electrostatic chuck is configured to fix a large area of ​​the back surface of the wafer, the entire wafer is firmly fixed to a certain extent. During high-temperature implantation, the wafer is heated by a heater or ion beam irradiation. At this time, if the wafer is firmly fixed, stress is generated in the wafer.

[0007] When the wafer is separated from the wafer fixture after the high-temperature implantation process, the thermal strain suppressed during the implantation process is released all at once as stress, resulting in unfavorable conditions such as wafer cracking or the wafer jumping off the wafer fixture.

[0008] Patent Documents 1 and 2 do not disclose any countermeasures for the above disadvantages. In addition, when an electrostatic chuck is used, a residual suction force is generated on the wafer. When the wafer is detached from the wafer fixing tool, complex voltage control is required to remove the residual suction force. Summary of the invention

[0009] The present invention aims to reduce the occurrence of adverse conditions caused by stress release generated when a wafer is detached.

[0010] A chip fixing method utilizes a chip fixing tool, wherein the chip fixing tool has a mechanical clamp and an electrostatic chuck and is used in a semiconductor manufacturing device. When the processing temperature of the chip is above a first reference temperature higher than room temperature, only the mechanical clamp is used to fix the chip.

[0011] Since only mechanical clamps are used to fix the wafer in the high temperature region where the thermal deformation of the wafer becomes relatively large, the stress released when the wafer is detached can be reduced. Thus, the occurrence of adverse conditions caused by the stress release generated when the wafer is detached can be reduced.

[0012] In order to reduce stress generated when the wafer is heated, it is preferred that the wafer is heated on the wafer fixing tool before the wafer is fixed so that the temperature of the wafer becomes higher than the processing temperature.

[0013] In order to reduce the stress generated in the chip during low-temperature processing, a chip fixing method is provided, which utilizes a chip fixing tool, wherein the chip fixing tool has a mechanical clamp and an electrostatic chuck and is used in a semiconductor manufacturing device. When the processing temperature of the chip is below a second reference temperature lower than room temperature, only the mechanical clamp is used to fix the chip.

[0014] Similar to the high temperature treatment, when the processing temperature of the wafer is lower than the second reference temperature lower than room temperature, the wafer can be fixed using only a mechanical clamp, thereby reducing the occurrence of disadvantages when the wafer is detached due to stress release.

[0015] A semiconductor manufacturing device comprises: a chip fixing tool that fixes the chip using a mechanical clamp and an electrostatic suction cup; and a control device that controls the fixation of the chip, wherein the control device only uses the mechanical clamp to fix the chip when the processing temperature of the chip is above a first reference temperature that is higher than room temperature.

[0016] According to the above structure, deformation of the wafer can be tolerated, and the occurrence of disadvantages when the wafer is detached due to stress release can be reduced.

[0017] In order to reduce stress generated when the wafer is heated, it is preferred that the control device heats the wafer on the wafer fixing tool before fixing the wafer so that the temperature of the wafer reaches the processing temperature.

[0018] Preferably, the mechanical clamp partially fixes a peripheral end portion of the wafer, and the semiconductor manufacturing apparatus includes a limiting member that limits radial movement of the wafer at a portion of the peripheral end portion of the wafer that is not fixed by the mechanical clamp.

[0019] If there is a restriction member, it is possible to more reliably prevent the wafer from falling off the mechanical clamp.

[0020] Since only a mechanical clamp is used to fix the wafer in the high temperature region where the thermal deformation of the wafer becomes relatively large, the stress released when the wafer is detached can be reduced, thereby reducing the occurrence of adverse conditions caused by the stress release. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic plan view showing a configuration example of a wafer fixing tool. Figure 2 It means that when viewed from the P direction Figure 1 A schematic top view of an example of the structure of a chip fixing tool. Figure 3 It is a schematic plan view showing a configuration example of an ion implantation apparatus including a wafer holding tool. Figure 4 This is a flowchart showing an example of a wafer fixing method. Figure 5 This is a flowchart showing another example of the wafer fixing method. Figure 6 This is a flowchart showing another example of the wafer fixing method. Figure 7 This is a flowchart showing another example of the wafer fixing method. Figure 8 It is a schematic plan view showing another configuration example of the wafer fixing tool. Fig. 9 It is a schematic plan view showing another configuration example of the wafer fixing tool. Description of Reference Numerals F Wafer Fixing Tool M Mechanical Fixture E Electrostatic Chuck W chip IM ion implantation device T1 first reference temperature T2 Second reference temperature C Control Device 6.36 Restricted parts DETAILED DESCRIPTION

[0022] Figure 1 It is a schematic plan view showing a configuration example of the wafer fixing tool F. Figure 2 It means that when viewed from the P direction Figure 1 1 is a top view schematically showing an example of the structure of the wafer fixing tool F, in which the limiting component 6 is omitted for simplicity of the drawing. A configuration example of the wafer fixing tool F will be described with reference to these drawings.

[0023] The wafer holding tool F includes a mechanical clamp M and an electrostatic chuck E. The wafer holding tool F is used when a wafer W is processed in a semiconductor manufacturing apparatus such as an ion implantation apparatus, an ion beam etching apparatus, and a film forming apparatus. The mechanical clamp M is a mechanical clamp that does not require a power mechanism, similar to Patent Document 2, for example. By changing the inclination of the wafer fixing tool F, the link mechanism 3, the rod 4, and the pressing part 5 are linked to realize the opening and closing action of the claw 1 pressing the upper surface end of the wafer.

[0024] The electrostatic chuck E has, for example, Figure 2 The electrostatic chuck E is a bipolar electrostatic chuck having a pair of electrodes 10a and 10b shown in the figure. The electrostatic chuck E attracts the wafer W by Coulomb force and Johnson-Rabbeck force by applying DC voltages of different polarities to the electrodes 10a and 10b.

[0025] The electrostatic chuck E is provided with a heater 11, and heats the wafer W to a predetermined processing temperature before processing. However, the electrostatic chuck E is not necessarily provided with the heater 11, and a heating device for heating the wafer W may be separately provided in addition to the electrostatic chuck E. For example, a heater composed of an LED lamp that can be moved to a position opposite to the surface of the wafer W may be used to heat the temperature of the wafer W to the processing temperature before processing the wafer W. Furthermore, the electrostatic chuck E may also include a temperature measuring device 31 (for example, a thermocouple) for measuring the temperature of the wafer W. Figure 2 In the embodiment, the temperature of the chip W is directly measured by the temperature measuring device 31, but the chip temperature can also be indirectly measured by measuring the temperature of another component related to the chip temperature (for example, the temperature of the electrostatic chuck E).

[0026] The support table 2 supports the electrostatic chuck E. For example, when the wafer fixing tool F is used in an ion implantation apparatus as a semiconductor manufacturing apparatus, the support table 2 is rotated and tilted by a driving mechanism (not shown) to adjust the irradiation angle of the ion beam to the device structure formed on the wafer W.

[0027] A limiting member 6 for limiting radial movement of the circular wafer W is provided around the electrostatic chuck E. When the wafer W is placed on the electrostatic chuck E, the limiting member 6 abuts against the periphery of the wafer W or is slightly separated from the periphery of the wafer W. The limiting member 6 is a rod-shaped heat-resistant member, and is made of a material such as glassy carbon, SiC, SiC sintered body, or tantalum.

[0028] The control device C has a storage function for storing data, a calculation function for calculating data, and a function for controlling each unit based on the calculation result and input data. The control device C receives input of the recipe information R, and compares the processing temperature of the wafer W included in the recipe information R with the reference temperature stored in the control device C. Then, the control device C outputs a command signal I to the wafer holding tool F based on the comparison result.

[0029] Figure 3 1 is a schematic plan view showing a configuration example of an ion implantation apparatus IM including a wafer holding tool F. Plasma serving as a basis for an ion beam IB is generated in a plasma chamber 21 . An extraction electrode 22 extracts the ion beam IB from the plasma generated in the plasma chamber 21 .

[0030] The ion beam IB extracted from the extraction electrode 22 includes a plurality of ions. The mass analysis electromagnet 23 analyzes the ions according to their mass in order to extract desired ions from the ion beam IB. The acceleration tube 24 accelerates and decelerates the ion beam IB analyzed by the mass analysis electromagnet 23 to convert it into an ion beam IB of desired energy.

[0031] An energy analysis electromagnet 25 is disposed at the rear stage of the accelerating tube 24. The energy analysis electromagnet 25 removes ions of unnecessary energy components generated by charge exchange between the mass analysis electromagnet 23 and the accelerating tube 24 or within the accelerating tube 24.

[0032] When the ion beam IB is cut by a plane perpendicular to the traveling direction of the ion beam IB, the cut surface of the ion beam IB extracted from the extraction electrode 22 is elliptical. Such an ion beam IB is called a spot beam. After passing through the energy analysis electromagnet 25, the ion beam IB is periodically scanned in one direction by the scanner 26, and is apparently converted into an ion beam having a wide width in the scanning direction.

[0033] The scanned ion beam IB is incident on the parallelizer 27, and is transformed into a parallel ion beam IB having a consistent forward direction by magnetic deflection in the parallelizer 27. The scanning width of the ion beam IB after passing through the parallelizer 27 is wider than the size of the wafer W in the scanning direction of the ion beam IB. For such an ion beam IB, the wafer W is moved in a direction parallel to the Y axis of the figure, thereby performing ion implantation processing.

[0034] A wafer W held by a wafer fixture F is arranged in the processing chamber 28. The support table 2 supporting the wafer fixture F is connected to a driving mechanism 30. The driving mechanism 30 rotates and tilts the support table 2, thereby adjusting the irradiation angle of the ion beam IB to the device structure formed on the wafer W.

[0035] When the wafer W is delivered to the wafer fixing tool F, the support table 2 is rotated about 90 degrees from the posture shown in the figure by the driving mechanism 30. At this time, the wafer supporting surface of the electrostatic chuck E faces upward in the figure. When the electrostatic chuck E is in this posture, the wafer W placed on the electrostatic chuck E is heated by the heater 11 to a temperature higher than the processing temperature.

[0036] A temperature measuring device 32 (eg, a radiation thermometer or a thermal imager) is installed on the ceiling of the processing chamber 28. The temperature measuring device 32 is used to measure the temperature of the wafer W placed on the electrostatic chuck E. The wafer temperature can be measured by any device as long as it can be performed. For example, when the temperature measuring device 31 is provided on the electrostatic chuck E, the temperature measuring device 32 may be removed from the ceiling of the processing chamber 28 .

[0037] The XYZ axes shown in the figure are depicted based on the trajectory of the ideal ion beam IB incident on the processing chamber 28 described later. The Z axis is an axis parallel to the advancing direction of the ion beam IB. The X axis and the Y axis are orthogonal to the Z axis. The directions of the XYZ axes change according to the position of the ion beam IB transported on the beam line. In the ion implantation device IM, the scanning direction of the ion beam IB in the scanner 26 is parallel to the X axis. In addition, the control device C may have a function of controlling each unit of the ion implantation apparatus IM.

[0038] Figure 4 It means by Figures 1 to 3 A flowchart of an example of a wafer fixing method implemented by the control device C described in FIG. The recipe information R is input to the control device C (process S1). Next, the processing temperature of the wafer W included in the recipe information R is compared with the first reference temperature T1 higher than the room temperature (about 25°C) stored in the control device C (process S2). Here, if the processing temperature is higher than the first reference temperature T1 (for example, 300°C or 500°C), the wafer W is fixed only using the mechanical clamp M (process S3). On the contrary, if the processing temperature is lower than the first reference temperature T1, at least the electrostatic chuck E is used in the wafer fixing tool F to fix the wafer W (process S4).

[0039] Generally speaking, the higher the processing temperature of the wafer W is, the greater the thermal deformation of the wafer W is. If the wafer W is fixed so firmly that thermal deformation of the wafer W is hardly tolerated, stress when the wafer W is released increases, and inconveniences such as cracking or jumping of the wafer W often occur.

[0040] The mechanical clamp used in the semiconductor manufacturing apparatus does not cover the entire area of ​​the wafer surface to be processed. The mechanical clamp fixes the wafer W at the entire peripheral end area of ​​the wafer W or a part of the peripheral end, or near the peripheral end. Since the mechanical clamp does not fix the back side of the wafer in a wide range like the electrostatic chuck E, it can allow thermal deformation of the wafer W. That is, the fixation of the wafer W by the mechanical clamp does not prevent thermal deformation of the wafer W except for a portion where the wafer W is pressed.

[0041] exist Figure 4 In the flowchart of FIG. 1 , only the mechanical clamp M is used to fix the wafer W in the high temperature region where the thermal deformation of the wafer W becomes relatively large. As a result, the stress released when the wafer is detached can be reduced, and the occurrence of adverse conditions caused by the stress release generated when the wafer is detached can be reduced. In addition, by using only the mechanical clamp M, complicated voltage control for removing the residual attraction force generated when the electrostatic chuck E is used is not required.

[0042] On the other hand, in order to perform semiconductor manufacturing processing with high precision, it is preferable to firmly fix the position of the wafer W. Therefore, in the case of a processing temperature lower than the first reference temperature T1, an electrostatic chuck E is used to fix the wafer W. At this time, a mechanical clamp M may be used in combination as needed. Although the problem of residual adsorption force occurs when using the electrostatic chuck E, if the processing temperature is lower than the first reference temperature T1, the thermal deformation of the wafer W is small, and the stress released when the wafer is detached also becomes small. Therefore, it is sufficient to focus on removing the residual adsorption force, so the control when the wafer is detached becomes simple.

[0043] Figure 5 FIG. 1 is a flowchart showing another example of a wafer fixing method. Figure 4 The same processing as in the flowchart is given the same reference numerals and detailed description is omitted. The same applies to the subsequent flowcharts.

[0044] Figure 5 The flowchart is in Figure 4 Process S5 and process S6 are added to the flowchart of FIG. After wafer W is placed on the electrostatic chuck E, wafer W is heated before wafer W is fixed (process S5). After wafer W is heated, the wafer temperature is measured, and the measurement result is compared with the process temperature obtained from recipe information R (process S6). According to the comparison result, when the wafer temperature is higher than the process temperature, the process proceeds to process S2. On the contrary, when the wafer temperature is lower than the process temperature, the process returns to process S5 and the wafer W is continued to be heated.

[0045] When the wafer W is heated to the processing temperature after being fixed, a large stress is generated in the wafer W. In particular, if the wafer W is warped, the amount of thermal deformation caused by the heating of the wafer W is large, and the stress when the wafer is released becomes very large. In order to reduce the generation of such stress, the wafer W is intentionally thermally deformed before being fixed, and the wafer W is fixed after a small amount of thermal deformation occurs.

[0046] The heating of the wafer W is performed on the wafer fixture F. However, it is not excluded that the wafer W is preheated at other positions other than the wafer fixture F. As long as the wafer temperature is finally heated to a temperature higher than the processing temperature on the wafer fixture F, the preheating may be performed before the wafer W is transported to the wafer fixture F.

[0047] In addition, the wafer fixing tool F mentioned above includes the wafer fixing tool F and the upper area of ​​the wafer fixing tool F. Before the wafer W is transferred to the electrostatic chuck E, the wafer W is held by the transfer arm of the transfer robot and transferred to the top of the electrostatic chuck E provided on the wafer fixing tool F. In this state, the wafer temperature can be heated to a temperature higher than the processing temperature. The reason for this is that even if the wafer W is heated, the distance between the heating position of the wafer W and the electrostatic chuck E is close, so the wafer temperature does not change substantially.

[0048] Figure 6 1 is a flowchart showing another example of a wafer fixing method. After the wafer temperature becomes equal to or higher than the processing temperature in process S6, the elapsed time is counted (process S7). Thereafter, the elapsed time is compared with a predetermined set time (TM) (process S8). After the elapsed time becomes equal to or higher than the set time, the wafer W is fixed by the mechanical clamp M and the electrostatic chuck E. The set time is the time until the thermal deformation of the wafer W is sufficiently reduced, and is a time obtained in advance through experiments, etc. The set time is stored in advance in the control device C, and the set time can be appropriately changed as needed. In addition, a data table can be prepared according to the type of wafer W and the processing temperature, and the set time can be switched according to the recipe information R. By setting a set time to sufficiently heat the wafer W, the amount of thermal deformation of the wafer W after wafer fixation can be reduced. In addition, as an additional effect, uniform heat distribution within the wafer surface can be achieved.

[0049] In the embodiments described so far, the high temperature treatment of the wafer W is targeted. However, even in the low temperature treatment, the wafer W may be cracked or jumped due to the stress release when the wafer is separated. Figure 7, which is a flowchart showing an example of a wafer fixing method in a low temperature process. Figure 4 The difference lies in the processing of S9. In step S9, the processing temperature of the wafer W obtained from the recipe information R is compared with a second reference temperature lower than room temperature (eg, about 25° C.). When the processing temperature is lower than the second reference temperature T2 according to the comparison result, the wafer W is fixed only by the mechanical clamp M.

[0050] Similar to high temperature processing, when the processing temperature of the chip W is below the second reference temperature T2 (for example, -50°C) lower than room temperature, the chip can be fixed only with the mechanical clamp M to reduce the occurrence of adverse conditions when the chip is detached due to stress release.

[0051] The electrostatic chuck E is provided with a wafer cooling device when cooling the wafer temperature instead of the heater 11 or in addition to the heater 11. Specific examples of the wafer cooling device include a flow channel for flowing a refrigerant such as GALDEN, and a Peltier element.

[0052] Figure 8 FIG. 2 is a schematic plan view showing another configuration example of the wafer fixing tool F. Figure 1 The wafer fixing tool F shown in FIG. 1 is different in that a restriction member 36 composed of a wide arc-shaped member is used instead of a rod-shaped member. When the tilt angle is set during the ion implantation process, the ion implantation process is performed in a state where the wafer holding tool F is tilted with respect to the advancing direction of the ion beam IB. In this case, the side surface of the electrostatic chuck E is exposed to the ion beam IB and is irradiated with the ion beam IB, so there is a concern that the electrostatic chuck E may malfunction.

[0053] Therefore, by adopting the arc-shaped limiting member 36 along the periphery of the electrostatic chuck E, the sputtering of the ion beam IB on the electrostatic chuck E can be prevented. The height of the limiting member 36 from the support table 2 (the dimension in the inside-outside direction of the paper of the drawing) is higher than the height of the electrostatic chuck E (the dimension in the inside-outside direction of the paper of the drawing). The limiting member 36 may be provided along the entire periphery of the electrostatic chuck E except for the pressing portion of the claws 1 of the mechanical clamp M, as long as it does not hinder the transfer of the wafer W.

[0054] The front end of the claw 1 of the mechanical clamp M may be a single claw that presses the end of the wafer W. However, in order to make the front end elastic, the portion that presses the end of the wafer W may be configured as a portion that branches into a plurality of portions. As for the material of the claw 1, glassy carbon, SiC, SiC sintered body, tantalum, etc., which are difficult to be sputtered and difficult to damage the wafer W, are used.

[0055] In the case where the claw 1 is difficult to process due to its complicated shape, the claw 1 may be made of multiple materials, and only the front end portion that contacts the wafer W may be made of glassy carbon, SiC, or other materials. In addition, in order to further make the front end portion of the claw 1 elastic, the front end portion of the claw 1 may be made of another contact member 7, and an elastic member such as a coil spring or a leaf spring may be installed there. Specifically, Fig. 9 Here, the elastic member is made of a material having excellent heat resistance such as austenitic stainless steel, Ni-based superalloy, or Ni—Co-based superalloy.

[0056] like Fig. 9 As shown, by installing an elastic member on the abutting member 7, the deformation of the wafer W can be fully allowed, and the stress of the wafer W can be further reduced. Fig. 9 The contact member 7 itself is constituted by an elastic member. In this case, the contact member 7 is made of a material that is difficult to adhere to the wafer W and has heat resistance.

[0057] The configuration of the mechanical clamp M in the above-described embodiment is an example, and a configuration including a power mechanism for realizing the opening and closing operation of the claw 1 may be adopted. Furthermore, the fixation of the wafer W by the mechanical clamp M can be performed in all wafer processing from the high temperature region to the low temperature region. The portion of the wafer W that is pressed by the mechanical clamp M is not limited to the upper surface of the wafer W, but may also be the side surface.

[0058] The electrostatic chuck E in the above embodiment uses a bipolar DC voltage to adsorb the wafer W, but the structure and adsorption method of the electrostatic chuck E are not limited to the configuration described in the above embodiment. For example, an electrostatic chuck that is provided with two or more electrodes and adsorbs the wafer W using a two-phase or more AC voltage or an electrostatic chuck that adsorbs the wafer W using a gradient force may be used. In addition, the electrostatic chuck E does not need to adsorb the entire back surface of the wafer W, but can adsorb a portion of the wafer W. As long as the adsorption area of ​​the electrostatic chuck E on the wafer W is larger than the fixing area of ​​the mechanical clamp M on the wafer W, the effect of implementing the wafer fixing method of the present invention can be obtained.

[0059] In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

Claims

1. A wafer fixing method, characterized in that: The wafer fixing method uses a wafer fixing tool, which has a mechanical clamp and an electrostatic chuck and is used in a semiconductor manufacturing device. When the processing temperature of the wafer is equal to or higher than a first reference temperature higher than room temperature, the wafer is fixed using only the mechanical clamp.

2. The wafer fixing method according to claim 1, characterized in that: Before the wafer is fixed, the wafer is heated on the wafer fixing tool so that the temperature of the wafer becomes higher than the processing temperature.

3. A wafer fixing method, characterized in that: The wafer fixing method uses a wafer fixing tool, which has a mechanical clamp and an electrostatic chuck and is used in a semiconductor manufacturing device. When the processing temperature of the wafer is lower than a second reference temperature lower than room temperature, the wafer is fixed using only the mechanical clamp.

4. A semiconductor manufacturing device, characterized in that: The semiconductor manufacturing apparatus comprises: A wafer holding tool that holds the wafer with a mechanical clamp and an electrostatic chuck; and a control device for controlling the fixing of the wafer, The control device fixes the wafer using only the mechanical clamp when the processing temperature of the wafer is equal to or higher than a first reference temperature higher than room temperature.

5. The semiconductor manufacturing device according to claim 4, characterized in that The control device heats the wafer on the wafer holding tool before fixing the wafer so that the temperature of the wafer becomes higher than a processing temperature.

6. The semiconductor manufacturing device according to claim 4 or 5, characterized in that: The mechanical clamp partially fixes the peripheral end portion of the wafer, The semiconductor manufacturing apparatus includes a restriction member that restricts radial movement of the wafer at a portion of a peripheral end portion of the wafer that is not fixed by the mechanical clamp.

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