CN116175361A

CN116175361A - Grinding workbench and wafer thinning equipment

Info

Publication number: CN116175361A
Application number: CN202211684504.0A
Authority: CN
Inventors: 赵德文; 刘远航; 靳凯强
Original assignee: Huahaiqingke Co Ltd
Current assignee: Huahaiqingke Co Ltd
Priority date: 2022-12-27
Filing date: 2022-12-27
Publication date: 2023-05-30

Abstract

The invention discloses a grinding workbench and wafer thinning equipment, wherein the grinding workbench comprises a mounting plate, a supporting ring and a base which are sequentially stacked from top to bottom, the mounting plate is used for bearing a sucker, the supporting ring is fixedly connected with the base, the mounting plate is connected with a driving mechanism in the base, an air floatation structure is arranged in the supporting ring so as to realize that the mounting plate rotates under the driving of the driving mechanism when suspended in the supporting ring, and three highest points with completely consistent heights are arranged on the upper surface of the supporting ring, so that the mounting plate is kept horizontal when being abutted against the upper surface of the supporting ring.

Description

Grinding workbench and wafer thinning equipment

Technical Field

The invention relates to the technical field of ultra-precise grinding of wafers, in particular to a grinding workbench and wafer thinning equipment.

Background

The semiconductor industry currently manufactures semiconductor chips by forming electronic circuits such as ICs (Integrated Circuit, integrated circuits) and LSIs (Large Scale Integration, large scale integrated circuits) on the surface of a semiconductor wafer. The back surface of the wafer, which is the opposite surface of the device surface on which the electronic circuits are formed, is ground by a grinding and thinning processing device, also referred to as a substrate, before the wafer is divided into semiconductor chips. Thinning (thinning) of the back surface of the wafer refers to Grinding various materials such as silicon wafers or compound semiconductors before packaging with high precision to reduce the thickness to a suitable ultra-thin form.

The grinding workbench consists of multiple layers, the surface flatness or levelness of each layer can influence the surface flatness or levelness of the uppermost layer, and particularly, the diameter and the perimeter of the grinding workbench are large, the actual precision is limited by precision capability or installation factors, the precision of the upper surface of the grinding workbench is possibly not ideal, random new assembly errors can occur, unstable contact states between different surfaces are caused, and finally, the consistency of ultra-precise grinding is influenced.

Disclosure of Invention

The embodiment of the invention provides a grinding workbench and wafer thinning equipment, which aim to at least solve one of the technical problems in the prior art.

The first aspect of the embodiment of the invention provides a grinding workbench, which comprises a mounting plate, a supporting ring and a base, wherein the mounting plate, the supporting ring and the base are sequentially stacked from top to bottom, the mounting plate is used for bearing a sucker, the supporting ring is fixedly connected with the base, the mounting plate is connected with a driving mechanism in the base, an air floatation structure is arranged in the supporting ring so as to realize that the mounting plate rotates under the driving of the driving mechanism when suspended in the supporting ring, and three highest points with completely consistent heights are arranged on the upper surface of the supporting ring, so that the mounting plate is kept horizontal when abutting against the upper surface of the supporting ring.

In one embodiment, the highest point of the upper surface of the support ring is 1-50 μm higher than the lowest point.

In one embodiment, the three highest points are evenly distributed along the circumference of the support ring.

In one embodiment, the upper surface of the support ring has a smooth transition between the highest point and the lowest point.

In one embodiment, the upper surface of the support ring has an undulating shape.

In one embodiment, the highest point is disposed in close proximity to the ground workpiece.

In one embodiment, the outer side wall of the supporting ring is provided with an air distribution joint, an air passage is arranged in the supporting ring, the upper surface of the supporting ring is provided with a plurality of air holes, air is introduced through the air distribution joint, and air is discharged from the air holes so as to float the mounting plate together with the suction disc thereon by utilizing air pressure.

In one embodiment, the support ring has a material hardness of between 2.5 and 5.

In one embodiment, the mounting plate includes an upper layer including a partition plate and a waterproof cover, and a lower layer including an index plate.

A second aspect of an embodiment of the present invention provides a wafer thinning apparatus, including:

the sucker is used for holding the wafer and driving the wafer to rotate;

the grinding workbench is used for bearing a preset number of suckers and driving all the suckers to integrally rotate;

and grinding the workpiece for grinding and thinning the wafer.

The beneficial effects of the embodiment of the invention include: the mounting plate can be ensured to be kept absolute horizontal or absolute flat when being abutted on the support ring, so that good contact stability and certainty are ensured.

Drawings

The advantages of the present invention will become more apparent and more readily appreciated from the detailed description given in conjunction with the following drawings, which are meant to be illustrative only and not limiting of the scope of the invention, wherein:

FIG. 1 illustrates a wafer thinning apparatus according to an embodiment of the present invention;

FIGS. 2 to 4 show a grinding table according to an embodiment of the present invention;

FIG. 5 illustrates a support ring and indexing disk provided in accordance with one embodiment of the present invention;

fig. 6 illustrates the relative positions of a support ring and a ground workpiece provided in an embodiment of the invention.

Detailed Description

The following describes the technical scheme of the present invention in detail with reference to specific embodiments and drawings thereof. The examples described herein are specific embodiments of the present invention for illustrating the concept of the present invention; the description is intended to be illustrative and exemplary in nature and should not be construed as limiting the scope of the invention in its aspects. It should be noted that, in the case of no conflict, the embodiments and features in the embodiments may be combined with each other. In addition to the embodiments described herein, those skilled in the art can adopt other obvious solutions based on the disclosure of the claims of the present application and the specification thereof, including those adopting any obvious substitutions and modifications to the embodiments described herein. It should be understood that the following description of the embodiments of the present invention, unless specifically stated otherwise, is established in the natural state of the relevant devices, apparatuses, components, etc. in which no external control signal or driving force is given, in order to facilitate understanding.

Furthermore, it is noted that terms such as front, back, upper, lower, left, right, top, bottom, front, back, horizontal, vertical, and the like used herein are merely used for ease of description to aid in understanding the relative position or orientation and are not intended to limit the orientation of any apparatus or structure.

In order to describe the technical solution according to the invention, reference will be made to the accompanying drawings and examples.

In this application, a wafer (wafer) is also referred to as a die, a silicon wafer, a substrate, or a substrate (substrate), etc., and its meaning and actual function are equivalent.

The wafer thinning equipment provided by the embodiment of the disclosure is mainly applied to thinning the back surface of a wafer, wherein the back surface refers to the surface of the wafer, on which devices are not laid, and is generally a substrate, and the substrate material can be silicon, silicon oxide, silicon nitride, silicon carbide, sapphire and the like.

Fig. 1 shows a wafer thinning apparatus according to an embodiment of the present invention, including:

the equipment front end module 1 is used for realizing the in-out of the wafer, and the equipment front end module 1 is arranged at the front end of the wafer thinning equipment. The front end module 1 is a transition module for transferring wafers from outside to inside of the equipment platform, and is used for realizing wafer in-and-out so as to realize 'dry in-and-dry out' of the wafers.

The grinding module 3 is used for grinding the wafer, the grinding comprises rough grinding and finish grinding, and the grinding module 3 is arranged at the tail end of the wafer thinning equipment;

and a polishing module 2 for performing chemical mechanical polishing on the wafer after the completion of the grinding, and having a function of transferring the wafer between the three modules (the equipment front-end module 1, the grinding module 3 and the polishing module 2), the polishing module 2 being disposed between the equipment front-end module 1 and the grinding module 3.

It will be appreciated that the wafer thinning apparatus shown in fig. 1 is only an example, and that the polishing module 2 may be omitted in other implementations, leaving only the apparatus front end module 1 and the grinding module 3, and that the grinding module 3 may include multiple grinding passes, such as 3 passes, 4 passes, 5 passes, etc. Embodiments like these modifications should fall within the scope of the present application as long as the thinning function of the wafer can be achieved.

Device front end module 1:

the equipment front-end module 1 comprises a wafer storage unit and a first transmission unit. The wafer storage unit is arranged at one side of the front end of the wafer thinning device, and the first transmission unit is arranged between the wafer storage unit and the polishing module 2 and is used for realizing the transmission of the wafer between the wafer storage unit and the polishing module 2.

The wafer storage unit is composed of a plurality of front opening unified pods (Front Opening Unified Pod, FOUPs), specifically two, three, etc.

The first transmission unit comprises a pick-and-place slice manipulator. The picking and placing manipulator can rotate, stretch or fold and shrink and can also move along the conveying track. The wafer taking and placing manipulator can take out wafers to be processed from the wafer storage unit through the door structure of the wafer conveying box and send the wafers to the polishing module 2, and can also receive the processed wafers from the polishing module 2 and place the wafers into the wafer conveying box.

Polishing module 2:

the polishing module 2 includes a second transfer unit, a third transfer unit, a chemical mechanical polishing unit, and a post-processing unit. The second transfer unit, the chemical mechanical polishing unit and the post-processing unit occupy the respective edges of the polishing module 2, respectively, and the third transfer unit is located at the center.

Specifically, the second transmission units are located at the edge side in the polishing module 2 and distributed along the length direction of the apparatus, and can communicate the apparatus front end module 1 and the grinding module 3. The chemical mechanical polishing unit is located at the other side edge of the polishing module 2 and adjacent to the grinding module 3 and the second transfer unit, respectively. The post-processing unit is located at the edge of the polishing module 2 on the other side and is adjacent to the equipment front-end module 1, the second transfer unit and the chemical mechanical polishing unit, respectively. The third transfer unit is located near the center of the polishing module 2, surrounded by the second transfer unit, the chemical mechanical polishing unit, and the post-processing unit, for achieving mutual transfer of wafers among the second transfer unit, the chemical mechanical polishing unit, and the post-processing unit.

In one embodiment, the second transfer unit includes a temporary storage section and a movement buffer section for temporarily storing and shipping the wafer. The temporary storage part is arranged at a position close to the front end module 1 of the equipment and is used for temporarily storing or transferring the wafer. The movement buffer part is arranged along the direction from the equipment front end module 1 to the grinding module 3 and can move bidirectionally.

In one embodiment, the third transfer unit includes a central robot for transferring the ground wafer from the mobile buffer to the chemical mechanical polishing unit, transferring the polished wafer from the chemical mechanical polishing unit to the post-processing unit, and transferring the cleaned wafer from the post-processing unit to the temporary storage.

The wafer is taken out from the front end module 1 of the equipment and then conveyed to the grinding module 3 for grinding through the second transmission unit; after finishing grinding in the grinding module 3, the wafer is conveyed to a chemical mechanical polishing unit for polishing through a second transmission unit and a third transmission unit; after polishing and cleaning, the wafer is transferred back to the equipment front-end module 1 via the third transfer unit and the second transfer unit.

The post-processing unit is used for cleaning and drying the polished wafer and can comprise a horizontal brushing device and a single-cavity cleaning device.

Grinding module 3:

the grinding module 3 includes a grinding unit 31, a cleaning unit, and a fourth transfer unit.

The grinding unit 31 is used to achieve wafer grinding and thickness measurement. As shown in fig. 1, the grinding unit 31 includes a grinding table 40, a suction cup 60 provided on the grinding table 40, and a grinding workpiece 50 corresponding to the position of the suction cup 60. The chuck 60 is used for holding and rotating a wafer, and may be a porous ceramic chuck, which is used for vacuum-sucking the wafer. The grinding table 40 is used for carrying a preset number of suction cups 60 and driving all suction cups 60 to integrally rotate. The grinding workpiece 50 is used for grinding and thinning a wafer.

The grinding table 40 is used for carrying a wafer and is rotatable about its vertical central axis. In one embodiment, as shown in FIG. 1, the suction cup 60 is provided with three, rotatable between a rough grinding station, a fine grinding station, and a loading and unloading station. The grinding workpiece 50 may be a grinding wheel, two grinding wheels in fig. 1 being used to achieve rough grinding and finish grinding, respectively. It will be appreciated that fig. 1 is only an example, and that the number of suction cups 60, grinding wheels may be other values, such as 1, 2, 4, 5, 6, etc. for suction cups 60, 1, 3, 4, etc. for grinding wheels.

The cleaning unit is used to perform chuck 60 cleaning, polishing and wafer cleaning.

The fourth transfer unit includes a grinding robot for transferring the wafer, which is a robot used in the grinding module 3, for transferring the wafer between the grinding unit 31 and the second transfer unit, specifically, for transferring the wafer between the suction cup 60 and the moving buffer portion corresponding to the loading and unloading station. The grinding manipulator takes the wafer from the movable buffer part of the second transmission unit and sends the wafer into the grinding unit 31 for grinding, and after grinding and cleaning are completed, the grinding manipulator takes the wafer from the grinding unit 31 and then places the wafer in the movable buffer part so as to facilitate the subsequent transmission of the wafer. The grinding manipulator is internally provided with a pipeline capable of vacuumizing so as to realize vacuum adsorption of the wafer. Alternatively, the grinding manipulator may also be realized by a jaw-carrying mechanism.

As shown in fig. 1, the centers of the 3 suction cups 60 are at an angle of 120 ° to the center line of the grinding table 40. The 3 suction cups 60 rotate among 3 stations, namely a rough grinding station, a fine grinding station and a loading and unloading station, wherein 2 stations opposite to the grinding wheel are used for rough grinding and finish grinding respectively, and 1 station is left for loading and unloading and cleaning of wafers. The rotation of the grinding table 40 can drive the 3 suction cups 60 to switch among the 3 stations, so as to realize the circular movement of the suction cups 60 carrying the wafer according to the sequence of loading and unloading stations, rough grinding stations, fine grinding stations and loading and unloading stations.

During the grinding process, the working process of the grinding table 40 includes

1) The grinding manipulator conveys the wafer to the grinding workbench 40, so that the wafer is fixed on the sucker 60 corresponding to the loading and unloading station;

2) The grinding workbench 40 rotates forward by 120 degrees, and the wafer moves to a rough grinding station for rough grinding;

3) After the rough grinding is finished, the grinding workbench 40 rotates forward by 120 degrees, and the wafer moves to the fine grinding station for fine grinding;

4) After finishing finish grinding, the grinding workbench 40 reversely rotates by 240 degrees, and the wafer moves to a loading and unloading station;

5) And the ground wafer is taken down by a grinding manipulator after being cleaned and spin-dried in loading and unloading work.

In this embodiment, the plurality of suction cups 60 are disposed on the grinding table 40, each suction cup 60 can rotate independently, and the grinding table 40 can rotate around its vertical central axis to make the grinding table 40 drive the plurality of suction cups 60 to rotate integrally, so as to realize the conversion position of the suction cups 60 between different stations, and after the suction cups 60 rotate and move to a preset position, the grinding table 40 needs to be kept stationary.

In order to achieve the air-floating structure and surface flatness requirements of the grinding table 40, as shown in fig. 2 to 4, an embodiment of the present invention provides a grinding table 40 including a mounting plate 41, a support ring 42 and a base 43 stacked in this order from top to bottom.

The mounting plate 41 is adapted to carry a suction cup 60. As shown in fig. 4, in one embodiment, the mounting plate 41 includes fixedly connected upper and lower layers, the upper layer including a partition plate 411 and a waterproof cover 412, and the lower layer including an index plate 413. The partition plate 411 is for dividing the upper surface of the circular grinding table 40 into a plurality of uniform sector areas in the radial direction, and the partition plate 411 is radially formed with a plurality of pieces in the radial direction. Each sector area is provided with a waterproof cover 412, the waterproof covers 412 are fixedly connected with two adjacent partition plates 411, the surface of each waterproof cover 412 is provided with a mounting hole for mounting the sucker 60, the periphery of the sucker 60 is covered with the waterproof covers 412, and the waterproof covers 412 can receive waste materials and waste liquid generated by grinding. The lower index plate 413 is circular and is matched with the upper index plate in size, and a plurality of uniformly distributed mounting holes are formed in the surface of the index plate 413 so that the suction disc 60 can pass through. The mounting plate 41 is connected to a driving mechanism in the base 43, specifically, a lower index plate 413 may be connected to a driving mechanism below, and the index plate 413 drives the partition plate 411, the waterproof cover 412, and the plurality of suction cups 60 to rotate synchronously under the driving of the driving mechanism.

The suction cup 60 is fixed in the following manner: the suction cup 60 is supported on the dividing disc 413 through three supporting points uniformly distributed on the circumference, one of the three points is fixedly supported, and the other two points are height-adjustable supports for adjusting the angle of the suction cup plane relative to the grinding wheel.

As shown in fig. 2 and 4, the support ring 42 is located under the mounting plate 41, the support ring 42 is sized to match the mounting plate 41, specifically, the outer diameter of the support ring is 1100-1300 mm, the ring width of the support ring is 80-120 mm, the material hardness of the support ring is between 2.5 and 5, the material is preferably marble or the like, and the surface roughness of the support ring is superior to ra0.8 μm.

The mounting plate 41 and the support ring 42 are not fixed so as to realize the air floatation function. When the air floatation is started, the support ring 42 supports the mounting plate 41 under the action of air floatation force, namely, the mounting plate 41 floats relative to the support ring 42, and a gap is formed between the lower surface of the mounting plate 41 and the upper surface of the support ring 42, so that the mounting plate 41 is driven to rotate by the driving mechanism. When the air floatation is stopped, the mounting plate 41 falls down and is abutted against the supporting ring 42, and under the action of the gravity of the mounting plate 41 and the sucking disc 60, the friction force between the lower surface of the mounting plate 41 and the upper surface of the supporting ring 42 is large enough to ensure that the mounting plate 41 is static and motionless in the grinding process so as to provide a stable supporting environment; furthermore, the mounting plate 41 can be still kept still in a vacuumizing mode, the air hole 422 of the air distribution joint 421 is communicated with a negative pressure source, and an adsorption force is further formed between the mounting plate 41 and the support ring 42, so that stability can be better ensured.

In one embodiment, the support ring 42 is fixedly coupled to the base 43. The upper surface of the base 43 has a convex annular boss of a size matching that of the support ring 42, and the bottom of the support ring 42 is fixed on the annular boss.

In one embodiment, an air bearing structure is provided inside the support ring 42 to enable the mounting plate 41 to rotate under the drive of the drive mechanism when suspended from the support ring 42.

As shown in fig. 5, the implementation manner of the air-floating structure includes: the outer side wall of the support ring 42 is provided with an air distribution joint 421, an air passage is arranged in the support ring 42, the upper surface of the support ring 42 is provided with a plurality of air holes 422, air is fed through the air distribution joint 421, and air is discharged from the air holes 422 so as to float the mounting plate 41 together with the suction disc 60 thereon by utilizing air pressure.

In order to achieve an absolute level or flatness when the mounting plate 41 abuts against the support ring 42, as shown in fig. 5, in one embodiment of the present invention, the upper surface of the support ring 42 has at least three highest points H of exactly uniform height, so that the mounting plate 41 remains level when abutting against the upper surface of the support ring 42.

The upper surface of the supporting ring 42 is used as an air floatation surface, and the precision requirement is high; however, the accuracy of the upper surface of the support ring 42 is not ideal due to the accuracy capability or the installation factor, and random errors occur in the new installation, in other words, no matter how much the upper surface of the support ring 42 is required to be extremely flat during the machining process, the surface roughness phenomenon is very likely to occur after the installation or operation. Therefore, the support ring 42 is specially processed to have 3 uniformly distributed highest points H, and based on the principle of three-point plane determination, the embodiment of the invention can ensure that the mounting plate 41 is kept absolutely horizontal or absolutely flat when being abutted on the support ring 42, thereby ensuring good contact stability and certainty.

The embodiment of the invention avoids an extreme phenomenon: if the support ring 42 is deformed in an arch shape along a certain diameter, the mounting plate 41 will be in an unstable contact state on the support ring 42, and during the grinding process, the mounting plate 41 and the suction cup 60 thereon will be in an unstable state under different grinding wheels, different processes and different loads, thereby destroying the consistency of wafer grinding.

As shown in fig. 5, the support ring 42 is ring-shaped, and the upper surface of the support ring 42 has a wavy shape, which may be wavy, wherein preferably there are three highest points H, and the heights of the three highest points H are completely uniform. It should be understood that the number of the highest points H with the same height may be 4, 5, 6, etc., and three-point determination planes may be realized, the specific number of the highest points H is not limited to the above-listed values, and may be designed according to practical needs in other embodiments, so as to achieve the technical ideas disclosed in the present application, and all the basic principles in the present application should be covered by the claims of the present invention.

The support ring 42 may be specifically treated in the following manner: during manufacturing and processing, the upper surface of the support ring 42 is directly processed into a wavy shape; or the upper surface of the support ring 42 is polished to the special shape by using a special tool; or the support ring 42 may be assembled in multiple parts with multiple high points, such as mounting surfaces with several shims added to form a protrusion.

In one embodiment, the highest point H of the upper surface of the support ring 42 is 1-50 μm higher than the lowest point L, preferably 2-8 μm, to ensure effective air bearing capacity. Further, as shown in fig. 5, the air holes 422 are provided at the highest point H and the lowest point L of the upper surface of the support ring 42, or the air holes 422 may be provided only at the highest point H, thereby achieving a uniform air-floating effect.

As shown in fig. 6, three highest points H are uniformly distributed along the circumferential direction of the support ring 42, and the lowest points L may be uniformly distributed, and both the highest points H and the lowest points L may be uniformly distributed. The center of each of the three highest points H is at an angle of 120 ° to the center line of the support ring 42. Further, as shown in fig. 6, the highest point H may be disposed as close as possible to the grinding workpiece 50, i.e., the grinding wheel.

As shown in fig. 5, the upper surface of the support ring 42 has a smooth transition between the highest point H and the lowest point L.

The embodiment of the invention can ensure the contact state of the grinding workbench 40 which is absolutely reliable and consistent with stability in the grinding process, and ensure the stability and reliability of the support of the sucker 60, thereby improving the grinding stability; when different suckers 60 rotate below the grinding workpiece 50, the supporting state of the suckers 60 can still be kept consistent, so that the stability of keeping the surface smooth and consistent during grinding of different wafers on different stations is improved. The drawings in the present specification are schematic views, which assist in explaining the concept of the present invention, and schematically show the shapes of the respective parts and their interrelationships. It should be understood that for the purpose of clearly showing the structure of various parts of embodiments of the present invention, the drawings are not drawn to the same scale and like reference numerals are used to designate like parts in the drawings.

In the description of the present specification, reference to the terms "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

While embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that: many changes, modifications, substitutions and variations may be made to the embodiments without departing from the spirit and principles of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. The utility model provides a grinding workbench, its characterized in that, includes mounting disc, supporting ring and base that stacks in proper order from top to bottom, and the mounting disc is used for bearing the sucking disc, supporting ring and base fixed connection, and the actuating mechanism in mounting disc and the base is connected, and the inside air supporting structure that is equipped with of supporting ring to realize that the mounting disc floats and rotates under actuating mechanism's drive when the supporting ring, the upper surface of supporting ring has three highly identical highest points completely, makes the mounting disc butt keep the level when supporting ring upper surface.

2. A grinding table according to claim 1, characterized in that the highest point of the upper surface of the support ring is 1-50 μm higher than the lowest point.

3. A grinding table according to claim 1, wherein three of said highest points are evenly distributed along the circumference of the support ring.

4. The grinding table of claim 1 wherein the upper surface of the support ring has a smooth transition between a highest point and a lowest point.

5. A grinding table according to claim 1, wherein the upper surface of the support ring has an undulating shape.

6. The grinding table of claim 1 wherein the highest point is disposed proximate to the ground workpiece.

7. The grinding table of claim 1, wherein the outer side wall of the support ring is provided with an air distribution joint, an air passage is formed in the support ring, a plurality of air holes are formed in the upper surface of the support ring, air is introduced through the air distribution joint, and air is discharged from the air holes so as to float the mounting plate together with the suction disc thereon by using air pressure.

8. A grinding table according to claim 1, characterized in that the material hardness of the support ring is between 2.5 and 5.

9. The grinding table of claim 1 wherein the mounting plate comprises an upper layer comprising the divider plate and the waterproof cover and a lower layer comprising the indexing plate.

10. A wafer thinning apparatus, comprising:

the sucker is used for holding the wafer and driving the wafer to rotate;

a grinding table according to any one of claims 1 to 9 for carrying a predetermined number of said suction cups and for imparting an overall rotation to all suction cups;

and grinding the workpiece for grinding and thinning the wafer.