JP2000135671A - Wafer polishing surface plate, manufacture therefor and wafer polishing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten flatness working time of the surface of a surface plate in surface plate manufacturing by forming a surface layer composed of a highly workable object material on the surface of a highly rigid disk-like member, and flatly working the surface of the surface layer. SOLUTION: A highly rigid disk-like member 21a is installed on a surface plate receiver 22. The upper surface of the highly rigid disk-like member 21a is not required to be flat, and may be the rugged surface. Next, a surface layer 21b composed of a highly workable object material is formed on the upper surface of the highly rigid disk-like member 21a by coating, etc. Since the surface of this surface layer 21b normally becomes the recess/projection surface, the surface of this surface layer 21a is flatly worked. A surface plate 21 is manufactured in this way. General purpose machine work such as cutting, grinding and lapping can be adopted as a method for flatly working the surface of this surface layer 21b, but when considering workability and a work environment, grinding work is desirable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention processes a thin plate, for example, a semiconductor wafer such as a silicon wafer or a quartz thin plate such as a quartz mask substrate (these thin plates are collectively referred to as a wafer in this specification) into a mirror-finished surface. The present invention relates to a novel surface plate used in a wafer polishing apparatus, a method of manufacturing the same, a novel wafer polishing device having the surface plate, and a processing method for improving the flatness of the surface plate of the wafer polishing device.

[0002]

2. Related Art FIG. 5 is a schematic sectional view of a conventional wafer polishing apparatus 10. As shown in FIG. In FIG.
Is adhered to the surface of a plate (wafer holding plate) 12 constituting the polishing head 11 with wax or the like, pressed against a polishing cloth 14 with a predetermined polishing load, supplied with an abrasive 18 from a nozzle 16 and at a predetermined speed. Rotating platen 20
The surface of the wafer W is mirror-finished by the rubbing motion generated between the polishing pad 14 and the polishing cloth 14 attached to the surface.

The surface plate 20 and the surface plate receiver 22 are firmly fixed with bolts or the like, and a gap 24 formed therebetween is used to remove frictional heat generated by the sliding movement of the polishing pad 14 and the wafer W. , A cooling water 28 is supplied via a rotary joint 26. In FIG. 5, 30 is a collecting gutter for the abrasive 18, 32 is a storage tank for the abrasive 18, 34 is a pump,
36 is a connection pipe connecting the storage tank 32 and the pump 34, and 38 is a circulation pipe connecting the pump 34 and the nozzle 16.

The most important quality imparted to a wafer in mirror polishing is flatness. However, with the recent increase in the degree of integration of semiconductor devices, the demand has been increasing.

[0005] Since the upper surface of the wafer is processed by a plate and the lower surface of the wafer is processed by being sandwiched between polishing cloths, the surface of the plate must be flat to obtain a highly flat wafer.
The polishing cloth has a uniform thickness and a uniform elasticity, and when the wafer is pressed by a load, the contact pressure between the wafer and the polishing cloth, that is, the polishing pressure becomes uniform, and the polishing plate has a polishing plate attached thereto. The surface needs to be flat.

When the plate is flat and the polishing cloth is even, the flatness of the wafer depends only on the flatness of the surface of the surface plate. If the surface of the surface plate is flat, the wafer becomes flat. However, when the surface of the surface plate is not flat, the wafer is processed into various shapes as shown in FIGS. 6 to 10 according to the surface shape of the surface plate. That is, the shape of the surface of the surface plate 20 is high at the center (at most several tens of μm).
In the case of a conical or spherical shape, the wafer W is processed into a spherical shape with a concave central portion (FIGS. 6 and 7).
When the surface has an inverted conical shape or an inverted spherical shape whose central portion is low, the wafer W is processed into a spherical shape whose central portion is protruded (FIGS. 8 and 9). In this case, the outer edge of the wafer W is sagged (FIG. 10).

As described above, various measures described below have been devised in order to enhance the flatness of the surface plate, which greatly affects the flatness of the wafer. (B) The surface plate is made of a material with high hardness and rigidity to enable high-precision planar processing, a material with high thermal conductivity to reduce thermal deformation and a small coefficient of linear expansion, and metal contamination on the wafer. Ceramics such as stainless steel, alumina, and silicon carbide are used as a material that does not cause the occurrence of cracks.

(B) If the surface plate and the surface plate receiver are fixed by using fixing means such as bolts after flattening the surface surface of the surface plate, the surface plate is elastically deformed and the surface flatness of the surface plate Will decrease. To avoid this, the surface plate and the surface plate receiver are flattened on the surface of the surface plate while being fixed in advance.

(C) In order to enhance the flatness when the surface plate is actually used, the flatness processing of the surface surface of the surface plate (b) is performed by the same heat treatment as when the surface surface is actually used. Perform under load.

[0010] In any of the above (a), (b) and (c), in order to make the surface of the surface plate highly flat, lapping using free abrasive grains is adopted as the final processing. However, since the surface plate is formed of a hard material, the workability is low and the processing time is long, or the measurement and the processing must be repeated many times until a predetermined flatness is obtained. There is a disadvantage that.

Further, in a wafer polishing apparatus which is installed at a production site and has been continuously operated for a long period of several years, even if the flatness of the platen at the time of production is high, the platen and the platen can be fixed. The flatness of the surface plate is reduced due to the release of the residual stress of the plate receiver or the rusting of the surface of the surface plate and the fastening surface of the surface plate receiver, and the flatness of the product wafer is also reduced. In such a case, in order to recover the flatness of the surface plate, the surface plate is removed from the polishing device, lapping-processed by a plane processing machine tool, and then attached to the polishing device.

[0012] However, this method requires a long term stoppage of the polishing apparatus, and there is an economic loss that production activities are hindered. Even if the surface plate is manufactured with high flatness, if it is used for a long time, the surface plate deforms due to release of residual stress of the surface plate or surface plate receiver, or the surface plate generated at the joint with the surface plate The flatness of the surface of the surface plate decreases due to the deformation of the surface plate caused by rusting of the tray.

In such a case, the platen is removed from the polishing apparatus and lapping is performed again to restore the flatness. However, if the polishing apparatus is stopped for a long period of time, production activities are stagnated, resulting in economic loss. There is a disadvantage that it is large.

On the other hand, as materials satisfying various requirements required for the surface plate, for example, high hardness, high rigidity, high thermal conductivity, low coefficient of linear expansion, and low metal elution, ceramics such as stainless steel, alumina, and silicon carbide are used. Are adopted, but all of them are materials that take a long time to process.

However, the portion to be flattened is only on the surface of the surface plate or in the vicinity of the surface, and the portion requiring high workability is also the same. It can be said that the environment of the surface of the surface plate or the vicinity of the surface when it is actually subjected to polishing is very gentle.

(1) Since the rubbing required for polishing is performed on the surface of the polishing cloth stuck on the surface plate, there is no rubbing load on the surface of the surface plate, and abrasion resistance is unnecessary. It is.

(2) The load applied to the polishing cloth during polishing is
These are a compressive force perpendicular to the surface caused by the polishing load and a shear force parallel to the surface caused by the frictional force generated by the sliding motion with the wafer. The same force is applied to the surface of the surface plate, but any force is at most 1 kg / cm ^2, which is slight.

(3) Although the surface temperature of the polishing pad rises due to the frictional force caused by the sliding motion between the wafer and the polishing pad, it does not reach 50 ° C. under normal polishing conditions. Since the surface plate is cooled by contacting the cooling water on the surface plate receiving side, the surface temperature of the surface plate is 50 ° C. or less.

(4) The base material of the double-sided tape used for attaching the polishing cloth to the surface plate is a sheet-like synthetic resin such as soft vinyl chloride or polyethylene, which is stable against abrasives. , Does not permeate the abrasive. Therefore, the surface of the platen is protected by the base material of the double-sided tape, and does not come into contact with the abrasive containing various chemical substances.

(5) The base material of the polishing cloth and the double-sided tape and the pressure-sensitive adhesive are rich in viscoelasticity, and the unevenness of the surface roughness existing on the surface of the surface plate is absorbed. Has no effect.

(6) In order to replace the polishing pad whose life has expired,
When peeling the polishing cloth from the surface of the surface plate, tensile load and shear load act on the surface of the surface plate in a complicated manner due to the adhesive force of the double-sided tape adhesive, but since the adhesive force itself is not large,
The force acting on the surface of the surface plate is not large.

As described in the above (1) to (6), the environment where the surface of the surface plate is placed may be very gentle mechanically, chemically, and thermally, and may have low surface roughness. There are many materials with high workability that can be used in such an environment.

[0023]

SUMMARY OF THE INVENTION The present inventor has conducted research on the construction of a new platen based on the above-described consideration of the current platen. By forming a thin surface layer made of a material with high workability and flattening the surface to make a surface plate, the flatness processing of the surface plate becomes easy and the processing time is shortened. And arrived at the present invention.

A first object of the present invention is to reduce the time required for processing the flatness of the surface of the surface plate during the production of the surface plate, facilitate the change of the surface shape of the surface plate, and provide an optimum surface roughness for obtaining a flat wafer. It is an object of the present invention to provide a novel wafer polishing surface plate that facilitates processing of a disk shape, a novel wafer polishing apparatus having the surface plate, and a wafer polishing method using the wafer polishing device.

A second object of the present invention is to provide a method for efficiently manufacturing the above-described novel wafer polishing table.

A third object of the present invention is to restore the flatness of the surface plate deteriorated by long-term use without removing the surface plate from the wafer polishing apparatus. Wafer polishing, which can reduce the operating time, prevent the reduction of the operating time, facilitate the change of the surface shape of the surface plate, and facilitate the processing of the optimum surface shape to obtain a flat wafer An object of the present invention is to provide a method of flattening a surface plate for use.

[0027]

In order to solve the above-mentioned problems, a wafer polishing surface plate according to the present invention is a surface surface plate used for polishing a wafer. It is characterized in that a surface layer made of a material having high properties is formed and the surface of the surface layer is processed to be flat.

Further, since the thickness of the layer made of a material having high workability is small, there is almost no influence on properties required for the surface plate other than workability, for example, compressive deformation and heat transfer ability due to load.
The requirements required for a highly rigid plate-like member are the same as the various requirements required for the surface plate described above, and ceramics such as stainless steel, alumina, and silicon carbide may be employed.

When a surface layer having high workability is formed on the surface of the surface plate, the selection range of the material of the surface layer or the method of forming the surface layer is wide. After various investigations, the material of the surface layer is preferably a synthetic resin material, for example, an epoxy resin or an epoxy resin containing a filler.

As the filler, calcium carbonate, Si metal powder, glass powder, quartz powder and the like can be adopted.
By controlling within the range of 〜60%, mechanical properties such as workability and compressive strength can be changed.

As a method for forming the surface layer, it is preferable to use a coating method in which the bonding strength of the surface layer to the plate body is high and the coating can be easily removed with a release agent if necessary. Is preferably 2 mm or less, at which cracks are less likely to occur.

Examples of the synthetic resin material of the surface layer of the surface plate of the present invention include a phenol resin, a polyester resin, a polyamide resin and a polycarbonate resin in addition to the epoxy resin, and also a filler such as calcium carbonate. Can be contained in these synthetic resin materials.

The wafer polishing apparatus of the present invention is provided with the above-mentioned novel surface plate.

In the wafer polishing method of the present invention, the surface of the wafer is polished flat by using the novel polishing apparatus.

The method for producing a surface plate for wafer polishing according to the present invention is a method for producing a surface plate used for polishing a wafer. A surface layer made of a material having high workability is formed on the surface of a highly rigid disk-shaped member. The method is characterized by comprising a step of forming and a step of flattening the surface of the surface layer.

Further, the flattening method of the platen for wafer polishing according to the present invention is a processing method for improving the flatness of the platen mounted on the wafer polishing apparatus. A surface layer made of a material having high workability is formed on the surface of the board, and the surface of the surface layer is flattened. This flattening may be performed by machining, but is preferably performed by flattening means that moves in a direction parallel to and perpendicular to the surface of the surface plate.

On the other hand, if the above-described novel platen structure of the present invention is adopted, the workability of the surface of the platen is improved.
Flatness processing can be performed without removing the surface plate from the polishing apparatus. As the flattening means, general-purpose machining such as cutting, grinding, and lapping can be employed. However, in consideration of workability, work environment, and the like, grinding is preferable.

As the flattening device, flattening means, for example, a grinding means such as a straight whetstone or a cup whetstone, and a driving means therefor are provided on a linear guide (XZ table) which moves in a direction parallel and perpendicular to the surface of the surface plate. It can be composed of a device.

The flattening process is performed by rotating a grinding means such as a straight grindstone or a cup grindstone on the surface of a surface plate and making a predetermined cut in a vertical direction (Z-axis direction) to grind the grinding means such as a cup grindstone. May be moved horizontally (X-axis direction) on the surface of the surface plate.

For a surface plate whose flatness has deteriorated due to long-term use, a layer having high workability is formed on the surface of the surface plate by the above-described method, and then flatness processing may be performed. Since there is no need to remove, there is an advantage that the stop time of the wafer polishing apparatus is reduced.

It is needless to say that the present invention can be applied not only to a platen used for a long time but also to a platen newly manufactured. That is, the high-rigidity disk-shaped member in the present invention includes a platen body before being a platen, a newly manufactured platen, and a platen used for a long time.

Further, as an additional effect, the workability of the surface layer is improved, and if necessary, the surface of the surface plate is formed into a shape other than a plane, for example, a spherical surface protruding by several tens of μm.
Alternatively, a thin groove can be easily formed.

[0043]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to FIGS. 1 to 4 in the accompanying drawings, but various modifications may be made to these embodiments without departing from the technical idea of the present invention. Of course is possible. FIG. 1 is a schematic cross-sectional explanatory view showing one embodiment of a polishing apparatus provided with a wafer polishing table according to the present invention. 2A and 2B are cross-sectional explanatory views showing one example of a manufacturing process of a surface plate according to the present invention in the order of processes, and FIG. 2A shows a highly rigid disk-shaped member (surface plate body) and a surface plate receiver before forming a surface layer; (B) shows a state in which a surface layer having high workability is formed on the surface of the surface plate, and (c) shows a state in which the surface of the formed surface layer is flattened.

FIG. 3 is a schematic sectional side view showing an example of the grinding means for grinding the surface layer of the surface plate of the present invention. FIG. 4 is a top view of FIG. 1 to 4, the same or similar members as those in FIG. 5 are denoted by the same reference numerals.

In FIG. 1, reference numeral 10a denotes a wafer polishing apparatus provided with a surface plate 21 according to the present invention. The wafer polishing apparatus 10a has the same basic configuration as the conventional wafer polishing apparatus 10 shown in FIG. 5 except that the surface plate 21 is different from the conventional surface plate 20. Will not be described again.

The surface plate 21 according to the present invention has a surface layer 2 made of a material having high workability on a surface of a highly rigid disk-shaped member 21a.
1b, and the surface of the surface layer 21b is processed to be flat.

FIGS. 2A, 2B and 2C show the surface plate 2 of the present invention.
One example of one manufacturing process will be shown in the order of processes. First, platen tray 2
2 is provided with a highly rigid disc-shaped member 21a [FIG.
(A)]. The upper surface of the high-rigidity disk-shaped member 21a does not need to be flat, but may be an uneven surface as shown.
Next, a surface layer 21b made of a material having high workability is formed on the upper surface of the highly rigid disk-shaped member 21a by coating or the like (FIG. 2B). Since the surface of the surface layer 21b is usually uneven as shown in the figure,
The surface of 1a is worked flat [FIG. 2 (c)]. Thus, the surface plate 21 of the present invention can be manufactured.

As a method of flattening the surface of the surface layer 21b, general-purpose machining such as cutting, grinding, and lapping can be employed, but grinding is preferable in consideration of workability and working environment.

The high rigid disk-shaped member 21a is shown in FIG.
In the example shown in (b) and (c), the platen is also a platen before being the platen 21, and includes a platen that has been used for a long period of time and has deteriorated flatness or a newly manufactured platen as described later. Things.

An example of a grinding apparatus suitably used for the grinding will be described with reference to FIGS. In FIG. 3, reference numeral 40 denotes a flattening device for flattening the surface layer 21b of the surface plate 21, and a grinding device in the illustrated example. The grinding device 4
0 is an XZ table (linear guide means) 42 and the X
A grinding means (a cup grindstone in the illustrated example) 44 attached to the -Z table 42 and moving in a direction parallel and perpendicular to the surface of the surface plate 21 is provided.

The XY table 42 has an X-axis linear guide 46. The linear guide 46 has a linear portion 4.
6a and both legs 4 suspended from both ends of the linear portion 46a
6b, 46b. Reference numeral 48 denotes an annular base provided with a space 50 in the center. The X-axis linear guide 46 is attached to the base 48 across the space 50 by screws 52 via the legs 46b, 46b.
And so on.

Reference numeral 54 denotes an X-axis ball screw, which is rotatably mounted below and in parallel with the linear portion 46a of the X-axis linear guide 46 via both legs 46b, 46b of the X-axis linear guide 46. I have. An X-axis slide member 56 is slidably inserted in the X-axis direction in the linear portion 46 a of the X-axis linear guide 46.
Is screwably mounted on the X-axis ball screw 54. An X-axis encoder 58 is provided at one end of the X-axis ball screw 54. The X-axis ball screw 54 is rotated by a command from the X-axis encoder 58,
The movement of the shaft slide member 56 in the X-axis direction is controlled.

Reference numeral 60 denotes a Z-axis linear guide which is attached to the X-axis slide member 56 and is movable in the X-axis direction together with the X-axis slide member 56. A Z-axis ball screw 62 is rotatably mounted on the Z-axis linear guide 60. A Z-axis slide member 64 is slidably attached to the Z-axis linear guide 60 in the Z-axis direction. Reference numeral 66 denotes a Z-axis encoder provided at the upper end of the Z-axis ball screw 62. The Z-axis ball screw 62 is rotated by a command from the Z-axis encoder 66,
The movement of the shaft slide member 64 in the Z-axis direction is controlled. 6
Reference numeral 8 denotes a drive motor for rotating the grinding means (cup grindstone) 44.

When the grinding device 40 having the above-described structure is used, the grinding means (cup grindstone) 44 is appropriately moved in the X-axis direction and the Z-axis direction while being rotationally driven, so that the upper surface of the surface layer 21b of the surface plate 21 is rotated. Can be freely ground.

[0055]

The present invention will be described in more detail with reference to the following examples, but it goes without saying that the following description is illustrative and should not be construed as limiting.

(Example 1) This is an example of the method of flattening a surface plate for polishing a wafer according to the present invention, and an attempt was made to recover the flatness of the surface plate, which was deteriorated by long-term use. The diameter of the platen is 700 mm, the material is stainless steel, and the platen made of cast iron is firmly fastened with bolts, a cooling water channel is formed between them, and a polishing cloth is directly coated with double-sided tape on the surface. It was pasted and used.

First, after cleaning the surface of the surface plate, a room temperature-curable epoxy resin containing 30% of calcium carbonate fine particles and a curing agent are kneaded with a vacuum defoaming kneader to make the composition uniform. The surface of the platen was coated with a thickness of about 1.5 mm and left to stand for one day to cure.

To process the surface of the cured product, a cup-shaped grindstone made of metal bond diamond is attached to a linear guide (XZ table), and is moved in parallel with the surface of the platen, and at the same time, while rotating the platen, Pure water at a temperature was supplied as a grinding liquid, and flatness processing was performed until the thickness of the coating layer became 0.9 mm.

According to the above-described method, it is easy to process the flatness of the surface of the surface plate, and it can be confirmed that the workability of the epoxy resin coating layer is high. Has a conical shape with a central portion protruding about 20 μm, but the flatness of the flat shape is improved to 3 μm.

Even with such processing, the attachment of the polishing cloth, the holding of the polishing cloth during wafer polishing, the peeling of the polishing cloth after use, etc. could be performed in exactly the same manner as in the prior art. In addition, the suspension period of the polishing apparatus by the above operation is four days, which is significantly shorter than the conventional one month.

As for the phenolic resin, polyester resin, polyamide resin, and polycarbonate resin, the surface of the platen was coated and flattened to improve the flatness in the same manner as the epoxy resin, but similar results were obtained. did it.

In the above description, polishing of a semiconductor wafer such as a silicon wafer has been described. However, it goes without saying that the present invention can be applied to a thin plate other than a semiconductor wafer, for example, a quartz thin plate such as a quartz mask substrate.

[0063]

As described above, according to the surface plate for wafer polishing of the present invention, the time required for processing the surface flatness during the production of the surface plate is shortened, the surface shape of the surface plate can be easily changed, and The effect that the processing of the optimal platen shape for obtaining the wafer is also facilitated is achieved.

According to the wafer polishing apparatus of the present invention, the surface shape of the surface plate can be easily changed.
Flexible polishing of the wafer can be realized.

According to the wafer polishing method of the present invention, the wafer can be polished flat.

According to the method for producing a wafer polishing surface plate of the present invention, the effect that the above-described novel wafer polishing surface plate of the present invention can be efficiently produced is achieved.

According to the method of flattening a surface plate for wafer polishing of the present invention, the deteriorated flatness of the surface plate can be repaired without removing the surface plate from the wafer polishing apparatus.
The downtime of the wafer polishing machine can be shortened, so that the operating time can be prevented from lowering, and the surface shape of the surface plate can be easily changed, making it easy to process the optimum surface shape to obtain a flat wafer. Is achieved.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional explanatory view showing one embodiment of a polishing apparatus provided with a wafer polishing surface plate of the present invention.

FIG. 2 is a cross-sectional explanatory view showing one example of a manufacturing process of a surface plate of the present invention in the order of processes.

FIG. 3 is a schematic cross-sectional side view showing an example of a grinding means for grinding a surface layer of a surface plate.

FIG. 4 is a top view of FIG. 3;

FIG. 5 is a schematic sectional view showing a polishing apparatus provided with a conventional wafer polishing surface plate.

FIG. 6 is an explanatory diagram of a first example showing a relationship between a surface shape of a surface plate and a surface shape of a processed wafer.

FIG. 7 is an explanatory diagram of a second example showing a relationship between a surface shape of a surface plate and a surface shape of a processed wafer.

FIG. 8 is an explanatory diagram of a third example showing the relationship between the surface shape of a surface plate and the surface shape of a processed wafer.

FIG. 9 is an explanatory diagram of a fourth example showing the relationship between the surface shape of the surface plate and the surface shape of a processed wafer.

FIG. 10 is an explanatory diagram of a fifth example showing the relationship between the surface shape of a surface plate and the surface shape of a processed wafer.

[Explanation of symbols]

10: Conventional wafer polishing apparatus, 10a: Wafer polishing apparatus of the present invention, 20, 21: Surface plate, 21a: High rigid disk-shaped member, 21b: Surface layer, 40: Flattening processing apparatus, 42:
XZ table, 44: grinding means, 46: X-axis linear guide, 46a: linear portion, 46b, 46b: both legs,
48: annular base, 50: space, 52: screw, 54: X
Shaft ball screw, 56: X-axis slide member, 58: X-axis encoder, 60: Z-axis linear guide, 62: Z-axis ball screw, 64: Z-axis slide member, 66: Z-axis encoder, 68: drive motor.

Claims (9)

[Claims] 1. A surface plate used for polishing a wafer, wherein a surface layer made of a material having high workability is formed on the surface of a highly rigid disk-shaped member, and the surface of the surface layer is flattened. A platen for polishing a wafer, characterized in that: 2. The wafer polishing table according to claim 1, wherein the material having high workability is a synthetic resin material or a synthetic resin material containing a filler. 3. The synthetic resin material is an epoxy resin, a phenol resin, a polyester resin, a polyamide resin or a polycarbonate resin.
The surface plate for wafer polishing as described. 4. A wafer polishing apparatus comprising the wafer polishing surface plate according to claim 1. 5. A wafer polishing method, wherein a wafer surface is polished flat by using the wafer polishing apparatus according to claim 4. 6. A method of manufacturing a surface plate used for polishing a wafer, comprising: forming a surface layer made of a material having high workability on a surface of a highly rigid disk-shaped member; and flattening the surface of the surface layer. A method of manufacturing a surface plate for polishing a wafer, comprising: 7. A processing method for improving the flatness of a surface plate mounted on a wafer polishing apparatus, wherein a surface layer made of a material having high workability is formed on the surface of the surface plate mounted on the polishing device. And flattening the surface of the surface layer. 8. The method according to claim 6, wherein the surface of the surface layer is flattened by flattening means moving in a direction parallel to and perpendicular to the surface of the surface plate. 9. The method according to claim 8, wherein said flattening means is a grinding means.