JP2006192511A - Wave removal polishing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wave removal polishing method effectively smoothing a wave and a surface roughness to a further long waveform without deforming the whole shape of a workpiece surface before machining. <P>SOLUTION: A change rate of a machining amount of a polishing tool in various positions on a workpiece surface per unit time is estimated under machining conditions that the diameter of a polishing tool is large and the machining amount on the workpiece surface is large, which are large merits for wave removal, a machining amount change rate map per unit time of the polishing tool is created, and machining parameters are corrected for uniformizing the machining amounts in the respective positions on the workpiece surface based on the map so as to effectively smooth the wave and the surface roughness to the further long wavelength without deforming the whole shape of the workpiece surface before the machining. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polishing method for precisely polishing (aspherical surface polishing) the surface of a hard and brittle material such as a semiconductor, glass, ceramics, a single metal or a single crystal of a metal oxide.

  Conventionally, synthetic quartz glass, low thermal expansion glass, and CVD-SiC materials are used as lenses and mirrors for high-energy short-wavelength light because of their excellent physicochemical characteristics despite their high cost. As the shape of these lenses and mirrors, there is an increasing demand for an aspherical shape other than a simple shape such as a flat surface or a spherical surface. As a typical structure of these optical elements, a synthetic quartz glass material will be described as an example. The process of making a synthetic quartz glass lens for short wavelength light consists of grinding to a shape close to the final shape and polishing to improve the surface quality by reducing the shape error, waviness, surface roughness, etc. of the grinding surface. .

  For polishing, a polishing tool that performs a constant polishing motion with a small diameter is brought into contact with the workpiece surface parallel to the workpiece surface on a predetermined scanning pattern of the polishing tool on the workpiece surface, and is directed in the direction of the tool axis center of the polishing tool. This is performed by continuously scanning while applying a load and sequentially moving the machining points.

  Here, as shown in FIG. 1 (Patent Document 1), a small polishing tool used for polishing has a convex spherical surface 31d provided on a polisher support member 31a for holding a polisher 31c and a load shaft for applying a processing force. Between the guide portions 35a provided on the polishing tool support member 33 at the end of 39, at least three spheres 35b arranged at intervals are sandwiched, and the center point of the convex spherical surface 31d becomes the polisher support member 31a. The polisher 31c is held so as to coincide with the center point O of the polishing surface. In this way, the posture change due to the generation of a moment accompanying the swing of the polisher 31c (assuming a spherical surface with a radius of curvature at a certain position of the tool on the surface to be processed, the polishing surface of the polishing tool swings along the spherical surface. A configuration has been proposed in which the change in the attitude of the tool in the pitching direction at the time can be ignored in principle.

  During actual polishing, shape measurement is performed on the entire surface of the work piece, and the dwell time distribution of the polishing tool is calculated so that a place with a large amount of error is polished for a long time. In general, scanning is performed so as to realize the predetermined residence time distribution described above, and a polishing liquid in which an abrasive such as cerium oxide, zirconium oxide fine powder, or diamond fine powder is dispersed in water is interposed.

  By this process, as reported in Non-Reference 1, the shape error whose wavelength is sufficiently longer than the diameter of the polishing tool is corrected by controlling the residence time distribution, and conversely from the diameter of the polishing tool. Waves with a sufficiently short wavelength and surface roughness were smoothed by a constant polishing motion of the polishing tool.

JP 07-75952 A Journal of Japan Society for Precision Engineering Vil.62No.3 (1996) P408 412

  However, in this process, waviness having a wavelength in the vicinity of the diameter of the polishing tool cannot be corrected or smoothed, and remains on the processed workpiece surface. Therefore, the target surface of the workpiece is finished to a target accuracy by combining a plurality of processes with different target correction processing or smoothing shape error, waviness, surface roughness wavelength and efficiency. In the process aiming at improving the surface roughness, the undulation and surface roughness can be smoothed more effectively to a longer wavelength without destroying the overall shape of the workpiece surface. It is necessary to do.

  According to Preston's empirical formula, the amount of polishing is proportional to the product of the polishing pressure applied to the workpiece surface by the polishing tool surface and the relative speed. Taking the polishing tool of FIG. 1 as an example, FIG. 2 shows the distribution of polishing pressure, relative speed, and processing amount in a region where the polishing tool and the workpiece surface are in contact with each other. Here, the processing amount per unit time of the polishing tool is proportional to the value obtained by integrating the processing amount distribution in the region where the polishing tool and the workpiece surface are in contact with each other.

  The distribution of the polishing pressure varies depending on the difference between the shape of the polishing tool surface and the shape of the workpiece surface. During actual machining, the shape of the polishing tool surface is constant, but when the shape of the workpiece surface is aspherical, the surface of the workpiece surface where the polishing tool contacts at each position on the workpiece surface is localized. Since the shape changes, the polishing pressure distribution also changes.

  The polishing load is generated by a mechanism such as an air cylinder that generates a constant force and the weight of the polishing tool. When the polishing tool surface is tilted so as to be in parallel with the workpiece surface, the self-weight component acting in the polishing load direction changes and the polishing load changes. Therefore, the distribution of the polishing pressure varies depending on the inclination angle of each position on the workpiece surface.

  The constant polishing movement of the polishing tool is a rotation movement. The distribution of the relative speed during actual machining is constant over the entire surface of the workpiece, and has a distribution that increases linearly toward the periphery with the polishing tool center being zero.

  For this reason, the distribution of the relative speed does not change at each position on the surface of the workpiece, but the distribution of the polishing pressure changes, so that the processing amount per unit time of the polishing tool changes. As a result, there is a problem that the overall shape of the surface of the workpiece before processing is greatly destroyed by the processing of K.

  As the diameter of the polishing tool increases, the effect can be expected when the polishing tool can be smoothed by a constant polishing motion up to a longer wavelength wave.

  Further, it can be expected that the waviness and the surface roughness can be smoothed as the amount of processing on the surface of the workpiece increases.

  However, as a disadvantage, as the diameter of the polishing tool increases, the change rate of the processing amount per unit time of the polishing tool at each position on the surface of the workpiece increases. Further, there is a problem in that the overall shape of the workpiece surface is greatly destroyed as the amount of processing on the workpiece surface increases.

  Therefore, in the present invention, the polishing tool at each position on the workpiece surface has a large merit for removing waviness, the machining tool has a large diameter and a large machining amount on the workpiece surface. By predicting the rate of change of the machining amount per unit time and correcting the machining parameters at each position on the workpiece surface based on the predicted value, the overall shape of the workpiece surface before machining can be obtained. It is an object of the present invention to provide a swell removal polishing method that smoothes swell and surface roughness to a longer wavelength without breaking down and more effectively.

  In order to solve the above-described problems and achieve the object, the waviness removal polishing method according to the present invention performs a constant polishing motion with a small diameter on a scanning pattern of a predetermined polishing tool on the surface of a workpiece. A polishing apparatus for continuously polishing a polishing tool while applying a load in the direction of the center of the tool axis of the polishing tool, and polishing a workpiece while polishing a processing point sequentially. The entire shape of the surface of the workpiece before processing is destroyed by scanning the polishing tool at a constant speed and polishing the entire area on the surface of the workpiece with a uniform processing amount, assuming that the processing amount is constant. In the polishing method aiming at smoothing the undulation and surface roughness, there are great merits for undulation removal, the diameter of the polishing tool is large, and the processing conditions are large on the workpiece surface On the workpiece surface. Predict the rate of change of the processing amount per unit time of the polishing tool, create a processing rate change rate map per unit time of the polishing tool, and based on this map, the processing amount at each position on the workpiece surface By correcting the machining parameters so that they are uniform, the undulation and surface roughness can be smoothed to a longer wavelength and more effectively without breaking the overall shape of the workpiece surface before machining. It is characterized by becoming.

  In the waviness removal polishing method according to the present invention, first, the polishing tool surface is given to the workpiece surface when the polishing tool is brought into contact with each position on the workpiece surface under the same conditions as the actual machining. The distribution of the polishing pressure is obtained from a contact analysis simulation using a general-purpose tool, and then the distribution of the relative speed that the polishing tool surface gives to the workpiece surface is determined from the constant polishing motion of the polishing tool. To obtain the product distribution of the polishing pressure and relative speed proportional to the work volume distribution in the area where the polishing tool surface and the workpiece surface are in contact with each other. By integrating in the region where the tool surface and the workpiece surface contact, the integral value of the distribution of the product of the polishing pressure and the relative speed proportional to the processing amount per unit time of the polishing tool is obtained, and a certain standard condition ( For example, polishing tool surface The workpiece pressure is the integral value of the product of the product of the polishing pressure and the relative velocity of the workpiece surface both flat and the normal vector of the polishing tool surface in the vertical direction). By dividing the integral value of the product distribution of the relative speed, the change rate of the processing amount per unit time of the polishing tool at each position on the workpiece surface is predicted.

  Further, the waviness removal polishing method according to the present invention provides a dwell time on the above-mentioned predicted workpiece surface, which indicates a speed at which the polishing tool is scanned at each position on the workpiece surface that is initially set uniformly. Divide by the rate of change of the processing amount per unit time of the polishing tool at each position, and the processing amount on the workpiece surface indicated by the product of the processing amount per unit time of the polishing tool and the residence time of the polishing tool is The processing parameters at each position on the workpiece surface are corrected by being uniform over the entire surface of the workpiece.

  According to the present invention, the overall shape of the surface of the workpiece before processing is great even under processing conditions in which the diameter of the polishing tool is large and the amount of processing on the surface of the workpiece is large, which is advantageous for removing waviness. By smoothing the swell and the surface roughness to a longer wavelength and more effectively without breaking down, more accurate precision polishing (aspheric surface polishing) is realized.

  First, a polishing apparatus for carrying out the swell removal polishing method of the present invention will be described with reference to FIG. FIG. 3A is a perspective view showing an embodiment of the polishing apparatus of the present invention.

  In FIG. 3A, reference numeral 50 denotes a bed, and a y table 52 that can reciprocate in the y direction relative to the bed 50 is attached on the bed 50. Reference numeral 54 denotes a motor for driving the movement of the y table 52, and an encoder 56 is attached to the motor 54. The encoder 56 detects the amount of movement in the y direction of the y table 52. Mounted on the y table 52 is an x table 58 that can reciprocate relative to the y table 52 in the orthogonal direction. Reference numeral 60 denotes a motor for driving the movement of the x table 58, and an encoder 62 is attached to the motor 60, and the amount of movement in the x direction of the x table 58 is detected by the encoder 62.

  A polishing tank 64 is fixed on the x table 58. A support 66 is fixed in the polishing tank 64, and an object holding body 70 is attached to the support 66 by a shaft 68. The holding body 70 has an L shape, and a shaft 68 is connected to a vertical surface portion thereof. The shaft 68 faces the x-axis direction, and therefore the holding body 70 can be rotated around the x-axis. A motor 72 is attached to the support 66, and its drive rotation shaft is coupled to a shaft 68. An encoder 73 is attached to the motor 72, and the encoder 73 detects an inclination angle (rotation amount) around the x axis. A rotary table 71 is installed on the holding body 70, and the rotary table is rotationally driven and a rotational position is detected by a motor and an encoder (not shown).

  On the other hand, a column 74 is fixed to the x table 58 outside the polishing tank 64. The column 74 is formed with a guide 76 in the vertical direction, that is, in the z direction, and a polishing tool head holding body 78 is attached so as to reciprocate in the vertical direction along the guide 76. A polishing head 80 is supported on the holding body 78. As shown in FIG. 3B, the polishing head 80 holds a rotating shaft 81, and a polishing tool 84 having a circular tool surface that is a small-diameter tool is held at the lower end of the rotating shaft 81. The rotating shaft 81 is rotationally driven by a driving unit (not shown) to rotate the polishing tool 84 held on the rotating shaft 81. The center line of the rotation shaft 81 is defined as a tool axis center line T. In the polishing tool 84, a polishing load obtained by adding the weight of the tool to a load generated by an air cylinder (not shown) or the like is brought into contact with the workpiece 100 via the rotating shaft 81.

  A motor 86 is attached to the holding body 78, and its drive rotation shaft is connected to the polishing head 80, and can drive the rotation of the polishing head 80 about the y-axis. The motor 86 is provided with an encoder 87, and the encoder 87 detects an inclination angle (rotation amount) about the y axis.

  Reference numeral 88 denotes a motor as a driving means for moving the holding body 78 in the vertical direction (Z direction) along the guide 76. The motor 88 is provided with an encoder 89, and the encoder 89 attaches the holding body 78 in the Z direction. The amount of movement is detected.

  When performing polishing using the above-described polishing apparatus, the workpiece 100 is stacked and fixed on the rotary table 71. It is assumed that the workpiece 100 is finished with a predetermined surface roughness and shape accuracy by appropriate pre-processing. An appropriate amount of polishing liquid 102 is injected into the polishing tank 64.

  Reference numeral 92 denotes a control device which receives the y table movement amount and the x table movement amount from the encoders 56 and 62, and the rotation table rotation amount from the encoder that detects the position of the rotation table 71. Signals are output to the motors 54 and 60, the air cylinder 88, and a motor of a rotary table (not shown) so that the tool center moves to each machining point in order. In the present embodiment, the moving direction (scanning direction) of the machining point is made to coincide with the x-axis direction. Further, at each machining point, rotation amounts about the x-axis and y-axis are input by the encoders 73 and 87, and the rotation amounts matching the inclination angle of the polishing tool with respect to the workpiece surface are output to the motors 72 and 86. Then, a polishing head drive motor (not shown) in the polishing head 80 and / or a motor that replaces the movement of the polishing head 80 are driven to swing in the swing direction, and a time tool determined at each processing point according to the execution command program. The entire surface of the workpiece is polished by staying and sequentially scanning to the next processing point.

  Next, a method for calculating an execution command program for driving the polishing apparatus will be described with reference to the flowchart of FIG. Further, FIG. 5 shows details of a method for deriving a change rate map of the machining amount per unit time of the polishing tool at each position on the workpiece surface.

  First, the design shape formula of the workpiece surface, the scanning pattern, the target machining amount of the workpiece surface, and the machining amount per unit time of the polishing tool under the reference conditions are stored in advance in the memory of the control device 92. Keep it.

  The design shape formula of the work surface is called from the storage means, the design shape curved surface of the work surface is calculated, and is stored as (x, y, z) three-dimensional data on the memory. At this time, x and y are the machining positions (feed pitch) of the tool derived from the scanning pattern, and are arranged at regular intervals on the xy plane. In addition, the processing amount when polishing the entire surface of the workpiece with a uniform processing amount, and the condition used as a reference for determining the rate of change of the processing amount per unit time of the polishing tool (for example, the polishing tool surface and Calculate and store the dwell time representing the moving speed of the tool at each machining point from the machining amount per unit time of the polishing tool when the workpiece surfaces are both flat and the normal vector of the polishing tool surface is vertical. It is memorized in the means.

  Next, a change rate map of the machining amount per unit time of the polishing tool at each position on the workpiece surface is derived.

  Using a general-purpose simulation tool capable of performing contact analysis, a simulation model including only the polishing tool 201 including only the polisher support member 201a, the elastic body 201b, and the polisher 201c illustrated in FIG. This model shows the workpiece surface in the area where the polishing load and the polishing tool come into contact when the polishing tool is brought into contact with each processing point on the workpiece surface by changing the position and posture of the polishing tool. From this shape, the pressure distribution when the polishing tool is brought into contact with the workpiece surface can be output as the distribution of the polishing pressure applied to the workpiece surface by the polishing tool.

  Using this model, the distribution of the polishing pressure at each processing point is obtained. From the constant polishing motion of the polishing tool, the distribution of the relative speed that the polishing tool gives to the workpiece surface is determined. The distribution of the product of the polishing pressure and the relative speed, which is proportional to the distribution of the processing amount in the region where the polishing tool is in contact with the workpiece surface by multiplying the distribution of the polishing pressure at each processing point by the distribution of the relative speed. Ask for. The distribution of the product of the polishing pressure and the relative speed at each processing point is integrated in a region where the polishing tool contacts the workpiece surface, and is proportional to the processing amount per unit time of the polishing tool; Find the integral value of the product distribution of relative speed. The integrated value of the distribution of the product of the polishing pressure and the relative speed at each processing point is a condition based on the above-mentioned reference (for example, the polishing tool surface and the workpiece surface are both flat, and the normal vector of the polishing tool surface Is divided by the value in the vertical direction) to derive a change rate map of the machining amount per unit time of the polishing tool at each position on the surface of the workpiece.

  The dwell time indicating the moving speed of the tool at each machining point is called from the storage means on the memory of the control device 92, and the rate of change of the machining amount per unit time of the polishing tool at each position on the workpiece surface at the machining point is obtained. Called from the map.

  The dwell time called for each processing point is divided by the change rate of the processing amount per unit time of the polishing tool at each position on the surface of the workpiece, and stored again in the storage means.

  Finally, set the speed of the tool from the scanning pattern, 3D data of each machining point, and the dwell time at each machining point stored in the storage means of the control device, and create an execution command program to control the machining device during machining By driving the above-described polishing apparatus according to this execution command program, the waviness and surface roughness can be increased to a longer wavelength and more effective without destroying the overall shape of the workpiece surface before processing. Realize high-precision precision polishing (non-spherical surface polishing) that smoothes the surface.

  Examples of the present invention are shown below.

  Axisymmetric aspherical shape, when the machining amount per unit time of the polishing tool at each machining point in the past is constant, and the rate of change of the machining amount per unit time of the polishing tool according to the present invention is predicted to stay The case where correction is added to time is shown in comparison.

  The target optical element has a light beam effective diameter of 160 mm, a concave aspheric surface, and the aspheric amount is about 1.8 mm.

  The diameter of the polishing tool is φ25 mm, the constant polishing motion of the polishing tool is a rotation motion, the speed is 20 Hz, and the polishing load is 750 gf.

  As a result of the calculation, the rate of change of the processing amount per unit time of the polishing tool is 1 at the maximum (the polishing tool surface and the workpiece surface are both flat and the normal vector of the polishing tool surface is vertical). It was 32 times. FIG. 7 shows a change rate map of the machining amount per unit time of the polishing tool at each machining position in the radial direction.

  FIG. 8 shows the difference in shape of the workpiece surface before and after processing when the processing amount per unit time of the polishing tool at each conventional processing point is constant. FIG. 8 shows the change rate of the processing amount per unit time of the polishing tool according to the present invention. FIG. 9 shows a difference shape on the surface of the workpiece before and after machining when the dwell time is predicted and the dwell time is corrected.

  When the overall shape before processing is not destroyed at all, the amount of processing at each position on the surface of the workpiece is uniform, so the difference shape is preferably closer to a plane.

  As a result of comparing both, the difference shape of the workpiece surface before and after machining in the conventional method of FIG. 8 has a profile very similar to the change rate map of the machining amount per unit time of the polishing tool shown in FIG. It became a convex shape. The difference between the maximum value and the minimum value of the processing amount at each position on the workpiece surface is as large as about 420 nm.

  On the other hand, the difference shape of the workpiece surface before and after machining by the method of the present invention in FIG. 9 is similar to the change rate map of the machining amount per unit time of the polishing tool shown in FIG. It can be confirmed that the difference between the maximum value and the minimum value of the processing amount at each position on the surface of the workpiece is about 200 nm, which is smaller than the conventional method.

  It was also confirmed that the method according to the present invention exhibited the same smoothing effect as the conventional method with respect to the waviness and surface roughness of the workpiece surface.

It is a block diagram of a polishing tool. It is a related figure of grinding pressure, relative speed, and processing amount. It is a perspective view of a polish device. It is a figure which shows the calculation method of an execution command program. It is a figure which shows the derivation | leading-out method of the change rate map of the processing amount per unit time of an abrasive tool. It is a figure which shows the simulation model of an abrasive tool. It is a change rate map of the processing amount per unit time of an abrasive tool. It is a figure which shows the shape collapse of the conventional method. It is a figure which shows the shape collapse of the method of this invention.

Explanation of symbols

50 bed 52 y table 56 encoder 58 x table 60 motor 62 encoder 64 polishing tank 66 support 70 holding body 71 rotating table 80 polishing head 81 rotating shaft 84 polishing tool 87 encoder 88 motor

Claims (3)

A polishing tool that performs a constant polishing motion with a small diameter on the workpiece surface on a predetermined polishing tool scanning pattern is continuously scanned while applying a load in the direction of the center of the tool axis of the polishing tool. A polishing device that polishes a workpiece while polishing a processing point, and the entire amount on the surface of the workpiece is scanned by scanning the polishing tool at a constant speed with a constant processing amount of the polishing tool per unit time. In the polishing method aimed at smoothing the undulation and surface roughness without breaking the overall shape of the workpiece surface before processing, by polishing with a uniform processing amount,
Predict the rate of change of the machining amount per unit time of the polishing tool at each position on the workpiece surface under machining conditions where the diameter of the abrasive tool is large and the machining amount on the workpiece surface is large. By correcting the processing parameters at each position on the workpiece surface based on the above, undulation, surface roughness to a longer wavelength without breaking the overall shape of the workpiece surface before processing, and A waviness removal polishing method characterized by smoothing more effectively.   The rate of change of the processing amount per unit time of the polishing tool at each position on the workpiece surface is contacted with the workpiece surface at each position on the workpiece surface under the same conditions as actual machining. 2. The undulation removal polishing method according to claim 1, wherein the polishing tool surface is predicted from the distribution of polishing pressure applied to the surface of the workpiece when the polishing tool surface is applied.   The correction of the processing parameters at each position on the workpiece surface is performed by changing the scanning speed of the polishing tool based on the predicted change rate of the processing amount per unit time of the polishing tool. The swell removal polishing method according to claim 1.

Images