JP6232172B2 - Convergent polishing method and convergent polishing system - Google Patents

Description

Cross-reference of related applications

  [0001] This application claims priority to US Provisional Patent Application No. 61 / 454,893, filed March 21, 1011, the entire disclosure of which is hereby incorporated by reference for all purposes. Included in the book.

Statement of invention rights made under government-backed research and development

  [0002] The United States government has rights to the invention in accordance with contract number DE-AC52-07NA27344 between the US Department of Energy and Lawrence Livermo National Security (LLC), which operates Lawrence Livermore National Laboratory.

  [0003] Conventional optical element manufacturing processes typically include 1) molding, 2) grinding, 3) edge grinding / polishing, 4) full aperture intermediate polishing or lapping, 4) full aperture finish polishing, and 5) small tool polishing. including. Since the demand for high optical quality materials with accurate surface shapes has increased dramatically and lasers have been invented, the accuracy of optical element manufacturing has made great progress. However, as is commonly done today, optical element manufacturing is an art rather than a science and technology. The skill of deterministically removing material during the grinding process has made great progress in recent decades based on advances in tools and real-time diagnostics using computer numerical control (CNC) grinding machines. Similarly, the advent of small tool polishing (eg, computer controlled optical polishing machine (CCOS)) and magnetorheological finishing (MRF) has transformed the polishing industry. However, full aperture lapping, which is still the most commonly used method for polishing glass and silicon wafers, and usually the most economical method, does not involve a deterministic process. Although incremental improvements have been made, conventional polishing still requires skilled optical technicians who can master craftsmanship. This type of polishing typically requires multiple iterative cycles, including polishing, measurement, and adjustment of parameters to converge to the desired surface shape (ie flatness or fit to a specific radius). To do. Therefore, there is a need in the art for improved methods and systems related to polishing optical elements.

  [0004] According to the present invention, techniques relating to optical systems are provided. More particularly, embodiments of the present invention relate to deterministic polishing of optical elements performed in only one iteration. By way of example only, the present invention has been applied to a single iterative polishing of an optical element under a fixed polishing parameter set regardless of the initial shape of the optical element. The methods and systems described herein are applicable to the processing and manufacturing of a wide variety of optical materials suitable for use with high power laser systems and amplifier systems.

  [0005] According to other embodiments of the present invention, methods and systems are provided for a "no" surface finish. Scratches on the surface of expensive optical elements used in high peak power laser systems are known to cause laser damage. Therefore, over the years, great efforts have been made to reduce the number and size of scratches formed during optical element manufacture. Scratches are caused, for example, by sliding rogue particles and ridges loaded on the surface of the optical element during cleaning, polishing and handling. Prior work to reduce rogue particles has modified and improved existing full aperture polishing. This method was sufficient to deal with external rogue particles that caused scratches. Current mitigation strategies are exclusively limited to performing cleansing during polishing. However, this strategy is extremely sensitive to the degree of cleansing performance, the operator skill set, and the complexity of the polisher. In addition, these strategies are difficult to implement efficiently. A second limitation in controlling rogue particles is that there is a limit to grasping and controlling the particle size distribution of the slurry during the polishing process. The state of the art is not optimized and is limited to the use of filtration techniques with many unclear points.

  [0006] Embodiments of the present invention suppress the uptake of rogue particles during polishing by performing one or more of the following steps. 1) creating a full polishing system that ensures that the rogue particles do not enter the workpiece / lap interface by hermetically sealing the polisher to prevent slurry drying; 2) may act as rogue particles; Providing a 100% humidity environment to prevent dry slurry agglomeration; 3) using a chemically stable abrasive slurry to minimize agglomeration of slurry particles inside; and 4) Utilizing an optimized filtration method that constantly removes any generated rogue particles within the polishing system.

  [0007] According to an embodiment of the present invention, a polishing system for polishing an optical element is provided. The polishing system includes a polishing pad having a radial dimension and a septum disposed on the polishing pad and configured to partially surround the optical element. The optical element contacts the polishing pad over a range of radial dimensions, and the pad wear rate of the polishing pad is substantially constant as a function of the radial dimension over a range of radial dimensions.

  [0008] According to another embodiment of the invention, a high humidity polishing system is provided. The high humidity polishing system includes a polishing unit that includes a polishing pad and a slurry supply system that is operable to supply slurry to the polishing pad. The high humidity polishing system also includes an enclosure that surrounds the polishing unit. The humidity within the enclosure is sufficient to prevent substantial drying of the slurry.

  [0009] According to certain embodiments of the invention, a slurry system for polishing an optical element is provided. The slurry system includes a solvent and an abrasive component supported in the solvent. The slurry system also includes a surfactant supported in the solvent.

  [0010] According to certain embodiments of the invention, a method for attaching a workpiece to a substrate is provided. The method includes determining a PV value and determining a value associated with the pitch area. The method further includes calculating a relative area of the pitch, calculating a button radius, and calculating a number of pitch buttons. The method further includes coupling N pitch buttons to the workpiece and coupling the N pitch buttons to the substrate.

[0011] Embodiments of the present invention provide an apparatus and method for polishing flat, spherical and rectangular surfaces on glass of various aspect ratios (diameter / thickness). The polishing system provided by embodiments of the present invention is also referred to as a convergent, initial surface independent, single-interaction, logue-particulate free (CISR) polisher. And provide one or more of the following features.
1) The polishing parameters are fixed (ie not variable) and the same polishing continues during / between polishing regardless of the initial surface shape of the workpiece;
2) Since the shape of the workpiece will converge to the desired shape that matches the shape of the lap, polishing can be performed in one iteration from the ground state; and 3) Polishing on the workpiece In an environment free of rogue particles with little or no scratching.
4) Polishing is performed using a highly controlled particle size distribution utilizing chemical stabilization and / or engineering filtration systems.

[0012] Embodiments of the present invention achieve a desired polishing process based on one or more of the following principles.
1) Basically, due to non-uniform spatial material removal on the optical element, other than a mismatch between the optical element and the wrap (ie, non-uniform physical separation between the optical element and the wrap) The surface shape of the optical element is converged to the desired shape (i.e., the shape of the wrap) by eliminating all the following elements; and 2) Rogue particle penetration into the grinder system and rogue within the grinder system By eliminating or aggressively removing the cause of particle formation, there is little or no scratching on the workpiece.

[0013] As described in greater detail throughout this specification, the engineering features provided by embodiments of the present invention may include one or more of the following.
1) Use a septum specifically designed for round and rectangular workpieces to prevent uneven pad wear;
2) Use specially designed septa to prevent uneven stress distribution and uneven material removal caused by viscoelasticity;
3) Use a specially designed septum to ensure a uniform distribution of slurry on the workpiece;
4) Use glass based rectangular septa to provide stability in uniform pad wear;
5) Use a CVO diamond-based rectangular septum to condition the pad to ensure a constant material removal rate with polishing time;
6) Use a wheel driven workpiece to prevent moment force from causing uneven stress distribution and uneven material removal;
7) Utilize low-z pivot point mounting to minimize moment force on the workpiece and septum;
8) Utilizing kinematics (workpiece and lap motion) to equalize the time average speed on the workpiece to prevent uneven material removal due to kinematics;
9) Using a chemically stable polishing slurry (eg, an anionic surfactant (eg, micro-90) or a cationic surfactant and a chelating agent at an appropriate pH and concentration in a cerium oxide (Hastite PO) slurry) To minimize particle agglomeration (causing rogue particles);
10) Use a 100% humidity hermetically sealed polishing chamber to a) prevent the formation of dry polishing slurries known to cause rogue particles that cause scratches, and b) be present in the environment Prevent external rogue particles from entering the polishing system;
11) Use strong button bonding technology (also called pitch button bonding (PBB)) to prevent workpiece deformation in high aspect ratio (thin) workpieces / optical elements;
12) Utilizing compliant button bonding technology (also referred to as foam button bonding (FBB)) to prevent residual stress from the grinding surface from causing uneven removal and workpiece bending;
13) A pre-etch technique (eg, HF or buffered oxide etch) is utilized on the ground workpiece to remove residual stresses that can cause non-uniform removal and workpiece bending;
14) A pre-etch technique (eg, HF or buffered oxide etch) is utilized on the ground workpiece to remove the source of glass rogue particles on the grinding surface that can cause scratches;
15) Using a fluorinated coating on the inside of the polisher housing and parts to facilitate cleaning of the slurry during repeated polishing and to minimize the generation of rogue particles, the adhesion of the slurry particles Reduce;
16) Use a grinder design that minimizes corners and cracks that collect slurry particles to minimize the occurrence of rogue particles; and / or 17) Efficiently remove rogue particles; And an active slurry filtration system that controls the particle size distribution of the slurry.

(Item 1)
A polishing system for polishing an optical element,
A polishing pad having a radial dimension;
A septum disposed on the polishing pad and configured to receive a septum load (σs) and partially surround the optical element;
The optical element is in contact with the polishing pad over a range of the radial dimension and receives an optical element load (σo), and the optical element load (σo) can be different from the septum load (σs). A polishing system wherein the pad wear rate of the polishing pad is substantially constant as a function of radial dimension over the range of radial dimensions.
(Item 2)
Item 2. The polishing system of item 1, wherein the optical element comprises a circular lens.
(Item 3)
The polishing system according to item 1, wherein the septum includes a structural layer, a compliance layer, and a polishing layer.
(Item 4)
Item 4. The polishing system of item 3, wherein the structural layer is characterized by a higher density than the compliance layer or the polishing layer.
(Item 5)
4. The polishing system according to item 3, wherein the optical element is made of an optical material, and the polishing layer is made of the optical material.
(Item 6)
6. A polishing system according to item 5, wherein the optical material includes fused silica.
(Item 7)
The polishing system of item 1, wherein the polishing pad is operable to receive a slurry comprising an abrasive component and an additive carried in a solvent.
(Item 8)
8. The polishing system according to item 7, wherein the additive includes a surfactant.
(Item 9)
Item 9. The polishing system according to Item 8, wherein the surfactant includes an anionic surfactant or a cationic surfactant.
(Item 10)
The polishing system of item 1, further comprising a chamber surrounding the polishing pad and characterized by a humidity higher than ambient humidity.
(Item 11)
Item 11. The polishing system according to Item 10, wherein the humidity is substantially 100%.
(Item 12)
Item 2. The polishing system of item 1, wherein the optical element has a PV value that stabilizes at a fixed value after a predetermined polishing time.
(Item 13)
The polishing system of item 1, wherein all portions of the septum disposed on the polishing pad are characterized by a flat shape.
(Item 14)
A polishing unit including a polishing pad;
A slurry supply system operable to supply slurry to the polishing pad;
An enclosure surrounding the polishing unit;
A high humidity polishing system wherein the humidity in the enclosure is sufficient to prevent substantial drying of the slurry.
(Item 15)
Item 15. The high-humidity polishing system of item 14, wherein the humidity is higher than ambient humidity.
(Item 16)
Item 16. The high-humidity polishing system according to Item 15, wherein the humidity is substantially 100%.
(Item 17)
Item 15. The high-humidity polishing system according to Item 14, wherein the slurry includes a solvent, an abrasive component supported in the solvent, and an additive supported in the solvent.
(Item 18)
The polishing unit further comprising a septum disposed adjacent to the polishing pad and configured to partially surround an optical element in contact with the polishing pad over a range of radial dimensions of the polishing pad; Item 15. The high humidity polishing system of item 14, wherein the pad wear rate of the polishing pad is substantially constant as a function of radial dimension over the range of the radial dimension.
(Item 19)
Item 19. The high-humidity polishing system according to Item 18, wherein the slurry includes a solvent, an abrasive component supported in the solvent, and a surfactant supported in the solvent.
(Item 20)
A solvent,
An abrasive component carried in the solvent;
A slurry system for polishing an optical element comprising a surfactant carried in the solvent.
(Item 21)
Item 21. The slurry system of item 20, wherein the solvent comprises water.
(Item 22)
Item 21. The slurry system of item 20, wherein the abrasive component comprises at least one of cerium oxide and Hastilite PO.
(Item 23)
Item 21. The slurry system of item 20, wherein the surfactant comprises an anionic surfactant.
(Item 24)
24. The slurry system of item 23, wherein the anionic surfactant comprises at least one of μ-90 and ammonium lauryl sulfate.
(Item 25)
Item 21. The slurry system of item 20, wherein the surfactant comprises a cationic surfactant.
(Item 26)
21. The slurry system of item 20, further comprising a chelating agent.
(Item 27)
A method for attaching a workpiece to a substrate, comprising:
Determining a PV value;
Determining a value related to the pitch area;
Calculating the relative area of the pitch;
Calculating the button radius;
Calculating the number of pitch buttons;
Coupling N pitch buttons to the workpiece;
Coupling the N pitch buttons to the substrate.
(Item 28)
28. A method according to item 27, wherein the PV value is based on measurement of a PV value associated with a solid pitch layer.
(Item 29)
28. A method according to item 27, wherein a relative area of the pitch is equal to a value obtained by dividing the PV value by a value related to the pitch area.
(Item 30)
28. A method according to item 27, wherein a distance between the N pitch buttons is substantially uniform.
(Item 31)
28. The method of item 27, wherein coupling the N pitch buttons to the workpiece comprises applying the N pitch buttons to a tape layer coupled to the workpiece.
(Item 32)
28. A method according to item 27, wherein the workpiece includes an optical element and the substrate includes an optical flat.
[0014] Many advantages are achieved by the present invention compared to the prior art. For example, embodiments of the present invention provide methods and systems suitable for polishing an optical element to a predetermined shape in a single iteration. These and other embodiments of the present invention, along with many of its advantages and features, will be described in more detail in conjunction with the following text and accompanying drawings.

It is a table | surface which shows the parameter which affects the polishing nonuniformity based on one Embodiment of this invention. (A) is a simplified plot showing the PV value (peak-to-valley height) as a function of polishing time for different polishing configurations according to one embodiment of the present invention; 1 shows a polishing configuration according to one embodiment of the invention, (C) shows another polishing configuration according to one embodiment of the present invention, and (D) shows yet another polishing configuration according to one embodiment of the present invention. (E) shows yet another polishing configuration according to an embodiment of the present invention, (F) shows yet another polishing configuration according to an embodiment of the present invention, and (G) 6 shows yet another polishing configuration according to an embodiment of the present invention. 4 is a simplified plot showing PV value as a function of polishing time for a polishing configuration, according to one embodiment of the present invention. 6 is a simplified plot showing polishing convergence (convergence) for a certain initial shape according to an embodiment of the present invention. 6 is a simplified plot showing polishing convergence for another initial shape, according to another embodiment of the present invention. 6 is a simplified plot showing polishing convergence for yet another initial shape, according to yet another embodiment of the present invention. 6 is a simplified plot showing polishing convergence for yet another initial shape, according to yet another embodiment of the present invention. (A) is a surface contour showing a PV value at a certain polishing time according to an embodiment of the present invention, and (B) is a PV value at another polishing time according to an embodiment of the present invention. (C) is a surface contour showing a PV value at yet another polishing time according to an embodiment of the present invention, and (D) is according to an embodiment of the present invention. The surface contour showing the PV value at yet another polishing time, and (E) is the surface contour showing the PV value at yet another polishing time according to an embodiment of the present invention. (A) is a simplified perspective view of a polishing septum according to an embodiment of the present invention, and (B) is a simplified cross-sectional view of a polishing septum according to an embodiment of the present invention. 6 is a simplified plot of pad wear rate as a function of radial distance, according to one embodiment of the present invention. 6 is a simplified plot of septum width as a function of radial distance, according to one embodiment of the present invention. (A) is a simplified plot of septum load as a function of radial distance according to one embodiment of the present invention, and (B) is a function of radial distance according to one embodiment of the present invention. FIG. 4C is a simplified plot of septum load, (C) is a simplified plot of septum load as a function of radial distance, according to one embodiment of the present invention. (A) is a simplified schematic diagram showing scratching by rogue particles, and (B) is an image showing scratching by rogue particles. 1 is a simplified perspective view of a high humidity polishing system according to an embodiment of the present invention. FIG. 5 is a simplified plan view of a part of a high-humidity polishing system according to another embodiment of the present invention. FIG. 6 shows a plot of normalized interface height as a function of time for various dilutions of polishing liquid according to an embodiment of the present invention. FIG. 6 shows a plot of normalized interface height for a polishing liquid under the influence of agitation, according to one embodiment of the present invention. FIG. 6 shows a plot of normalized interface height as a function of time for a plurality of polishing liquids according to an embodiment of the present invention. 4 is a simplified plot of relative interface height as a function of time for a stabilized polishing liquid and an unstabilized polishing liquid, according to one embodiment of the present invention. FIG. 4 is the amount of polishing liquid as a function of particle size for a stabilized polishing liquid and an unstabilized polishing liquid according to an embodiment of the present invention. 6 is a simplified flowchart illustrating a method of polishing a set of optical elements according to an embodiment of the present invention. It is an image which shows the surface curvature before grinding based on one Embodiment of this invention. It is an image which shows the surface curvature after grinding based on one Embodiment of this invention. It is an image which shows the surface curvature after chemical etching based on one Embodiment of this invention. (A) is a simplified schematic diagram showing a method for performing pitch button bonding according to an embodiment of the present invention, and (B) is for performing pitch button bonding according to an embodiment of the present invention. (C) is a simplified schematic diagram showing a method for performing pitch button bonding according to an embodiment of the present invention, and (D) is an implementation of the present invention. (E) is a simplified schematic diagram showing a method for performing pitch button bonding according to an embodiment of the present invention, (F) is a simplified schematic diagram illustrating a method for performing pitch button bonding according to an embodiment of the present invention. 2 is a plot showing the measured surface shape change of fused silica and phosphate glass in various PPB configurations according to one embodiment of the present invention. 2 is a plot of measured thermal expansion of pitch, in accordance with one embodiment of the present invention. (A) is a plot of workpiece PV value as a function of pitch subcooling for one button and three buttons, according to one embodiment of the present invention, and (B) is the present invention. Is a plot of workpiece PV value as a function of pitch button radius, according to one embodiment, wherein (C) is a normalized workpiece PV value as a function of pitch button offset, according to one embodiment of the present invention. (D) is a plot of the workpiece PV value as a function of the relative total pitch button area, according to one embodiment of the present invention. FIG. 6 is a simplified schematic diagram showing pitch button bonding parameters according to an embodiment of the present invention. (A) is a diagram showing an optimized pitch button bonding pattern for an optical element according to an embodiment of the present invention, and (B) is an optimization for an optical element according to an embodiment of the present invention. It is a figure which shows the made pitch button bonding pattern. (A) is a plot of the workpiece PV value as a function of the spacing between buttons, according to one embodiment of the present invention, and (B) as a function of area ratio, according to one embodiment of the present invention. Is a plot of the workpiece PV value. It is a simple flowchart which shows the determination method of the pitch button bonding parameter based on one Embodiment of this invention.

  [0049] FIG. 1 is a table showing parameters affecting polishing unevenness according to one embodiment of the present invention. Embodiments of the present invention provide techniques and systems for reducing polishing unevenness. The control of some of the parameters shown in FIG. 1 is described in more detail in US patent application Ser. No. 12,695 / 986, filed Jan. 28, 2010, the entire disclosure of which is incorporated herein by reference. This is incorporated herein. As described in further detail throughout this application, embodiments of the present invention use a mirror septum to provide an elastic lap response (4.2) and viscoelastic effects (4). .5) and provide constant lap wear (4.6.1). The optical element shape is then used to converge the polishing process (4.6.4).

  [0050] As shown in FIG. 1, various parameters except for one variable element are reduced or eliminated, which then causes the optical element to converge to a desired shape in a full aperture polishing system. use. This is in sharp contrast to previous techniques in which the optical engineer changed various parameters simultaneously in real time to produce the desired shape. Since the inventors have determined and reduced or eliminated the parameters affecting the material removal shown in FIG. 1, only one variable, the mismatch between the optical element and the wrap, remains. The optical element is then polished in a manner that converges to match the shape of the wrap using a wrap having the final desired shape of the optical element.

  [0051] FIG. 2-1 (A) is a simplified plot showing PV values (peak-to-valley height) as a function of polishing time for different polishing configurations, according to one embodiment of the present invention. . As shown in FIG. 2-1 (A), a plot of the PV value of the optical element as a function of polishing time is shown for various polishing configurations. As shown in FIG. 2A, since the present inventors have dealt with the parameters shown in FIG. 1, the optical element is polished in a state where the PV value as a function of the polishing time is hardly changed from a state where it is hardly changed. be able to. As shown by the “pad wear reduction” curve (No. 5), we change the shape of the pad during polishing so that the shape changes the pressure distribution on the optical element as it is polished. I found out. Thus, embodiments of the present invention use a septum to contact the wrap and reduce pad wear by achieving essentially the opposite (ie, spatially opposite) wear caused by the optical element. Compensates for spatially uniform pad wear. Thus, embodiments of the present invention reduce the effect of viscoelasticity, as shown in curve # 6, resulting in a nearly constant PV value as a function of time.

  [0052] FIGS. 2-1 (B) to 2-1 (G) illustrate different polishing configurations according to one embodiment of the present invention. The parameters shown in FIGS. 2-1 (B) to 2-1 (G) correspond to the curve numbers in FIG. 2-1 (A).

  [0053] Although the present inventors have shown a stable PV value with plot number 6 (reduced viscoelasticity (77)) in Fig. 2-1 (A), the PV value is higher when 100 hours are exceeded. It has been found that the PV value as a function of time can rise, rising to a higher level. FIG. 2-2 is a simplified plot showing the PV value as a function of polishing time for a polishing configuration, according to one embodiment of the present invention. As shown in FIG. 2-2, the convergence point moves from 0 hour to about 150 hours. While embodiments of the present invention are not limited, we have found that one mechanism that can potentially cause convergence point movement is the balance between pad wear due to optical elements and pad wear due to septum. It is considered that an increase in PV value is caused by the disturbance of.

  [0054] Embodiments of the present invention adjust the convergence point by removing the optical element being polished and operating the polishing system using only the septum. By operating the system with only the septum, the convergence point is adjusted to return the PV value to a value smaller than a predetermined level. This is because the movement of the convergence point occurs over a longer period than that associated with the polishing operation for a single optical element. As an example, the polishing time for an optical element can be 10 hours, while simultaneously increasing the number of optical elements (eg, 15 optical elements) until the PV value increases to greater than 2.5 μm. Polishing within about 150 hours shown in 2-2. Therefore, the adjustment of the convergence point is performed over a long period of time as compared with a general period used for polishing a single optical element. Although the embodiment of the present invention is not limited to such adjustment of the convergence point over a long period of time, the following example is presented from the viewpoint of the period accompanying the polishing process for a plurality of optical elements.

  [0055] Referring to FIG. 2-2, the convergence point initially moved from about 0.5 μm to about 2.5 μm during the first 150 hours of polishing. In order to reduce the value of the convergence point, embodiments of the present invention remove the optical element to operate the polishing system for a while using only the septum to reduce the PV value to a predetermined level. As shown in FIG. 2-2, indicated by the PV value falling to 0.48 mm by running the polishing system using only 0.6 psi septum in the period from about 200 hours to about 400 hours. As a result, the convergence point decreases. As shown in the figure, the rate of decrease in PV value is relatively slow (eg, about 1.7 μm / 212 hours = 7.3 nm / hour) but is controllable and nearly linear. For periods after about 400 hours, both septa and optical elements are used. Thereby, a new drift occurs at the convergence point between about 400 hours and about 500 hours. As indicated by the last few data points, polishing using only the septum after about 500 hours reduces the convergence point as expected. Therefore, in the embodiment, a process of polishing using only a septum (for example, a 0.6 psi septum) without using an optical element is used as a method for finely adjusting the convergence point. Those skilled in the art will recognize many variations, modifications, and alternatives.

  [0056] Convergent pad polishing is published in March 2012, International Journal of Applied Glass Science, Special Issue: Part 1, The Flow and Fluctuation of Advanced 14 It is further described in “Convergent Pad Polishing of Amorphous Silica” by Tayab Suratwala, Rusty Steele, Michael Feit, Richard Desjardin and DanMason, described on page 28, for all purposes, To incorporate the entire contents herein.

  [0057] FIG. 17 is a simplified flowchart illustrating a method of polishing a set of optical elements according to an embodiment of the present invention. The method 1700 includes polishing a first subset of optical elements using a septum and a polishing process characterized by a PV value less than a first predetermined value (1710). The method also includes determining whether the PV value is greater than or equal to a first predetermined value (1712). Once the PV value reaches the first predetermined value, the last optical element of the first subset of optical elements is removed and the polishing system is operated for a period of time without the optical elements (1714). After that period, it is determined whether the PV value has decreased to less than a second predetermined value (1716). In some embodiments, the second predetermined value is less than the first predetermined value. In other embodiments, the second predetermined value is equal to the first predetermined value.

  [0058] In some embodiments, the operation of the polishing system during adjustment of the convergence point uses a septum without an optical element or one of the optical elements (as described in other embodiments below). Or use a device or optical element other than a septum. Therefore, shapes other than the septum for adjusting the convergence point are included in the scope of the present invention.

  [0059] Once the PV value reaches a second predetermined value, a septum is used to polish a second subset of optical elements by a polishing process characterized by a PV value less than the first predetermined value (1718). ).

  [0060] In other embodiments in which the PV value increases in a negative direction as a function of polishing time, the septum is removed and one optical element or dummy optical element in the set of optical elements without using a septum The embodiment shown in FIG. 17 can be modified to change the process 1714 by polishing. Therefore, this complementary correction method can be used to adjust the PV value that increases in the negative direction.

  [0061] FIG. 3A is a simplified plot showing polishing convergence when the initial shape is changed, according to one embodiment of the invention. In FIG. 3A, the optical element is a low aspect ratio circular workpiece. As shown in FIG. 3A, the PV value (unevenness difference) in the workpiece (ie, the optical element) is about 7 μm (ie, about 14 waves) for Experiment 79, and about −1 μm (ie, 2 waves). For experiment 80, the original PV value of about −7 μm drops to about −1.5 μm. Therefore, the embodiment of the present invention provides a convergent polishing technique that is optical polishing that converges to a uniform smoothness regardless of the difference in the original shape of the optical element. The convergence band here shown in FIG. 3A is characterized by a negative PV value with a width of less than about 0.5 μm, but the present invention is not limited to this particular PV value, rather than centering on other zeros. Narrow PV value bands are also included within the scope of the present invention.

  [0062] Initial-surface-independent polishing process performed in one iteration by performing shape convergence using mismatches between the wrap and optical element shape using embodiments of the present invention. process). During polishing, the optical element converges to the same shape as the wrap due to the normalization of the mismatch between the pressure optic and the wrap. Accordingly, embodiments of the present invention allow the polishing process to converge to a band while maintaining the PV value in a band characterized by a predetermined PV value for an extended period of time.

  [0063] One of the advantages provided by the present invention is that the convergent polishing process ends at a constant PV value and stays at this convergence value for a long time. In contrast to conventional polishing techniques where polishing must be completed in a precise time to prevent overpolishing, convergent polishing techniques are self-terminating in a single iteration and are free of wraps that are independent of the initial surface of the optical element. Provide a desired shape based on the shape.

  [0064] FIG. 3B is a simplified plot showing polishing convergence for a low aspect ratio circular optical element, according to one embodiment of the invention. The polishing pad used in the polishing process shown in FIG. 3B (ie, the Dow Chemical Company IC1000 ™ polishing pad in Midland, Michigan) is the polishing pad used in the polishing process shown in FIG. 3A (ie, Suba from Dow Chemical Company). (Trademark) 550 polishing pad). FIG. 3C is a simplified plot showing polishing convergence for a square optical element, according to one embodiment of the present invention. FIG. 3D is a simplified plot showing polishing convergence for a high aspect ratio circular optical element, according to one embodiment of the present invention.

  [0065] As shown in FIGS. 3A-3D, in workpieces with different initial surface shapes and in four different configurations, the PV value converges to a predetermined band as a function of polishing time. As an example, in FIG. 3C, a low aspect ratio square workpiece is polished using an IC1000 ™ polishing pad, and in FIG. 3D, a high aspect ratio (ground or polished) circular workpiece is IC1000 ™. ) Polished using a polishing pad. In the polishing operations shown in these figures, a plurality of workpieces characterized by changing the initial surface shape are similarly polished while converging all the workpieces into a final, nominally flat shape. Convergent full aperture polishing was demonstrated.

  [0066] FIGS. 4A through 4E are surface contours showing PV values at various polishing times, according to one embodiment of the present invention. The original surface is shown in 4A, and the PV value is 6.5 μm. The surface after polishing for 1 hour is shown in FIG. 4B, where PV = 4.64 μm. The subsequent polishing time is shown in FIG. 4 (C) to FIG. 4 (E). After polishing for 2 hours, PV = 3.59 μm (FIG. 4 (C)), and after polishing for 6 hours, PV = −1. 0.04 μm (FIG. 4D), and PV = −0.95 μm after polishing for 24 hours (FIG. 4E). As shown in FIGS. 4D and 4E, the convergent polishing process ends with a fixed PV value after a predetermined period.

  [0067] FIG. 5A is a simplified perspective view of a polishing septum according to one embodiment of the present invention. The septum 500 in the embodiment shown in FIG. 5A includes a curve 510 shaped to receive a circular optical element. In other embodiments, the septum can be a sacrificial workpiece that also causes pad wear that counteracts the spatially non-uniform pad wear caused by the optical element being polished, such as a square optical element or a rectangular optical element. Modifications are made to accept optical elements that may be referred to as workpieces having different shapes. The septum 500 includes a structural material 520 formed from a laminated material such as that shown in FIG. 5B, for example, 25 mm stainless steel or other material with sufficient stiffness and density, and a 3 mm rubber or other A compliance layer 522 formed from a compliant material and a polishing layer 524 formed from, for example, 1.1 mm fused silica or other material equivalent to the optical element being polished can be included. is there. Depending on the desired mass of the septum, the materials used in the different layers can be modified to provide stiffness / mass, conformability and polishing similarity functions. As an example, the structural layer can also be formed from aluminum, including laminated materials, or other high density materials, preferably in order to reduce the septum to a low aspect ratio. The septum shown in FIG. 5A is suitable for polishing circular optical elements, but other shapes including square and rectangular optical elements are within the scope of the present invention.

  [0068] The septum thus provides a flat shape with a compliance layer that can normalize the pressure when the septum is applied to the pad and keep it constant throughout the septum. A material other than rubber, for example, a soft polymer, a foamed material, silicone, or a composite material thereof can also be used. The compliance layer can be adhered to the structural layer using epoxies or other adhesives as required. Using the same material for the polishing layer as the optical element being polished is beneficial to provide the same pad wear rate. However, other materials can be used as appropriate for a specific application. As will be apparent to those skilled in the art, the use of a different material than the optical element being polished results in a different septum shape. In the septum design shown in FIG. 5A, the pressure (ie, load) on the wrap by the septum (ie, 0.3 psi septum) matches the pressure by the optical element. In other designs, specifying the pressure between the septum and the optical element to another pressure creates another septum shape.

  [0069] FIG. 6 is a simplified plot of pad wear rate as a function of radial distance, according to one embodiment of the present invention. Referring to FIG. 6, the wear by the workpiece (i.e., the wear rate of the polishing pad) is shown in a region with parallel lines in the right direction as a function of distance from the center of the polisher. This area under the curve simply shows how much the pad wears when the optical element is placed on the wrapping pad. With respect to the illustrated graph, an optical element (also called optical element, optical) is placed 25 mm from the center of a 100 mm diameter polisher. Pad wear does not occur where the optical element does not contact the pad (for example, 0 to 25 mm), and the wear rate is zero in this region. Similar zero pad wear is shown where the distance exceeds 125 mm. Given the wear rate shown, over time, the inverse of this curve becomes the shape of the wear groove in the pad being polished.

  [0070] Polishing pad wear due to the septum (shown as complement wear by the parallel lined region in the left direction) is provided, giving a constant overall pad wear rate (C) as a function of distance. Generate. As will be apparent to those skilled in the art, the difference between the constant value and the wear by the workpiece is a criterion for designing the shape of the septum that produces the wear rate shown.

[0071] The pad wear rate can be expressed by the following equation by the combination of the pad wear due to the workpiece (ie, the optical element) and the pad wear due to the septum.

Where r = distance from the center of the pad s = displacement of the optical element from the center of the pad k _L = Preston coefficient (if the optical element and the polished surface of the septum are the same material, they are the same value)
μ = coefficient of friction σ = load (pressure), which can be the same value for both optical element and septum, circumferential width of the optical element at f _O (r) = r f _S (r) = rection of the septum at r Relative velocity between the optical element and the pad in the direction width V _rO = r, and if R _O = _RL , then R
Relative velocity between septum and pad at V _rS = r equal to _O s, and if R _O = R _L then R
R _o equal to _L r = rotational speed of the optical element R _L = rotational speed of the wrap (ie pad)

[0072] Constant pad wear rate

The shape of the septum for is determined by calculating the width of the septum with the following formula:

For a circular optical element, calculation is performed using the following equation.

  [0073] Septa provided by embodiments of the present invention, as shown in FIG. 5A, provide advantages that cannot be obtained using conventional polishing techniques. The septum shape is designed using the Preston equation as described with respect to FIGS. 6 and 7 to keep the pad wear rate constant as a function of position on the optical element. In some embodiments, pad wear uniformity is characterized by a pad wear rate value that is less than 5% different across the portion of the pad that is in contact with the optical element. In other embodiments, the difference in pad wear rate values is less than 2%, less than 1.5%, less than 1%, less than 0.75%, less than 0.5%, less than 0.4%, 0.3 %, Less than 0.3%, less than 0.2%, less than 0.1%, less than 0.05%, less than 0.025%, or less than 0.01%. For example, a first pad wear rate as a function of position is associated with wear by an optical element, and a second pad wear rate as a function of position is associated with wear by a septum (see FIG. 6). ). The sum of these pad wear rates is a substantially constant rate at a plurality of portions on the pad in contact with the optical element. In some embodiments, pad wear rate uniformity is less than one wave, but as will be apparent to those skilled in the art, pad wear rate uniformity is more than one on a pad that is not in contact with an optical element. (For example, in FIG. 6, where the radial distance is less than 25 mm and greater than 125 mm), the uniformity is not in contact with these optical elements as appropriate, depending on the particular application. Spans part. Those skilled in the art will recognize many variations, modifications, and alternatives.

  [0074] In addition to improving spatial pad wear uniformity by using a septum as described herein, there are other mutual benefits due to other phenomena that vary proportionally with speed and pressure. Note that this can also result in Examples of such phenomena include pad compression and viscoelasticity, pad glazing, temperature effects that cause friction, and slurry distribution.

  [0075] By utilizing embodiments of the present invention, pad wear as a function of position is substantially uniform as a function of position, eg, less than a few waves for multiple optical elements. In one embodiment, the pad wear uniformity is within one wave across the radial direction in contact with the optical element. However, pad wear uniformity may be characterized by even more uniform pad wear, eg, less than one wave.

  [0076] FIG. 7 is a simplified plot of septum width as a function of radial distance, according to one embodiment of the present invention. The septum width (ie, circumferential width) in FIG. 7 is the maximum thickness set by the outer circumference of the wrap at a particular radial distance (at shorter radial distances, the outer circumference that can be covered by the septum is limited. And the minimum thickness set by the influence of viscoelasticity and rigid punch. With a softer pad, the effect of viscoelasticity becomes more prominent, whereas with a harder pad, the effect of a rigid punch becomes more prominent.

[0077] In FIG. 7, the shape of the septum is determined for a circular optical element, but other shapes for that optical element are within the scope of the present invention. Using the formulas described here, the septum load can be different from the optical element load, for example, even a non-uniform load. The function f _O (s) is calculated for a predetermined shape, such as a circular or rectangular optical element, and then substituted into this equation to define the septum shape. The functions f _O (r) and f _S (r), referred to herein as the circumferential width, are defined as the ratio of the outer circumference of the wrap at the radius r covered by the optical element or septum, respectively.

  [0078] Of course, as the radial distance decreases, the radial velocity tends to zero, so in the portion near the center of the septum grinder (the short radial distance on the lap) in FIG. It may not be possible to achieve the constant value shown. However, since the optical element is located at a finite distance from the center (eg, 25 mm in the illustrated embodiment), the septum design described here satisfies this design constraint. Since the optical element does not overlap with a radial distance of less than 25 mm, design freedom to provide a constant pad wear rate is provided at a radial distance that does not overlap the optical element.

  [0079] FIGS. 8A-8C are simplified plots of septum load as a function of radial distance, according to one embodiment of the present invention. Embodiments of the present invention may include a septum that is uniformly loaded (FIG. 8A), a septum that is differentially loaded (FIG. 8B), or a septum that is continuously loaded (FIG. 8). 8 (C)) can be used. The plots in FIGS. 8A-8C show that the load (σ (r)) can be a function of position, as described for the pad wear rate.

  [0080] FIG. 9A is a simplified schematic diagram showing scratching by rogue particles. For example, some optical element polishing applications can place strict requirements on scratch density for high energy laser and amplifier systems. While embodiments of the present invention are not limited, the inventors have found that some scratches are rogue particles that are larger than other particles present in the polishing slurry, such as foreign particles or other particles in the slurry. It is considered that any of the particles having a particle size distribution larger than the average particle size of the particles is generated during polishing. As shown in FIG. 9 (A), rogue particles impose a large load on the optical element and form a scratch or a series of scratches. Attempts to remove rogue particles by filtration still show scratches on the optical element. FIG. 9B is an image showing scratching by rogue particles.

  [0081] Without limiting embodiments of the present invention, the inventors believe that a further cause of producing rogue particles is drying of the slurry. For example, in a polishing compound containing cerium oxide, not only the slurry is completely dried, but also when the slurry is dried, the slurry undergoes a chemical reaction with the slurry itself to generate hard aggregated particles from soft aggregated particles. The agglomerated particles may then form scratches as shown in FIG. 9 (B).

  [0082] Embodiments of the present invention prevent the slurry from drying by encapsulating the polishing system. This not only prevents the entry of foreign particles from the outside, but also provides a high humidity environment to prevent the slurry from drying out. In order to prevent slurry from remaining on the optical element, a rinse system can be used to clean the optical element as it is removed from the system. As a result, since the slurry is removed during the rinsing process, the cleaned optical element can be dried after removal without accompanying the drying of the slurry. To further facilitate flushing of the slurry from the system, system components can be coated with a fluorinated polymer layer to reduce adhesion between the slurry and various system components.

  [0083] FIG. 10 is a simplified perspective view of a high humidity polishing system according to an embodiment of the present invention. The high humidity polishing system 1000 includes a polishing surface 1010 and an optical element 1012 partially surrounded by a septum 1014, which can be a polishing pad. The movable cover 1020 can be positioned to contact the enclosure 1022 to form a management environment that surrounds the polishing surface. An inlet and outlet (not shown) for the polishing slurry and an inlet 1030 and outlet 1032 for humidity gas (eg, water vapor) are provided as part of the system.

  [0084] In the embodiment shown in FIG. 10, the humidity inside the polishing system is higher than the ambient humidity, for example, higher than 80%, higher than 85%, higher than 90%, higher than 95%, 97 %, Higher than 98%, higher than 99% and up to 100%. In some embodiments, the humidity is at a high level to prevent significant drying of the slurry in the system. The absence of drying in the environment prevents the formation of hard agglomerated particles and the associated scratching.

  [0085] FIG. 11 is a simplified plan view of a portion of a high humidity polishing system that differs in some respects from the high humidity polishing system shown in FIG. As shown in FIG. 11, the optical element 1105 is placed on a wrap 1110 and is spatially controlled using a guide wheel. In the embodiment shown in FIG. 11, another septum design is used and a mirror septum 1120 is provided to create uniform pad wear. A 100% humidity supply 1030 is provided adjacent to the wrap 1110 so that the polishing environment can provide the desired controlled high humidity atmosphere in a manner similar to a sealed chamber (eg, hermetic chamber 1107). The embodiment of the present invention is not limited to the design shown in FIG. 11, and this embodiment is shown only as an example.

  [0086] FIG. 12 shows a plot of normalized interface height as a function of time for various dilutions of polishing liquid, according to one embodiment of the present invention. Since the height of the interface between the slurry and the solvent that separates from the slurry decreases as the slurry settles to the bottom of the vessel (eg, graduated cylinder), the normalized interface height as a function of time is It is a clue to understand the sedimentation characteristics of the slurry. Hastilite PO mixed with deionized water (DI water) at a ratio of 1: 4 settles fastest and the normalized interface height decreases to 30% within about 25 minutes. When Hastilite PO and ion-exchanged water are mixed at a ratio of 1: 1, the settling time is increased by an order of magnitude, reaching a normalized interface height of 30% in about 300 minutes. With undiluted Hastilite PO, the precipitation time is the longest and takes 300 minutes to decrease to about 60%. Thus, dilution of the slurry, eg Hastilite PO, affects the settling time.

  [0087] FIG. 13 shows a plot of normalized interface height for a polishing liquid under the influence of agitation, according to one embodiment of the present invention. As shown in FIG. 13, agitation makes little difference in the settling time of the slurry diluted with either ion exchange water or tap water.

  [0088] In addition to the effects of dilution, the inventors have found that the addition of an additive that prevents agglomeration to the slurry can achieve slurry stability and increased sedimentation time. FIG. 14 shows a plot of normalized interface height as a function of time for a plurality of polishing liquids according to one embodiment of the present invention. The slurry shown in FIG. 14 is Hastilite PO, but other slurries are within the scope of the present invention. Referring to FIG. 14, Hastelite PO (square) diluted to 9 degrees Baume is characterized by the fastest settling time and decreases to 10% of the original normalized interface height in about 30 minutes. In Hastilite PO (cross-shaped) that has not been diluted with water, the precipitation time becomes longer as in the case of the influence of dilution shown in FIG. Addition of additives to prevent agglomeration results in the longest settling time, as shown by Hastilite PO (diamonds) diluted to 9 ° Baume and added with 1% by volume surfactant μ-90 . As shown in FIG. 14, the addition of the surfactant increases the precipitation time, resulting in a normalized interface height of about 90% of the original height in 800 minutes.

  [0089] Examples of anionic surfactants include, in addition to μ-90, alkyl sulfates (eg, sodium dodecyl sulfate, ammonium lauryl sulfate, etc.), alkyl sulfonates (eg, dodecylbenzene sulfonic acid, Sulfonic 100, Calimulse EM-99), alkyl ether phosphates (eg, Triton H66, Triton QS44, etc.), alkyl carboxylates (eg, sodium stearate, etc.), or other suitable anionic surfactants. Note that surfactants that can be used as additives to the polishing slurry can include sodium, ammonium, or potassium salts where the counterion is not active. The inventors have found that polishing results are improved by stabilizing the slurry with a surfactant (in some embodiments, at an appropriate pH).

  [0090] Embodiments of the present invention are not limited to the use of anionic surfactants, and cationic surfactants can also be used. Examples of cationic surfactants include trimethylalkylammonium chlorides (eg, cetyltrimethylammonium bromide (CTAB), distearyldimethylammonium chloride, etc.), benzalkonium chlorides, alkylpyridinium chlorides (eg, cetylpyridinium chloride). Etc.), or other suitable cationic surfactants. Note that surfactants can include chloride or bromide salts where the counterion is not active.

  [0091] In some embodiments, the surfactant is effective to separate the polishing byproduct from the optical element being polished. The presence of by-products can reduce the stability of the polishing slurry. In some embodiments, the surfactant is effective to improve the long-term stability of the polishing slurry by preventing agglomeration of by-products. Examples of by-products include cations (K +, Na +, Ca2 +, Mg2 +, etc.) generated from glass that has been polished and removed during polishing.

  [0092] In some embodiments, the water used to prepare the slurry may contain various concentrations of metal cations. In addition, cations such as Na +, B3 +, Fe2 +, Ca2 +, Mg2 + and Al3 + are released into the slurry as by-products of the polishing process. The presence of these ions can interfere with the electrostatic interaction between the slurry particles and the stabilizer molecules and reduce the effectiveness of the stabilizer.

  [0093] Chelating agents (eg, glycine, citric acid, ethylenediaminetetraacetic acid (EDTA), etc.) form favorable complexes with cations and trap cations in solution to prevent interaction with slurry particles. To do. Therefore, by adding a chelating agent to the slurry, the initial stabilization efficiency can be improved, and the stability can be maintained when the polishing byproduct is deposited. Thus, embodiments of the present invention use a chelating agent to improve the stability of the slurry.

  [0094] FIG. 15 is a simplified plot of relative interface height as a function of time for a stabilized polishing liquid and an unstabilized polishing liquid, according to one embodiment of the present invention. As shown in FIG. 15, for example, by adding an anionic surfactant additive to stabilize the slurry, the precipitation time is significantly increased.

  [0095] FIG. 16 is the amount of polishing liquid as a function of particle size for a stabilized polishing liquid and an unstabilized polishing liquid according to one embodiment of the present invention. In addition to extending the settling time and stabilizing the slurry, adding additives to the slurry reduces the particle size of the particles in the slurry. Referring to FIG. 15, the particle size distribution of the stabilizing slurry is characterized in that the majority is particles having a size of less than 1 μm and the distribution peak is about 0.25 μm. For unstabilized slurries, the distribution peak is centered around 1 μm and half or most of the particles are characterized by a larger particle size distribution. During the polishing process, the smaller the particle size distribution, the smaller the particles in the slurry, so the stabilized slurry improves the polishing results. The inventors have demonstrated that adding additives can also provide a slurry characterized by acceptable material removal rates and improved microroughness.

  [0096] In some optical finishing operations that utilize grinding prior to the polishing of thin optical elements, the inventors have experienced stresses caused by grinding, causing the optical elements to undergo stress caused by grinding. I found out that they tend to bend. In order to suppress the stress, the stress existing in the optical element is reduced by chemically etching the optical element to remove the surface layer. As an example, the optical element can be ground and then exposed to an acid or other suitable etchant (eg, immersed in an acid bath) to remove a predetermined surface area of the optical element.

  [0097] In one embodiment, an optical element characterized by a first bending rate is ground. After grinding, the optical element is characterized by a second bending rate that is greater than the first bending rate. In some embodiments, the bending rate is increased by the stress exerted on the workpiece during the grinding process. The optical element is chemically etched to remove a predetermined portion of the optical element. After chemical etching, the optical element is characterized by a third bending rate that is less than the second bending rate. In some embodiments, the third bending rate is less than or equal to the first bending rate, and the optical element is returned to the bending rate characterizing the optical element prior to the start of the finishing process. Thus, the chemical etching process reduces the stresses introduced during grinding in some embodiments.

  [0098] FIGS. 18A-18C are images showing surface curvature before grinding, after grinding, and after chemical etching, according to one embodiment of the present invention. As shown in FIG. 18A, the surface curvature before grinding is characterized by a PV value of 1.29 μm. Since stress is dictated by the grinding process, the surface curvature after grinding is characterized by a PV value of 3.65 μm as shown in FIG. 18B. Embodiments of the present invention utilize chemical etching of the grinding surface to remove residual stresses present after the grinding process and return the shape to its original shape, as shown in FIG. 18C. The surface curvature after post-grinding etching is characterized by a PV value of 1.16 μm. Accordingly, embodiments of the present invention provide methods and systems where chemical etching is a mitigation technique useful for suppressing or eliminating residual stresses that cause mismatches between optical elements and laps.

[0099] The inventors have found that in some polishing applications, pitch button bonding (PPB) technology can be utilized to improve the quality of the finished optical element. Table 1 is an overview of the PBB process (also referred to as pitch button blocking process), process parameters and changes in measured surface shape, such as changes in PV value (ΔPV) of the workpiece (also referred to as optical element) before and after blocking. It shows. As described herein, PBB includes a mounting technique that utilizes a small pitch island structure between the workpiece and the mount that is cooled from the softening temperature of the pitch. At room temperature, the workpiece-pitch button-mount system is stiff and the workpiece substantially maintains its initial surface shape. After polishing using PBB technology, embodiments of the present invention are not limited to this, but do not bend the workpiece and dominate the mismatch between the workpiece and the lap due to the influence of the workpiece shape pressure distribution. As a result, the workpiece converges to the shape of the wrap.

  [0100] The inventors have determined that an optical element (eg, fused silica or phosphate glass optical element) can be adhered to a substrate (eg, a stainless steel material) using a pitch button. As will be described in more detail below, the pitch button geometry prevents deflection of the workpiece due to the difference in coefficient of thermal expansion between the workpiece and the substrate. In some embodiments, isothermal cooling is utilized to reduce or eliminate workpiece deflection.

[0101] FIGS. 19A through 19F are simplified schematic diagrams illustrating a method for performing pitch button bonding, according to one embodiment of the present invention. As shown in FIG. 19 (A), an adhesion / protection layer that enhances the adhesion between the glass and pitch interface and protects the glass surface from dirt due to contact with the pitch or contact with the residual slurry is provided on the workpiece. The reflected wavefront is measured through the surface S1. The adhesive / protective layer is a tape in some embodiments, and the tape is optional in some embodiments. As shown in FIG. 19B, the pitch button is applied to the surface of the tape (or the surface S2 of the workpiece). The application of the pitch button is performed in consideration of several variable factors including the type of pitch, the pitch button radius (r _p ), the pitch button thickness (t _p ), the pitch button spacing, and the like. As shown in FIG. 19C, the pitch button is annealed, for example, using a heating element (eg, in a furnace). The temperature of the annealing process, which can vary as a function of time, raises the entire system (eg, glass, block and pitch) to near the pitch temperature (denoted Tg) when the pitch begins to undergo significant stress relaxation. Then, the system is selected to cool as isothermally as possible to prevent or mitigate the effects of residual stresses that deform the shape of the workpiece.

  [0102] FIG. 19D shows a preheated substrate (eg, an aluminum block or stainless steel) having sufficient rigidity and mechanical properties for use during a polishing operation of a workpiece having an annealed pitch button. Shows the attachment to the block. A shim having a predetermined thickness (eg, 1.25 mm) may be used when attaching the workpiece to the substrate. Thereafter, the mounting structure is placed in the center of the substrate, and cooled by, for example, air cooling, and the shim is removed as shown in FIG. In order to clarify the optical characteristics of the workpiece after attachment, as shown in FIG. 19 (F), the reflected wavefront can be measured through the surface S1.

  [0103] FIG. 20 is a plot showing the measured surface shape change of fused silica and phosphate glass in various PPB configurations, according to one embodiment of the present invention. As shown in FIG. 20, changes in the surface shape of the fused silica (FS) optical element and the phosphate glass (PG) optical element under three conditions are shown. The workpiece used in the measurement shown in FIG. 20 has a diameter of 100 mm and a thickness of 2.2 mm. In order to adhere to the solid layer of pitch, the relative surface height of the FS varies from about 7.5 μm at the outer periphery to about 3.7 μm at the center, and the relative surface height of the PG is at the outer periphery. It varies from about 10.1 μm to about 4.2 μm at the center.

  [0104] By using PBB technology to attach the workpiece, it is also possible to first use non-optimized PBB technology with variations of convex curvature (FS) and concave curvature (PS) of about half a micron. The variation in surface height was greatly reduced. By optimizing the PBB process as described herein, the variation in relative surface height was effectively reduced to zero as shown in FIG.

[0105] FIG. 21 is a plot of measured thermal expansion of pitch, according to one embodiment of the present invention. Referring to FIG. 21, the thermal expansion of two pitches (Universal Photonics Blocking Pitch-1 Black and Cycad Products Cycad Blackgold optical polishing pitch) measured using thermomechanical analysis is shown. As the temperature increases, the pitch dimension increases, and the measured coefficient of thermal expansion of BP1 is 37 × 10 ⁻⁶ ° C.− ¹ and that of Cycad is 43 × 10 ⁻⁶ ° C.− ¹ .

[0106] FIG. 22 (A) is a plot of workpiece PV value as a function of pitch supercooling for one button and three buttons, according to one embodiment of the present invention. 22 (A) to (D), fused silica workpieces (diameter d _w = 100 mm, t _w = 2.2 mm, E _p = 73 GPa, α = 5.4 × 10 ⁻⁷ ° C. ⁻¹ ) were used. . The PV value of the workpiece after PPB is plotted for one button (r = 25 mm) and three buttons (s = 50 mm, r = 10 mm) as a function of pitch supercooling. Using multiple pitch buttons significantly reduced the PV value of the workpiece.

  [0107] FIG. 22B is a plot of workpiece PV value as a function of pitch button radius, according to one embodiment of the present invention. As shown in FIG. 22B, the radius of the pitch button affects the PV value of the workpiece. In FIG. 22B, PV values are plotted against the elastic modulus, thickness, and thermal expansion coefficient (ΔT = 54 ° C.) of various pitches for the case of one button.

[0108] FIG. 22C is a plot of the normalized workpiece PV value as a function of pitch button offset, according to one embodiment of the present invention. In FIG. 22C, the PV values after PBB are normalized by dividing by the relative total button area in microns and the spacing between the buttons measured in millimeters for the case of 3 buttons and 9 buttons. Plotted as a function of distance (d _m ). In the case of three buttons, the size of the pitch button is changed in the range of a radius of 10 mm to a radius of 20 mm. The pitch button parameters were ΔT = 54 ° C., t = 1 mm, E _p = 0.22 GPa, α _p = 54 × 10 ⁻⁶ ° C. ⁻¹ . The curve in FIG. 22C represents an experimental curve that matches the calculated data.

  [0109] FIG. 22 (D) is a plot of workpiece PV value as a function of relative total pitch button area, according to one embodiment of the present invention. FIG. 22D shows the case of 3 buttons and the case of 9 buttons after PPB, and the distance between the buttons is maintained at a value larger than 20 mm. The straight line in FIG. 22D represents an experimental curve that fits the calculated data.

[0110] FIG. 23 is a simplified schematic diagram illustrating pitch button bonding parameters according to an embodiment of the present invention. The PBB parameters include the elastic modulus (E _p and E _w ), the coefficient of thermal expansion (α _p and α _w ) and the thickness (t _p and t _w ) for each of the pitch button and the workpiece. The PBB parameters also include the pitch button radius (r _p ), the workpiece radius (r _w ), the center-to-center distance (s) between the pitch buttons, and the pitch button spacing (d _m ). The total number of pitch buttons is represented by N. The pitch buttons can be of uniform dimensions or different dimensions depending on the specific application. Thus, both material parameters and geometric parameters are used in the methods and systems described herein.

  [0111] FIGS. 24A and 24B are diagrams illustrating optimized pitch button bonding patterns for an optical element according to one embodiment of the present invention. FIG. 24 (A) shows an optimized PBB pattern for a fused silica workpiece of 100 mm diameter (ie, samples S18-S20). FIG. 24B shows an optimized PBB pattern for a 100 mm diameter phosphate glass workpiece (samples P1 and P2).

[0112] FIG. 25 (A) is a plot of the workpiece PV value as a function of the spacing between buttons, according to one embodiment of the present invention. FIG. 25 (A) shows the change in surface shape of a fused silica workpiece (ie, FS optical element with a diameter of 100 mm and a thickness of 2.2 mm) in various PPB configurations using pitch material, with the button spacing (d _m ) As a function. FIG. 25B is a plot of workpiece PV value as a function of area ratio, according to one embodiment of the present invention. Changes in the surface shape of both fused silica workpieces and phosphate glass workpieces (ie, workpieces with a diameter of 100 mm and a thickness of 2.2 mm) are observed in various PBB configurations (N = 11 and d _m > 20 mm). , Shown as a function of area ratio. In FIG. 25 (B), a point represents measurement data, and a line represents an approximate curve using α _p = 2.4 × 10 ⁻⁶ ° C.− ¹ .

[0113] FIG. 26 is a simplified flowchart illustrating a method for determining pitch button bonding parameters, according to one embodiment of the present invention. The method includes PV value determination (2610) and pitch area related value determination (2612). In one embodiment, the PV value is a minimum acceptable PV value measured for a dimension (eg, 0.05 μm) or a minimum acceptable PV value measured for a wavelength of light transmitted through an optical element (eg, λ / 10). The value associated with the pitch area (which can be referred to as the area constant) can be approximated by the PV value associated with the solid layer of pitch (eg C = 1.0 μm). The area constant C, in some embodiments, can be determined by measuring the PV value associated with the solid layer of pitch, the processing conditions (ΔT = 54 ° C.), the thermal expansion coefficient (eg, α = 54 × 10 ⁻⁷ ° C ⁻¹ ), elastic modulus (eg, E _p = 0.22 GPa), pitch thickness (t _p = 1.0 mm), etc., can be varied for a particular material system. Usually, the area constant value is set as follows.

Δα is a change in thermal expansion coefficient between the pitch and the workpiece material, [Delta] T is a drop in temperature to room temperature Tg, t _p is the thickness of the pitch. As will be apparent to those skilled in the art, by changing the pitch or workpiece, for example, phosphate glass has a higher coefficient of thermal expansion than fused silica, so the area constant C and the observed deflection are Change. It should also be noted that the deflection of fused silica is convex, while the deflection of phosphate glass is concave. In some embodiments, it is beneficial to maximize the area coverage to increase the interfacial strength between the workpiece and the substrate.

[0114] The method also includes calculating (2614) the relative area (A _r ) of the pitch and is calculated as follows.

As an example, PV _s = 0.05 μm, C = 1.0 μm, and A _r = 0.05.

[0115] The method further button calculation of the radius (r _{p) (2616)} also comprises, is calculated as follows.

Following the above example, d _m = 23.1 mm and r _p = 3.4 mm.

  [0116] The method further includes calculating (2618) the number of pitch buttons (N), and the pitch buttons may be arranged at equal intervals. Thereafter, N pitch buttons are applied to the workpiece (2620) in accordance with the parameters calculated using the present method to couple the pitch buttons to the workpiece. As shown in FIG. 19B, the pitch button can be applied to an adhesive and / or protective material such as tape or other suitable material bonded to the workpiece. The workpiece is then attached to the substrate by bonding the N pitch buttons to the substrate, for example, an optical flat made of stanless steel, aluminum, or a composite of these (2622).

  [0117] It will be appreciated that the specific steps shown in FIG. 26 provide a particular method for determining pitch button bonding parameters, according to one embodiment of the present invention. Other sequential steps may be performed according to other embodiments. For example, other embodiments of the invention may perform the steps outlined above in a different order. Furthermore, each step shown in FIG. 26 may include a plurality of sub-steps, and the plurality of sub-steps may be appropriately performed in various orders depending on each step. Furthermore, additional steps may be added or deleted depending on the particular application. Those skilled in the art will recognize many variations, modifications, and alternatives.

  [0118] The examples and embodiments described herein are for illustrative purposes only, and various changes or modifications in light of them will be suggested to those skilled in the art, and within the spirit and scope of this application, And is to be included within the scope of the appended claims.

Claims (6)

A method for attaching a workpiece to a substrate, comprising:
Determining a PV value associated with the workpiece ;
Determining a value related to a pitch area for attaching the workpiece to the substrate ;
Calculating a relative area of the pitch based on the PV value and a value related to the pitch area ;
Calculating a button radius based on the relative area of the pitch ;
Calculating the number of pitch buttons (N) based on the relative area of the pitch and the button radius ;
A step of combining the N pitch buttons to the workpiece,
Coupling the N pitch buttons to the substrate.   The method of claim 1, wherein the PV value is based on a PV value measurement associated with a solid pitch layer.   The method of claim 1, wherein the relative area of the pitch is equal to the PV value divided by a value related to the pitch area.   The method of claim 1, wherein the spacing between the N pitch buttons is substantially uniform.   The method of claim 1, wherein coupling the N pitch buttons to the workpiece includes applying the N pitch buttons to a tape layer coupled to the workpiece.   The method of claim 1, wherein the workpiece includes an optical element and the substrate includes an optical flat.