WO1988001070A2

WO1988001070A2 - A method for making a precision patterned mirror

Info

Publication number: WO1988001070A2
Application number: PCT/US1987/001742
Authority: WO
Inventors: Albert L. Schmidt; Edward E. Kovach; Gary W. Sherwin
Original assignee: Westinghouse Electric Corporation
Priority date: 1986-08-06
Filing date: 1987-07-17
Publication date: 1988-02-11
Also published as: WO1988001070A3

Abstract

A precision patterned mirror can be created having a pattern resolution as high as 5 microns using an etchable photoresist mask. The mirror is created by producing a high resolution photomask using standard photomask production techniques. A photoresist is coated onto a mirror blank which includes reflecting layers on a glass substrate. The photomask is used to contact print or expose the photoresist. The resist is then developed and the exposed portions removed using standard photoresist development techniques. The mirror is then ion etched to remove the exposed portions of the reflective layer. The resist is then completely removed using a dry plasma etching process resulting in a completed mirror.

Description

A METHOD FOR MAKING A PRECISION PATTERNED MIRROR

The present invention is directed to a method for making a precision patterned mirror which is suitable for visual testing systems.

Conventionally, a patterned mirror such as a mirror checkerboard, has been created using a vapor deposi¬ tion technique. This technique involves creating an electric discharge machining template. The machining template is a metal machining electrode on which a machin¬ ing surface is created in the pattern which is desired on the finished mirror. The metal template is then used to electric discharge machine a metal mask through which the desired pattern is visible as holes. The metal mask is then placed in contact with a substrate such as glass and a reflective material such as chrome is vapor deposited through the metal mask. The metal mask can then be cleaned and used again. This process results in a patterned mirror which has a resolution no better than 0.635 cm ( inch). That is, if a checkerboard mirror is created using the prior art method, checks on the checkerboard can be no smaller than 0.635 cm (3-j of an inch) on each side. Even at this resolution, the corners of the check patterns are rounded, producing a mirror of very low quality. Another drawback to the prior art method is that the machining template can only be used a single time to create a single mask because the template is partially destroyed when it is used to electrically etch the metal mask. The machining of the machining template is very expensive and time consum¬ ing resulting in an overall mirror production process which is very expensive. The mass production of complex mirror patterns having varying size and shaped mirror elements becomes practically impossible due to the limita¬ tions in creation of the machining template. Each time the metal mask is used it must be cleaned with an acid solution and eventually the mask wears out. Precision mirrors are impossible to make with the prior art methods. It is a principal object of the invention to provide an economical process for producing patterned mirrors having high resolution.

Accordingly, with, the present object in view, the present invention resides in a method for creating a patterned mirror from a mirror blank having a substrate and a reflective layer formed on the substrate characterized by the steps of: (a) forming and developing a patterned resist on the reflective layer; (b) etching the reflective layer through, the patterned resist; and (c) removing the patterned resist.

The preferred embodiment of the invention will be described, by way of example, with, reference to the accom¬ panying drawings, in which:

Figs. l(a)-l(e) illustrate the steps used in producing a mirror having a chrome reflecting surface; and

Figs. 2(a)-2(f) illustrate the steps used in producing a mirror having an aluminum reflecting surface.

The process of creating a precision patterned mirror in accordance with the present invention, begins with determining what type of pattern is desired. For example, the mirror pattern could be a checkerboard pattern with all the checks of the same size or a complex spiral checkerboard pattern with the checks decreasing in size toward the center of the spiral. Once the mirror pattern is determined, the pattern is created by a person of ordinary skill in the mask design art using a computer aided mask design system, such as an Applicon Model 860 from Applicon, Inc. of Burlington, Massachusetts. This particular computer aided mask design system is capable of producing a mirror or mask pattern having a resolution as high as 5 microns. Once the desired mask pattern is completed using the design system, the pattern is converted into a mask generator control program which is stored on a magnetic tape. The magnetic tape is read by a mask pattern generator such as a Mann 3600F pattern generator manufactured by G.C.A. of Massachu- setts. The Mann pattern generator produces a 10.16 * 12.70 cm (4 x 5 inch) photoemulsion master mask with a 5 micron resolution. This mask is then used to create the mirror using photoresist etching techniques as discussed in detail below. Aluminum is the preferred embodiment for a mirror reflecting medium since it has a wider, more uniform reflection spectrum than competing materials. It is not possible to coat a glass substrate directly with aluminum, however, because the aluminum does not bond adequately to the glass, creates pinholes and is susceptible to easy damage and degradation.

To obtain an aluminum patterned mirror with the appropriate reflection spectrum and adequate bonding, a coating of aluminum is formed over a chrome mirror blank. The chrome mirror blank is mounted in a Sloan SL1800 vacuum coating system, manufactured by Sloan, Inc. of Santa Barbara, California, which is pumped down to 5 e-7 Torr. The aluminum is then electron beam evaporated onto the chrome mirror at 25 Angstroms per second to a thickness of approximately 5000 Angstroms producing a mirror blank, as illustrated in Fig. 2(a). Aluminum layer 20 is coated with a photoresist 16, as illustrated in Fig. 2(b) and the photoresist is exposed, as illustrated in Fig. 2(c) and developed as illustrated in Fig. 2(d) and baked at 165°C for 15 minutes. Once the exposed portions of the resist are removed, the mirror is dry etched in an ion milling machine using an argon atmosphere for approximately twelve minutes, producing an etched mirror, as illustrated in Fig. 2(c). The photoresist is then stripped in a dry plasma etcher, in an oxygen atmosphere for approximately 20 minutes leaving behind the completed mirror, as illustrated in Fig. 2(f).

An alternative embodiment is a mirror with, a chrome reflecting surface. A 10.16 * 12.70 cm (4 x 5 inch) chrome mirror blank, suitable for creating a chrome mirror can be purchased from Telic Corp. of Santa Monica, Califor- nia. The chrome mirror blank includes a substrate 12 of glass and a layer 14 of chrome approximately 1 micron thick. The mirror 10 is coated with either a negative or positive photoresist 16 such as AZ1350J (positive) manufac¬ tured by Shipley, as illustrated in Fig. 1(a). The photoresist 16 is spun onto the chrome surface as a layer approximately 5000 angstroms thick and baked at a tempera¬ ture of 90°C for 30 minutes.

The master photomask (not shown) is then placed in contact with the photoresist 16 and exposed through, the mask using a well known visible light contact print tech¬ nique resulting in exposed portions 18, as illustrated in Fig. 1(b). The resist is then developed by immersion in a mixture of a 50% resist developer (positive or negative depending on the resist originally deposited), such, as Waycoat Developer from Hunt Chemical Corp. of Palisades Park, N.J., and 50% water at room temperature for 30 seconds resulting in the mirror of Fig. 1(C) in which the exposed portion 18 have been removed.

The developed resist coated mirror is baked at 165°C for 15 minutes and then dry etched in an ion milling machine such as a Veco ion milling machine manufactured by Veco Inc. of New York using an argon atmosphere for approx¬ imately 12 minutes to remove the chrome exposed outside of the resist as illustrated in Fig. 1(d). The photoresist 16 is ^•then stripped in a dry plasma etcher, such as L.F.E. Corp. manufactured by L.F.E. Corp. in an atmosphere of oxygen for approximately 20 minutes leaving behind the completed mirror as illustrated in Fig. 1(e). Care must be taken to ensure that the chrome is not removed in what is called a finish cleaning bath which is normally conducted in a semiconductor manufacturing process, since the chrome must remain on the glass substrate to produce a mirror. A chrome mirror is not the best mirror for vision testing system, however, because it adversely absorbs portions of the human vision spectrum and, therefore, does not reflect true colors. It is possible to use other reflecting materials depending on the desired portion of the spectrum to be reflected. Instead of the ion milling step other metal etching methods could be used such as etching with hydrofluoric acid. Rather than remove the resist using dry plasma etching another resist removal technique could be used such as using a resist removal solution.

If the single layer mirror is created such as the chrome mirror previously discussed, either side of the reflecting mirror can be used as the reflecting surface. However, if a double layer mirror such a^*s the aluminum reflecting surface mirror is created, the reflecting surface will necessarily be the side of the mirror includ¬ ing the aluminum coating.

Even though the description herein is directed to producing a mirror, other optical elements could be pro¬ duced such as neutral density filters and diffusion masks. In fact, if light is shown from the back side of a pat¬ terned mirror with a sufficiently fine pattern, the mirror would act as neutral density filter. Matte filters could also be created.

The patterned mirror is particularly useful in human visual assessment systems in which acuity, contrast and other vision tests are performed. The mirror is used to provide an alternating pattern for perception by the human visual system. The increased resolution of the mirror allows more accurate acuity, refraction and astigmatism testing as well as enhancing other vision tests and permitting miniaturization of other optical elements. If higher resolution is desired, a higher resolu¬ tion digital mask can be created and the resist can be exposed using a known electron beam exposure method.

Claims

CLAIMS:

1. A method for creating a patterned mirror from a mirror blank having a substrate (12) and a reflective layer (14) formed on the substrate characterized by the steps of: (a) forming and developing a patterned resist

(16) on the reflective layer;

(b) etching the reflective layer through the patterned resist; and

(c) removing the patterned resist.

2. A method as recited in claim 1 characterized by step (a) including the steps of:

(al) creating a patterned mask;

(a2) applying a resist to the reflective layer;

(a3) exposing the resist through the patterned mask; and

(a4) removing unexposed resist.

3. A method as recited in claim 1 characterized by step (b) comprising the step of:

(bl) ion milling the reflective layer through the resist.

4. The method of claim 1 characterized by the step of forming a layer of a second reflective material over the reflective layer of the mirror blank prior to the step of forming and developing said layer of said resist.

5. The method of claim 1 characterized by said second reflective material being of aluminum.