US3755023A

US3755023A - Technique for etch polishing single rare earth crystal synthetic garnet

Info

Publication number: US3755023A
Application number: US00206529A
Authority: US
Inventors: D Miller
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1971-12-09
Filing date: 1971-12-09
Publication date: 1973-08-28
Anticipated expiration: 1990-08-28

Abstract

A technique for etch polishing single crystal synthetic garnet involves etch polishing the crystal in hot phosphoric acid saturated with steam. The described technique significantly enhances reproducibility and results in crystals manifesting an amplitude of surface undulations of + OR - 0.25 microns or less with a frequency of 100 or greater.

Description

United States Patent Miller Aug. 28, 1973 TECHNIQUE FOR ETCH POLISHING [56] References Cited SINGLE RARE EARTH CRYSTAL UNITED STATES PATENTS SYNTHETIC GARNET 3,080,320 3/1963 Vickery 252/625 Inventor: David Christopher Miller,

Millington, NJ.

Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, Berkeley Heights, NJ.

Filed: Dec. 9, 1971 Appl. No.: 206,529

US. Cl 156/2, 156/20, 252/625 Int. Cl. H011 7/50, H011 7/62 Primary Examiner-Jacob l-l. Steinberg Attorney-W. L. Keefauver et a1.

[57] ABSTRACT A technique for etch polishing single crystal synthetic garnet involves etch polishing the crystal in hot phosphoric acid saturated with steam. The described technique significantly enhances reproducibility and results in crystals manifesting an amplitude of surface undulations of $0.25 microns or less with a frequency of 100 or greater.

8 Claims, No Drawings TECHNIQUE FOR ETCI-I POLISHING SINGLE RARE EARTH CRYSTAL SYNTHETIC GARNET This invention relates to a technique for etching single crystal synthetic garnet. More particularly, the present invention relates to a technique for etch polishing single crystal synthetic garnet in phosphoric acid.

DESCRIPTION OF THE PRIOR ART In recent years, the most promising commercial applications of synthetic garnet have been in microwave devices and in bubble domain devices. In each of these cases, the garnet selected should desirably be flat within 1 micron, smooth and highly polished. Heretofore, this end has been attained by conventional mechanical polishing techniques which are inordinately time consuming, economically expensive and not readily adaptable to manufacturing techniques. Workers in the art have utilized heated phosphoric acid to etch a plurality of single crystal oxides but limitations have been imposed by precipitate formation. and the transition of the acid to unsatisfactory etchants. Unfortunately, this procedure when applied to synthetic garnet has proven to be unsatisfactory in that the aforementioned difficulties are greatly increased. Additionally, studies have revealed that such processes are not reproducible and, therefore, unsatisfactory for commercial applications.

SUMMARY OF THE INVENTION In accordance with the present invention, these prior art limitations are effectively obviated by a novel technique utilizing steam saturated phosphoric acid as the etchant. Briefly, the inventive technique involves etch polishing the crystal in hot phosphoric acid saturated with steam and subsequently quenching the crystal in oil. Synthetic garnet treated in the foregoing manner manifests an amplitude of surface undulations of 10.25 micron with a frequency of 100 or greater.

DETAILED DESCRIPTION OF THE INVENTION I The synthetic garnet materials considered here can be represented by the general formula M,Me,,0 where Ois oxygen, Me is a trivalent metal, and M is yttrium or one of the rare earth elements of atomic number between 62 and 71 or a mixture of these rare earth elements with each other. Me may be trivalent iron or one of the elements selected from among gallium, aluminum, scandium, or mixtures thereof.

In order to aid in the understanding of the present invention, an outline of the procedure employed in the etching of synthetic garnet will now be given.

The first step in the practice of the present invention optionally involves pre-etching the garnet of interest with phosphoric acid. The garnet materials described above are conveniently obtained from commercial sources or are grown in accordance with standard techniques. The as-grown crystals are cut in the desired size and then pre-etched in phosphoric acid at.a temperature within the range of 155 180C. The purpose of this pre-etching step is to reduce the statistical likelihood of precipitate formation and to dissolve the highly damaged surface layer with its accompanying grit. Studies have revealed that the use of temperatures less than the noted 155 minimum will not yield a satisfactory etch rate, whereas the-use of temperatures in excess of 180C result in excessive pitting of crystal surfaces. Etching in accordance with this schedule is continued for a time period ranging from 5 to 10 minutes, the minimum and maximum being dictated by the same criteria noted above with respect to temperature.

The next step in the practice of the present invention involves immersing the pre-etched crystal in phosphoric acid which is heated to a temperature within the range of 250 450C and to which steam heat to a temperature within the range of 200C has been added in an amount sufficient to saturate the acid, thereby establishing a moisture level which can be conveniently maintained throughout a plurality of processing sequences. Immersion is continued for a time period ranging from 2 to 5 minutes, the duration of immersion being dictated by considerations relating to the amount of material to be removed.

It has been determined that the etch character is a function of the temperature as well as the moisture content of the etching bath. At temperatures in excess of the noted 450 maximum, crucible attack has been noted. The use of temperatures less than 250C results in a high frequency of undulations. However, processing in accordance with the foregoing schedule of pa rameters results in the formation of high-quality polished surfaces with a stabilized etch rate and character and a concurrent high degree of reproducibility. The rate of removal of material in the processing sequences described hereinis approximately of the order of 1 mil per minute.

The phosphoric acid selected for use in the practice of the present invention is orthophosphoric acid having a concentration of 85 percent, such being readily available from commercial sources.

Following, the etched crystal is quenched in oil and subsequently cleansed with any conventional detergent. The particular oil selected is not critical, mechanical pump oil being found particularly suitable for this purpose. In order to obviate the likelihood of cracking during the quenching operation, it is convenient to heat the oil, temperatures of the order of 60C being found suitable for this purpose.

Examples of the application of the present invention are set forth below. They are intended merely as illustration, and it is to be appreciated that the methods described may be varied by one skilled in the art without departing from the spirit and scope of the present invention.

The examples are in tabular form for convenience and brevity. Each example in the table is to be considered separately, since each set of data was obtained in a separate process. The procedure followed in each of these examples is as follows:

The synthetic garnet crystal of interest, obtained by standard growth processes, was initially cut in discs approximately 2 centimeters in diameter by 1 millimeter in thickness and placed in a platinum crucible. Then, orthophosphoric acid (85 percent concentration) was introduced into the crucible and heating initiated at a temperature of C for 5 minutes. Then, the preetched crystal was heated in the presence of the phosphoric acid .to 390C and steam heated to 200C introduced into the phosphoric acid to effect saturation thereof, saturation being indicated by the presence of an equilibrium, that is, the volume of steam entering solution is equivalent to the volume evolved from the acid. Immersion in this solution was continued for 2 minutes. Next, the crystal was removed from the crucible and immersed in mechanical pump oil heated to a net crystal evidenced precipitate formation, surface temperature of 60C. Finally, the crystal was cleansed cracking and lack of reproducibility from step to step with a conventional detergent and potassium hydroxide as the acid deteriorates. and the quality of the solution etched surface studied. What is claimed is:

1. Technique for etch polishing single crystal syng a g gg iggi thetic garnet of the general formula M Me O where Talysurf Surface 0 is oxygen, Me a trivalent metal, and M is yttrium Example l Measurements or one of the rare earth elements of atomic number be- 1 Gadolinium Gallium Gamer Amplitude f urf undulations tween 62 and 71 or mixtures thereof which comprises of less than $015 micro" will 10 (a) etch polishing the crystal in phosphoric acid satua frequency of 100 or greater 2 Yttrium Aluminum rated with steam for a time period ranging from 2-5 Garnet Amplitude of Surface undulations minutes, the temperature of the phosphoric acid being g eate within the range of 250-450C, and (b) quenching the 3 Holmium Gallium etched polished crystal in oil.

4 Dyspwsh'l l5 2. Technique in accordance with claim 1 wherein Gallium Garnet said garnet is pre-etched in phosphoric acid at a tem- 5 gfigi perature within the range of l55-l80C for a time pe- 6 S -i Gadolinium riod ranging from 5-10 minutes prior to etch polishing.

7 S3L2L gg2 g 3. Technique in accordance with claim 1 wherein the Gamum Game steam is heated to a temperature within the range of 8 Gadolinium Scandium lO5-200C.

9 r. 4. Technique in accordance with claim 1 wherein Garnet quenching is effected in oil heated to a temperature of g ggfl gg fgf the order of 60C.

l1 N odymium Scandium 5. Technique in accordance with claim 2 wherein l2 gzgiggfix said pre-etching is conducted at a temperature of Scandium Aluminum Garnet 165C and the etch polishing is conducted at 390C.

6. Technique in accordance with claim 4 wherein Additionally, the crystal of Example 2 as etched could said single crystal garnet is gadolinium gallium garnet. not be distinguished from a mechanically polished sur- 7. Technique in accordance with claim 4 wherein face, For comparative purposes, the procedure desaid single crystal garnet .is yttrium aluminum garnet. scribed above was reiterated with the exception that 8. Technique in accordance with claim 4 wherein steam was not added to the heated phosphoric acid in said single crystal garnet is holmium gallium garnet. the etching step. The resultant gadolinium gallium gar- I

Claims

2. Technique in accordance with claim 1 wherein said garnet is pre-etched in phosphoric acid at a temperature within the range of 155*-180*C for a time period ranging from 5-10 minutes prior to etch polishing.
3. Technique in accordance with claim 1 wherein the steam is heated to a temperature within the range of 105*-200*C.
4. Technique in accordance with claim 1 wherein quenching is effected in oil heated to a temperature of the order of 60*C.
5. Technique in accordance with claim 2 wherein said pre-etching is conducted at a temperature of 165*C and the etch polishing is conducted at 390*C.
6. Technique in accordance with claim 4 wherein said single crystal garnet is gadolinium gallium garnet.
7. Technique in accordance with claim 4 wherein said single crystal garnet is yttrium aluminum garnet.
8. Technique in accordance with claim 4 wherein said single crystal garnet is holmium gallium garnet.