US3838031A

US3838031A - Means and method for depositing recrystallized ferroelectric material

Info

Publication number: US3838031A
Application number: US00289480A
Authority: US
Inventors: A Snaper
Original assignee: Individual
Current assignee: INSTITUTE FOR SOCIAL AND SCIENTIFIC DEVELOPMENT 376 EAST 400 SOUTH NO 315 SALT LAKE CITY UTAH 84111 A CORP OF UTAH; SOLOMON JACK D; STRATUM Inc 1000 E BONANZA LAS VEGAS NEVADA 89106
Priority date: 1972-09-15
Filing date: 1972-09-15
Publication date: 1974-09-24
Anticipated expiration: 1991-09-24

Abstract

RECRYSTALLIZED FERROELECTRICAL MATERIAL IS DEPOSITED BY SPUTTERING ON A HEATED SUBSTRATE BY APPLYING RADIO-FREQUENCY VOLTAGE TO A TARGET HOLDER SUPPORTING A TARGET OF FERROELCTRIC MATERIAL HELD IN PROXIMITY TO THE SUBSTRATE WITHIN A CHAMBER WHICH IS EVACULATED EXCEPT FOR THE PRESENCE OF A SMALL AMOUNT OF INERT GAS, AND OPTIONALLY WITH A SMALL AMOUNT OF OXYGEN. THE RADIO-FREQUENCY VOLTAGE CAUSES IONS OF THE GAS TO STRIKE THE TARGET MATERIAL WITH SUCH FORCE AS TO BREAK OFF MOLECULES OR ATOMS WHICH BOND THEMSELVES TO THE SUBSTRATE. THE TARGET HOLDER IS PROVIDES WITH FLUID COOLANT CHANNELS WHICH PROTECT THE TARGET FROM DETERIORATION.

Description

A. A. SNAPER MEANS AND METHOD FOR DEPOSITING RECRYSTALLIZED FERROELEGTRIC MATERIAL 2 Sheets-Sheet 1 Filed Sept. 15, 1972 WIIIIIIIIII,

'4 Hal f A. A. SNAPER Sept. 24, 1914 MEANS AND METHOD FOR DEPOSITING RECRYSTALLIZED FERROELECTRIC MATERIAL 2 Sheets-Sheet 2 Filed Sept. 15, 1972 United States Patent 3,838,031 MEANS AND METHOD FOR DEPOSITING RECRYS- TALLIZED FERROELECTRIC MATERIAL Alvin A. Snaper, 2800 Cameo Circle, Las Vegas, Nev. 89107 Filed Sept. 15, 1972, Ser. No. 289,480 Int. Cl. C23c /00 US. Cl. 204-192 8 Claims ABSTRACT OF THE DISCLOSURE Recrystallized ferroelectric material is deposited by sputtering on a heated substrate by applying radio-frequency voltage to a target holder supporting a target of ferroelectric material held in proximity to the substrate within a chamber which is evacuated except for the presence of a small amount of inert gas, and optionally with a small amount of oxygen. The radio-frequency voltage causes ions of the gas to strike the target material with such force as to break off molecules or atoms which bond themselves to the substrate. The target holder is provided with fluid coolant channels which protect the target from deterioration.

This invention relates to the sputtering deposition of recrystallized ferroelectric material and more particularly to a method and apparatus capable of producing such a deposit.

Ferroelectric materials such as gadolinium titanate, gadolinium molybdate, gadolinium niobate, barium titanate, and bismuth titanate have been known. Such ferroelectric materials when in crystal form possess birefringent properties permitting their use as light modulators, enabling them to affect transmission of light in various ways, for example, the production of color effects, or the passing, reducing or blocking of light transmission in ways dependent on such factors as the physical dimensions of the material, the application of an electric field or voltage and the relationship of the light to the material.

It has heretofore been known to deposit ferroelectric materials but the deposited material has been amorphous in structure.

It has heretofore been known to create crystals of ferroelectric material. One such method has been a seeding process by which a relatively large crystal is grown from a much smaller seed crystal. Another prior known method has been the so-called flux process, by which small pieces of crystal material are obtained from a flux.

It is an object of the present invention to provide a method and means for obtaining recrystallized ferroelectric material by deposition.

A related object is to create such a recrystallized ferroelectric deposit by sputtering on a substrate or the like.

A further related object is to provide a fluid-cooled target holder for carrying the ferroelectric material used as the target in the sputtering operation.

The invention is carried out by the application of radiofrequency (R.F.) electrical power to a target holder containing a ferroelectric target material, in a high vacuum to which there have been introduced minor amounts of inert gas, preferably a relatively heavy inert gas such as argon, xenon, neon or krypton or mixtures thereof. The RF. energy acting upon the gas creates a plasma. A heated substrate or the like spaced somewhat from the target material receives a layer of recrystallized ferroelectric material bonded to it due to the sputtering of this material from the target by the actions of ions of the plasma striking the target material in response to the radio frequency energy. Although lighter inert gases such as nitrogen are usable, they are not as efficient as the heavier ones since ions of the lighter gases do not possess as much kinetic energy as ions of the heavier gases.

3,838,031 Patented Sept. 24, 1974 In the sputtering deposition, there is a high temperature build-up which creates risk of the target material breaking, cracking, or exploding, or becoming contaminated. Under such circumstance, it is desired to avoid or minimize this risk by providing cooling for the target material. According to a feature of the invention, the target holder may be cooled by fluid, ordinarily water. Novel aspects reside in the configuration and arrangement of the coolant channels at the target holder.

Recrystallized ferroelectric material can be used in such equipment as: computers or data processing systems, such as that of Snaper Pat. 3,348,217, issued Oct. 17, 1967; polarized film structures such as those of Snaper Pat. 3,- 376,135, issued Apr. 2, 1968; color-producing tubes such as that illustrated in Snaper Pat. 3,391,296, issued July 2, 1968; electro-optical devices such as that illustrated in Snaper Pat. 3,445,666, issued May 20, 1969; color-producing equipment such as that illustrated in Snaper Pat. 3,- 488,106, issued Jan. 6, 1970; and planar random access ferroelectrie computer memories such as that illustrated in Snaper et al. Pat. 3,675,220, issued July 4, 1972. These are only some of the many possible uses for the material.

The invention will be better understood from the following detailed description and the accompanying drawing, of which:

FIG. 1 is a cross-section view showing equipment for carrying out an RF. sputtering deposition according to this invention;

FIG. 2 is an elevation view in cross-section showing a target holder which may be used in the arrangement of FIG. 1;

FIG. 3 is a front view of a portion of the head of the target holder of FIG. 1;

FIG, 4 is a side view of the portion shown in FIG. 3;

FIG. 5 is a top view of the portion shown in FIGS. 3 and 4;

FIG. 6 is a side view of another portion of the head of the target holder of FIG. 1; and

FIG. 7 is a bottom view of the portion shown in FIG. 6.

Referring to FIG. 1, there is shown a vacuum bell jar I mounted with an O-ring seal In on a base member 2 provided with an opening 3 for connection to the inlet of a vacuum pump. The bell jar will ordinarily be circular in its top view, although it may be of a different shape. A bleed conduit 4 provided with a bleed valve 5 is adapted for connection to a source of gas which it may be desired to introduce into the evacuated bell jar. The base 2 is provided with a centrally located opening 6 to the wall in which there is attached an upstanding hollow tubular member 7 of an electrical conducting material extending upwardly into the bell jar. The upper end of this tubular member is provided with an annular head 8 having an outwardly and upwardly beveled surface 9 which is attached to a corresponding beveled surface 10 of a target holder 11 through an electrical insulating member 12 sealed to both the target holder and the head 8. The top surface of the target holder is flat and horizontal and preferably circular in its top view. The target holder has attached to its underside a stem 13 extending down through, and spaced from, the tubular member 7 to the exterior of the bell jar. The

members

2, 7, 8, 11, and 13 are of electrical conducting material, ordinarily metal, and the material of the bell jar 1 is electrically insulating. There is bonded to the top of the target 11 a layer of ferroelectric target material 14 which will ordinarily be in the amorphous form. The bonding may be done by use of an electrically conductive bonding material such as 0 a silver conductive paste and the application of heat and There is supported within the bell jar at a position above the target material a horizontally disposed plate of electrical-conducting material, ordinarily metal, pro vided with an opening 16 at a position above the target material 14, There is supported within this opening a substrate 17, ordinarily of an optically transparent electrically-insulating material which should ordinarily be compatible with transparent conductors and with opaque high dielectric materials. Typical substrate materials can be Pyrex glass or quartz or the like. Before a sputtering operation, the substrate should be made very clean, which can be done by cleaning ultrasonically and baking in a high temperature oven, or by reverse sputter etching. Then the substrate is mounted to a matrix mask 18, the configuration or position of which will determine the area or areas of the deposition on the substrate. Above the substrate, there are supported heating elements 18 ordinarily in the form of high-temperature quartz lamps which direct their heat down upon the top of the substrate.

To provide radio frequency power for the sputtering operation, a source 20 of radio frequency voltage is applied over

respective conductors

21 and 22 between the

grounded elements

2, 15 and the electrode 13', which thus acts as an antenna radiating electromagnetic energy into the bell jar.

To perform a sputtering operation with the system of FIG. 1, a high vacuum pump is attached at the entrance 3 to the bell jar and vacuum pumped to a very high vacuum of about 10- to 10' torr while the heaters 19 are turned on to preheat the substrate 17 to a high temperature which, though not critical, should be between about 200 degrees and 600 degrees C. When the desired temperature and vacuum levels are reached, the vacuum pump is throttled to a partially closed condition by means of a valve 23 at a vacuum port 3 Opening and the inert gas, ordinarily argon, is bled into the chamber through tube 4 past valve 5. The amount of the inert gas such as argon, xenon, or neon or the like bled into the system is not critical, and may be between 1.0x 10 torr and 40x10 torr. Argon is preferred over xenon or neon for the inert gas, as argon is usually less costly.

It may, in many instances, be desirable to bleed into the chamber along with the inert gas, some amount of oxygen to act as a control factor. A reason for this is that ferroelectric materials commonly contain oxygen in the binder, some more than others. If a ferroelectric material deficient in oxygen relative to others is being used for the sputtering, it is found desirable to bleed into the chamber an amount of oxygen sufiicient to make up the oxygen deficiency in the material. In the case of the use of gadolinium molybdate as the ferroelectric material and with the use of argon as the inert gas, it has been found beneficial to introduce with the inert gas an amount of oxygen between about 1.0x 10' torr and about 10.0)(10' torr, the precise amount of oxygen not being critical.

After this evacuation and gas bleeding operation has been performed, and while the heating elements 19 remain operating, the radio frequency electrical energy from source 20 is turned on to apply it to the electrode 11, 13, the radiation from which thereby ionizes the argon (or other inert) gas. The negative cycle portions of the RP. energy cause positive ions, represented in FIG. 1 by circles 55 containing plus signs, to strike the target material, thus knocking olf particles in the form of atoms and molecules (probably mostly molecules) of the target material toward the under surface of the substrate. These particles striking the substrate through the mask 8 have enough energy to bond themselves into the substrate.

The radio frequency energy must be of at least a high enough frequency to create the plasma and this minimum frequency is believed to be around 5 megacycles per second. A normal range would be about 5 to 25 megacycles per second, which is below the microwave range. A proper R.F. power level would be between about 1 and 10 watts per square centimeter surface of the target material. The

time of the sputtering operation will be dependent on the thickness of the recrystallized ferroelectric layer desired on the substrate, the longer the time, the greater the thickness.

A sputtering deposition using gadolinium molybdate as the target material was performed under the following test condition:

Substrate material: quartz; target to substrate distance 1% inches; argon gas pressure 4X10 torr; oxygen gas pressure 1.0x 10' torr; quartz substrate temperature 400 centigrade using quartz heating lamps. Starting with a very clean vacuum system and chamber, the system was pumped onto a very high vacuum, and then the vacuum pump Was throttled as the argon and oxygen were backfilled into the chamber to the foregoing pressure. During this process, the heating lamps were kept on, and holding the substrate temperature at 400 C. At this time, the RF. power of a frequency of 12.7 megacycles per second was applied to the target, igniting an RF. plasma. The radio frequency power was increased slowly to a power level of 7 watts per square cm. of target material surface area. Sputtering under these conditions for approximately 8 hours achieved a thickness of 20,000 angstroms (7.874 l0- inches), which is a typical thickness. The radio frequency power and the substrate temperature were then both decreased slowly for four hours until the substrate temperature dropped to 60 C., whereupon the system was shut down. The substrate was then removed and placed in a furnace, which heated it to a temperature of 700 C. The substrate was then cooled slowly, thereby removing all stress from the crystalline material which had been sputtered onto it. It was found that the sputtered material was of gadolinium molybdate crystalline structure and had excellent birefringence.

For any selected RF. power level the thickness of the deposit is about proportional to the time of the sputtering.

Inasmuch as the uses to which recrystallized material deposited according to this invention will ordinarily be put, will generally involve its effects in response to the application of an electric field, there will generally be applied to the substrate a substance which acts as an electrical conductor. This may be done, for example, by vacuum depositing a film of conductive stannic oxide on the substrate prior to the ferroelectric sputtering operation. Such a conductive stannic oxide deposit may be made in a well-known manner. Then when the substrate bearing the conductive stannic oxide film on its undersurface is placed in the bell jar with the matrix mask, the crystalline ferroelectric layer from the sputtering will bond to the parts of the substrate not covered by the conductive stannic oxide film and also over the conductive stannic oxide film. Thus, the conductive stannic oxide constitutes an electrically conductive surface adjacent to the ferroelectric material to which an electrical lead or conductor may be brought in a suitable manner to apply voltage over the ferroelectric layer. Such a stannic oxide film may be formed over the entire ferroelectric layer or, if desired, only over part of it, as for example, in strips, according to the effects which are desired. The stannic oxide is not only electrically conductive, but is transparent and compatible with such substrate material as Pyrex glass and quartz and is compatible with ferroelectric material which is a ceramic type of dielectric. Stannic oxide is mentioned merely as an example of a compatible transparent conductor which may be used for the purpose. Thin metallic films and other materials may also be utilized.

The ferroelectric deposit has been referred to herein as recrystallized ferroelectric material, which is the term usually applied to it even though the ferroelectric material constituting the target is amorphous and not crystalline.

FIGS. 2 to 7 illustrate a fluid-cooled target holder which can be used as the target holder of FIG. 1. Referring particularly to FIG. 2, the stem 13 constituting the RF. elec trode is in the form of a hollow conduit 30, terminating at its upper end in a flanged collar 31 having attached to it a circular head 32 of greater diameter than that of tube 30.

The target holder 11 comprises a plate 33, shown in detail in FIGS. 3, 4 and 5, and an upper plate 34 shown in detail in FIGS. 6 and 7. FIG. 6 is a front elevation view and FIG. 7 is a bottom view, of the upper plate 34, from which it is seen that the plate is flat and circular with upper and lower surfaces which are horizontal and with a side peripheral surface which is beveled inwardly in a downward direction from the top. The lower surface is flat and horizontal excepting for an arrangement of connected generally circular grooves 35 machined into the plate from the bottom as shown in FIG. 7. This groove arrangement comprises a central recess 35a located at the central axis of the plate, around which are recesses arranged in generally concentric circles. The central recess 35a communi cates through an opening 36 into the end of the innermost concentric circle 35b, the end of which communicates through an opening 37 into the next concentric circle 350, the end of which in turn communicates through passageway 38 into the outermost concentric circle 35d. The end of the outermost concentric circle connects with a circular recess 39.

The lower plate 33 of the target holder 11, illustrated in detail in FIGS. 3, 4, and 5 is a flat circular plate of about the same thickness as plate 34, having horizontal upper and lower surfaces with a perpheral surface beveled at the same angle as plate 34. The diameter of the upper surface of plate 33 coincides with the diameter of the lower surface of plate 34. Plate 33 has formed through it at its central axis, an opening 40 of somewhat smaller diameter than recess 35a of plate 34, and adapted to register with recess 35a.

The underside of plate 33 has machined into it a concentric circular recess 41 shown dotted in FIG. 3, and also in FIG. 5, the top view of the plate. From one side of recess 41, there is formed a channel 42 communicating from the recess 41 to a circular opening 43 extending from the top of the plate to meet the end of channel 42. Plates 33 and 34 are fastened together as by welding around the beveled periphery.

As shown in FIG. 2 the electrode 13 is assembled to target holder 11 by use of a hollow pipe or conduit 44 extending through and spaced from the inner wall of tube 30. The upper end of pipe 44 is provided with outside threads 45 which threads into corresponding internalthreads 46 in opening 40 of plate 33, and also to threads 47 at opening 48 of head 32.

To assemble the target holder to the electrode 13 as shown in FIG. 2, the head 32 of the electrode will be threaded onto the threads 45 of pipe 44 until the end of the threading is reached to secure the head 32 firmly on the pipe. This will leave a substantial part of the pipe threading protruding above head 32, so that the threads 46 of plate 33 of target holder 11 can then be threaded until the plate 33 assumes a position firmly against the top of head 32. When thus assembled, the concentric recess 41 of plate 33 aligns with the vertically extending bores 49 of head 32 which communicate with the annular passage 50 between pipe 44 and tubular electrode 30. In this assembly, the passage 50 within pipe 44 is in communication with recess 35a of the upper plate 34. Owing to the close fit between plates 33 and 34, there is no fluid leakage from the

concentric channels

35b, 35c and 35d between these two plates. To insure against fluid leakage be- 6 tween head 32 and the bottom of plate 33, the head 32 is provided with concentric recesses 52 and 53 containing resilient O-rings to act as seals.

Fluid, ordinarily a liquid, preferably water, can be circulated through the target holder in the direction of arrows 54, by a suitable pump or pressure means. Thus, fluid is forced upwardly through conduit 44 into recess 35a from which it passes through the

concentric recesses

35b, 35c and 350! to channel 42 and downward through the concentric passage 50.

It will be understood that the embodiments of the invention illustrated and described herein are given by way of illustration and not of limitation, and that modifications or equivalents or alternatives within the scope of the invention may suggest themselves to those skilled in the art.

I claim:

1. Method of depositing recrystallized ferroelectric material which comprises applying radio frequency power to a target comprising ferroelectric material in proximity to a substrate heated to a temperature between 200 C. and 600 C. within an enclosure substantially evacuated, but containing some inert ionizable gas, said voltage being of a frequency and intensity sufficient to create a plasma comprising ions derived from the gas, causing at least some of said ions to strike the ferroelectric material with such force as to sputter particles of it to the substrate, with sufficient velocity to bond it to the substrate in crystalline form.

2. Method according to claim 1 in which the sputtered particles comprise molecules or parts thereof, or both.

3. Method according to claim 1 in which the frequency is between about 5 and 25 megacycles per second.

4. Method according to claim 1 in which the inert gas is selected from the group consisting of argon, xenon, neon, and krypton, and mixtures thereof.

5. Method according to claim 1 in which the inert gas is present in an amount between about 1.0 10- torr and 40 X 10* torr.

6. Method according to claim 5 in which oxygen is present in an amount between about 1.0 10 torr and 10.0X l0 torr.

7. Method according to claim 1 in which the ferroelectric material is selected from the group consisting of gadolinium titanate, gadolinium molybdate, gadolinium niobate, barium titanate and bismuth titanate.

8. Method according to claim 1 in which the ferroelectric material is gadolinium molybdate.

References Cited UNITED STATES PATENTS 8/1972 Vogel 204-192 8/1970 Davidse et al. 204298 X 3,730,867 5/1973 Albers, Jr. et al. 204-298 X 3,707,452 12/1972 Lester et al. 204-298 X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S. Cl. X.R.