US3699916A

US3699916A - An apparatus for monitoring of the deposition of metallic films

Info

Publication number: US3699916A
Application number: US61154A
Authority: US
Inventors: Desmond F Sheahan; George C Callander
Original assignee: GTE Automatic Electric Laboratories Inc
Current assignee: AG Communication Systems Corp
Priority date: 1970-08-05
Filing date: 1970-08-05
Publication date: 1972-10-24
Anticipated expiration: 1989-10-24
Also published as: CA946505A; BE770704A

Abstract

A blank crystal is sandwiched between two electrodes in a holder in a manner such that the only contact of the electrodes with the crystal is along the outer circumference of the crystal. A preliminary evaporation of metal onto the exposed surface of the crystal establishes the desired initial resonant frequency of the crystal. A substrate is then placed in the evaporation chamber proximate the crystal and a metal film is vapor deposited on the substrate and crystal until the resonant frequency of the crystal changes by a predetermined amount. The crystal holder is designed to provide easy removal of the crystal therefrom, thereby facilitating reclamation of the crystal after metal film buildup thereon by removing the crystal from the holder and dipping the crystal in a suitable metal etchant solution.

Description

United States Patent Sheahan et al. 1 Oct. 24, 1972 [-541 APPARATUS", FOR hm 2,479,264 8/1949 Rosenberg ..310/91 OF THE DEPOSITION OF METALLIC 2,711,646 6/ 1955 Mendousse ..310/8.7

FILMS Inventors: Desmond F. Sheahan, San Carlos; George C. Callander, Palo Alto, both of Calif.

GTE Automatic Electric Laboratories Incorporated Filed: Aug. 5, 1970 Appl. No.: 61,154

Assignee:

References Cited UNITED STATES PATENTS 4/ 1 95 2 Wolfskill ..1 18/9 5/1949 Bach ..118/9 5/1968 Unzicker et al. ..118/9 8/1956 l-lansell ..310/8.3

Primary Examiner-Mervin Stein Assistant Examiner-Leo Millstein Attorney-Kurt Mullerheim, Russel A. Cannon and Leonard R. Cool ABSTRACT A blank crystal is sandwiched between two electrodes in a holder in a manner such that the only contact of the electrodes with the crystal is along the outer circumference of the crystal. A preliminary evaporation of metal onto the exposed surface of the crystal establishes the desired initial resonant frequency of the crystal. A substrate is then placed in the evaporation chamber proximate the crystal and a metal film is vapor deposited on the substrate and crystal until the resonant frequency of the crystal changes by a predetermined amount. The crystal holder is designed to provide easy removal of the crystal therefrom, thereby facilitating reclamation of the crystal after metal film buildup thereon by removing the crystal from the holder and dipping the crystal in a suitable metal etchant solution.

2 Claims, 4 Drawing Figures PATENTEU um 24 I972 25 FIG. 2

FIG. 4

FIG. 3

INVENTORS DESMOND F SHEAHAN BY GEORGE C, CALLANDER APPARATUS FOR MONITORING or THE DEPOSITION or METALLIC FILMS BACKGROUND OF THE INVENTION Thin metallic films may be deposited on substrates by a vapor deposition technique wherein the substrate and a charge of the desired metal are placed in an evacuated chamber. Heat applied to the charge vaporizes the metal. The metal vapor on striking the cooler surface of the substrate condenses thereon and thereby builds up a thin metallic film. Conventional methods of monitoring the thickness and rate of the vapor deposition of such a film involve placing a crystal monitor proximate the substrate, with a crystal surface exposed whereby the change in crystal frequency with the build up of metal deposited thereon can be directly related to the thickness of the metal film deposited on the substrate.

The known crystal mounting techniques and subsequent methods for monitoring metallic vapor deposition have not been completely satisfactory because of expense, lack of accuracy, or lack of repeatability from one substrate to another. For example, one technique uses a conventional can mounted crystal. Initially the can protecting the crystal must be removed and drilled through on each side in a position centered on the electrode of the crystal. The can is then reinserted over the crystal and support, and appropriately positioned to monitor the build up of the metallic film. The difficulty with this technique is that the crystals must be preplated and soldered to wire leads for mounting in the can. Furthermore, in order to reclaim the crystal for subsequent use after a given amount of metal has built up on the crystal surface, the can unit must be disassembled to remove the crystal therefrom so that it may be placed in a suitable solution to remove the film deposited on the surface of the crystal. Additional care must be exercised to assure that the electrodes connected to the crystal are not damaged or possibly the electrodes will have to be unsoldered prior to placing the crystal in the cleaning solution. Also, with this technique the repeatability suffers since the angle at which the crystal is mounted may vary from one can unit to another.

Another technique uses a flat crystal plated on both sides which is then mounted on a flat base plate, with the base plate forming one electrode. A spring clip makes tensioned contact with the other crystal surface and forms the other electrode. A primary difficulty with this technique and device is a basic instability which may arise due to heating of the crystal and base plate which results in changes in the contact pressure of the spring clip. This in turn may cause so-called bowing of the crystal resulting in jumps in the resonant frequency of the crystal monitor, thus adversely affecting calibration and control of the thickness of the deposited metal. Also, this technique requires preplated crystals.

OBJECTS AND SUMMARY OF THE INVENTION It is a principal object to provide an improved apparatus for monitoring the deposition of metallic film on a substrate.

Another object of the invention is to provide an improved crystal holder for a crystal monitor adapted to monitor the deposition of metallic film on a substrate.

A further object of the invention is to provide an improved crystal holder of minimum complexity which keeps most of both sides of the crystal free.

Still another object of the invention is to provide a holder for a crystal monitor which does not necessitate preplating of the crystal.

Yet another object of the invention is to provide an improved crystal monitoring apparatus for monitoring the deposition of metallic film on a substrate which permits inexpensive reclamation of the monitor s crystal.

According to one aspect of the invention a flat cylindrical crystal is supported on its extreme top and bottom circumferences between concentric electrodes in a manner such that the majority of both crystal 'faces are free of contact with other surfaces and so that one surface of the crystal is exposed. The crystal electrode for the non-exposed crystal surface is the surface of an associated concentric electrode which is separated from the crystal surface by a small air gap over most of its area which in effect introduces a small capacitance in series with the crystal.

The crystal electrode for the exposed crystal surface is completed by building up on the exposed crystal surface an initial metal film by vapor deposition, for example, until a crystal resonant frequency in a predetermined range results. The crystal holder of the monitor is then placed proximate the substrate to be plated in a suitably evacuated container and metal is vapor deposited on the substrate and crystal until a predetermined change in frequency of the crystal monitor indicates that the desired thickness of metal film has been deposited on the substrate.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified representation, partially in section, of the system adapted to provide vapor deposition of a metal film on a substrate and monitor crystal;

FIG. 2 is a section view taken along line 2-2 of FIG. 1 of a portion of a crystal holder and crystal according to the present invention;

FIG. 3 is a section view of a portion of a crystal holder and crystal according to the present invention; and

FIG. 4 is a schematic circuit diagram of the equivalent circuit of a crystal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention together with other and further objects and advantages thereof, reference is made to the following description in conjunction with the above described Figures.

Referring first to FIG. 1, therein is shown a simplified system adapted to provide vapor deposition of a metal film on a substrate. A bell jar 11 rests on a platform 13. A filament 15 on which is placed a metal charge (not shown) and the filament is connected to a suitable power source 17. Above the filament 15 is a heat shield 16 having an opening therein. Mounted on a support 18 is a substrate 19 and a crystal monitor 21. The crystal monitor is connected by a pair of

leads

45 and 47 to a frequency determining circuit 23. The crystal surface 25 in the monitor 21 and the surface 27 of the substrate 19 to be plated are directed toward the metal charged filament 15.

In operation the monitor is first calibrated to obtain an initial resonant frequency of the crystal as determined by the frequency determining circuit 23. Power from the source 17 is then applied to the metal charged filament 15 causing the filament to heat and give off a metal vapor. The emitted vapor migrates toward the

surfaces

25 and 27 of the monitor crystal and substrate 19, respectively, and upon striking these cool surfaces condenses thereon forming a metal film. As the metal film builds up on the surface 25 in the monitor 21, the resonant frequency of the monitor crystal changes, and since the monitor crystal is proximate the substrate 19, the change in crystal frequency also reflects the build up of the metal film on the substrate. Thus, when the frequency of the monitor crystal changes by a predetermined amount, the desired thickness of metal film has been deposited on the substrate, the power is removed from the filament 15 and the substrate is removed from the bell jar 11.

While not absolutely necessary, the heat shield 16 does serve to prevent excessive heat from radiating toward the substrate 19 and monitor 21, thereby insuring that the monitor is subject to minimal temperature changes. In operation it may also be desirable to maintain the substrate 19 and the monitor 21 equal distances from the filament 15 so that the thickness of metal deposited on the substrate is the same as that deposited on the crystal in face in the mirror.

Depending on the crystal characteristics and the thickness of metal desired on the substrate, a single crystal may be used to control the vapor deposition of a number of consecutive substrates. However, the crystal will eventually become non-oscillatory (or non-linear) after an excessive build up of the metal on the exposed surface. It is then desirable to be able to reclaim the crystal, for example, by removing it from the holder and dipping it in a metal etchant solution, thereby permitting repeated use of the crystal.

With a crystal and holder configuration as illustrated in FIG. 2, it is possible to accomplish the desired abovementioned reclamation with minimal expense and relative ease. Here the crystal 31 is sandwiched between two

electrodes

33 and 35, with the only direct contact between the

electrodes

33, 35 and the crystal being along the narrow outer circumference of the crystal. The outer electrode 35 is in effect hollowed outwith an inwardly protruding edge 37 at the lower end thereof. The inner surface of electrode 35 is threaded at the other end thereof. The inner electrode 33 has a protrusion 39 along its lower, outer circumference and a flat surface 41 displaced by an air space 43 from the top surface of the crystal 31. Leads 45 and 47 connect the

electrodes

33 and 35, respectively, to the voltage source and frequency determining circuit 23. The circumference of cap 36 is threaded for engaging the threads in the end of electrode 35 and facilitating removal of the inner electrode 33 and crystal 31 therefrom.

In operation since the top electrode 33 is separated from the crystal by the gap 43, the crystal is free to vibrate in thickness shear mode. As illustrate in FIG. 3, during the initial stages of the metal deposition, the metal film 49 accumulates on the bottom surface of the crystal 31, forming the electrical contact between the l i "i 3.5ii1 us t r g th g iii v ri i g t h e crystal oscillator as formed by the crystal mounting of the present invention; The equivalent circuit of the crystal 31 is the capacitor 51 in series with an inductor 53 and the series LC circuit is then connected in parallel with a second capacitor 55. The effect of the air gap 43 of FIG. 2 is to add a small capacitance 57 in series with the equivalent circuit of the crystal.

One system which utilizes the present invention, begins with blank AT cut quartz crystals etched to 8,000 kHz i 1 kHz. The blank crystal is inserted in the holder and the exposed surface of the crystal is backplated by vapor deposition of the metal to about 7,940 kHz. The calibrated crystal in the holder is then placed proximate the substrate to be plated and the vapor deposition is commenced. As an initial approximation it is known that the thickness of the metal deposition on the substrate is proportional to the change in resonant frequency of the crystal divided by the density of the metal being deposited and that the relationship generally remains linear within a frequency change of kHz 200 kHz as long as the ambient temperature does not change significantly.

Thus it can be seen that in accordance with the present invention, no pre-plating or elaborate individual mounting of the crystal is required; the crystal may be reclaimed by simply dipping it in a metal etchant solution; and, since the crystal faces are not in contact with other surfaces, they are free to vibrate and are not subject to temperature induced contact pressure changes. Further, while there has been shown and described what is at present, considered the preferred embodiment of the invention, it will be recognized that changes and variations may be made therein without departing from the spirit of the invention. It is therefore intended that the invention not be limited to the specifics of the foregoing description, but rather is to embrace the full scope of the following claims.

What is claimed is: v

1. A frequency controlling element for an oscillator circuit adapted to monitor the thickness of vapor deposited metallic films, said element comprising a Atcut crystal having a resonant frequency in a predetermined range, and first and second electrode terminals supporting said crystal along its outer circumference with one side thereof exposed and the other side thereof separated over other than the circumferential supporting area from the adjacent electrode by an air gap that frees the crystal to vibrate in its thickness shear mode, said first electrode being a hollow conductive sleeve having one end protruding inwardly to provide a circumferential support member for the outer circumference of the exposed side of said crystal, said second electrode being formed to be inserted within said sleeve and having a downwardly extending circumferential protuberance adapted to contact and support the outer circumference of the unexposed side of said crystal.

2. The invention according to claim 1 wherein said first and second electrode terminals are formed to permit easy withdrawal of the second terminal from within said first terminal thereby facilitating removal. and replacement of said crystal.

Claims

1. A frequency controlling element for an oscillator circuit adapted to monitor the thickness of vapor deposited metallic films, said element comprising a At-cut crystal having a resonant frequency in a predetermined range, and first and second electrode terminals supporting said crystal along its outer circumference with one side thereof exposed and the other side thereof separated over other than the circumferential supporting area from the adjacent electrode by an air gap that frees the crystal to vibrate in its thickness shear mode, said first electrode being a hollow conductive sleeve having one end protruding inwardly to provide a circumferential support member for the outer circumference of the exposed side of said crystal, said second electrode being formed to be inserted within said sleeve and having a downwardly extending circumferential protuberance adapted to contact and support the outer circumference of the unexposed side of said crystal.

2. The invention according to claim 1 wherein said first and second electrode terminals are formed to permit easy withdrawal of the second terminal from within said first terminal thereby facilitating removal and replacement of said crystal.