Quartz Deposition Rate Monitor

One great asset in a deposition system is a deposition rate monitor.

This is crystal of quartz that is set to resonate at its natural frequency.

As material is deposited onto the crystal, this frequency changes,

and from the rate of frequency change, a deposition rate can be

calculated. In many cases even a relative deposition rate is of great

use; to identify if a rate has dropped off after a deposition cycle for

example, or simply if there is any beam there!

For the rate monitoring specialist crystals are commonly available for

around $3 per piece (see figure).

They consist of a thin slab of a single quartz crystal with (typically)

gold or silver contacts on front and back. The challenge with

operating the crystals is that the front (solid contact) surface is

usually in contact with a metal crystal holder and grounded to the rest

of the deposition system. This avoids any build-up of charge on the

surface when used in sputter systems for example, but makes the

electronics of any oscillator more complicated.

The Oscillator Circuit

The figure shows a simple free-running crystal-driver circuit based on

the MC100EL16 or MC10H116 differential line driver ICs. The sine

wave output from the drivers are converted to a TTL square wave, via

a TLV3501 high-speed comparator, for subsequent processing.

Counting the Pulses

To calculate a deposition rate, the frequency change over time must

be measured. The simplest way of doing this is to just count the

number of oscillations per second. However, for low density materials

and low deposition rates, the frequency change that needs to be

measured can be less than 1Hz, which can ultimately lead to long

measurement times. To slightly speed this process up, an alternative

is to “gate” a second higher speed counter with a certain number of

pulses from the crystal. Such a circuit is shown in the figure to the

right.

The 74LV8154 IC’s are both used in 32-bit (4-byte) counting mode.

The first IC is set to count, and transfer the counts to the output

buffer, with every incoming crystal pulse. It is set up to output the 3^rd 

byte of data on its outputs. A set of jumpers allows a particular counter

bit to be selected e.g. for the 22nd bit corresponding to 4,194,304 counts – which will clock in less than 1 second with a 6MHz

crystal.

A second 74LV8154 IC is implemented and clocked from an active oscillator module. (In this case 30Mhz – 5 times the crystal

frequency). Counts from this counter however, are only transferred to the output buffer when an incoming pulse is received from

the first 74LV8154 IC. The result gives; the number of 30MHz pulses in every 4,194,304 crystal oscillations, from which the

crystal frequency can be calculated – but at 5 times the speed of

counting the crystal pulses directly. The second counter can simply

be left running and software used to determine when the counter has

rolled-over (and started from zero again).

Interfacing to the two counters is done with an Atmel AVR-8 (atmega-

168) microcontroller. This does the basic housekeeping for the

timers, resetting them, collecting the bytes of data and sending them

to wherever they are needed. In our case, they are transmitted via a

simple RS-485 interface to a computer which controls the whole

vacuum system, where the deposition rate can be calculated and

plotted. 

The figure to the right shows the final oscillator and counter module

with microcontroller. (The RJ-45 connector is used as a cost effective

interface for supplying power and RS-485 connections, allowing several units to be daisy chained together).

The Mechanics

A considerable expense with installing a deposition rate monitor is

the mechanics; the crystal housing, wiring and electrical feed-through

into the vacuum system. One great draw-back is that water cooling is

often implemented to prevent any shift in the crystal frequency as a

result of temperature change. (This is particularly the case when it is

desirable to move the deposition rate monitor in and out of a beam).

For thermal deposition systems, this temperature variation may be a

significant source of error (typically of the order of 1ppm / oC).

Compensation systems, such as temperature monitoring, or using a

hidden reference crystal could be implemented as an alternative to

the water cooling, but if the likely thermal drift is acceptable, then the

whole system can be made much more flexible and simple, without

compensation or water pipes. (Increasing the “thermal mass” of the

crystal holder can also be used to help reduce the effect where only

intermittent rate checking is needed).

The first two figures show a simple deposition monitor housing.

Small, plated, spring loaded “pogo-pins” are used to hold the crystal

in place and provide electrical connection to the back of the crystal. A

printed circuit board, allows for a very cost effective way of providing

an electrical connection, in this case through an MCX connector .

(Though not ideal for Ultra High Vacuum (UHV) systems, the

component outgassing rate has been found  insignificant for our own

HV system particularly when used in conjunction with lead free

solder.)

Finally for connecting the crystal through the vacuum chamber and to

the outside world, today’s large selection of small RF connectors are

idea for the application. The insulators of such connectors are usually

PTFE and whilst it’s doubtful that the metal parts are gold plated and

not simply titanium nitride coated, they are equally compatible with a

vacuum system.

The last figure shows a few such connectors, the threaded SMA style

to the left and the smaller MCX to the right. Smaller cables and

connectors are available, but the ones shown, area a nice size to

use, being compact and space saving but not too fiddly. For the cable

connectors the “windowed” style is highly recommended. These have

the central pins moulded in-place and the central wire can be

soldered directly to it via an access window in the connector housing.

So there is no trouble with having to cut the various cores to precise

lengths and insert the central pin afterwards. To the rear of the photo

a length of RG-316 cable is shown, which is used in conjunction with

these connectors. This can be purchased with silver plated wire, and

PTFE or FEP insulator (around $1 /m). In some cases, to minimise

virtual leaks, the outer jacket of the cable can be removed, however

this does adversely effect the flexibility of the cable and has been

found unnecessary in our own vacuum system.