TRANSAT CORP.

Crystal Etch Monitor CEM-1000 DATA SHEET

The Crystal Etch Monitor (CEM) monitors the etching of quartz crystals to a target thickness and provides a signal upon reaching target. It can be used for "shallow" and "deep" etching of flat blanks or high-frequency inverted mesa blanks, and for SC and AT cuts. It operates in all common etchants except (for now) in high-temperature (180°C) sodium hydroxide.

The CEM measures and displays frequency (in MHz) and frequency change (in MHz) and defines thickness change as DT = (F2-F1)/F1xF2. It can also monitor and display the etch rate (in DT per hour). Further, it can be used in conjunction with an external computer for external control and for plotting real-time frequency, frequency change, and etch rate. In this configuration, the system can also be used as an etch rate monitor to simultaneously control multiple batches of pre-etched blanks.

Operating Principle

The CEM measures the frequency of a "monitor blank" that is packaged in a "monitor probe" and immersed in an etch tank together with the etch load. The probe exposes one face of the monitor blank to the etchant. Since, during etching, all blank surfaces undergo the same thickness removal, the monitor thickness removal is half the load thickness removal, and the CEM can control the etching of load blanks via the monitor blank. This allows control that is independent of etch rate variations. For lapped blanks, both monitor and load blanks need to have the same surface finish. For pre-etched blanks, both monitor and load blanks need to be pre-etched into the linear etch range.

Monitor Probe

The probe consists of two parts: A "signal adapter", which houses the electronics, and a "monitor cartridge", which holds the monitor blank and plugs into the signal adapter. The cartridge accepts a monitor blank of 8.2 mm diameter and a (recommended) frequency of about 30 MHz. The blank is easily replaced by removing a cap nut. Two different signal adapters are available: The medium frequency adapter covers monitor frequencies from 10 to 30 MHz and load frequencies up to 60 MHz; the high frequency adapter covers monitor frequencies from 20 to 155 MHz and load frequencies up to 200 MHz. The probe is mounted in a "support plate" 20x5x1cm and it's immersion depth in the etch tank can be adjusted from 3 cm to 9 cm.

Accuracy

Generally, there is a "Load Frequency Error" between the final load frequency (after etching) and the Target Load Frequency. The main sources for the error are:

A) The repeatability of monitor and load frequency measurements. This error can be calculated, as shown in reference 1.

B) Different etch rates for monitor and load blanks. The etch rates of monitor and load blanks are affected by the etchants access to the blanks. The access to the monitor is limited because the blank is recessed in the probe head. The access to the load is limited by the mounting rack. Depending on the type of mounting rack, the etch rate for the load may be higher or lower then the monitor etch rate. Different rates produce a consistent offset between the final load frequency and the targeted load frequency. Once the user knows the offset, they can compensate by offsetting the target frequency.

An additional consistent frequency offset may occur if the monitor quartz material is different from the load material.

A typical error evaluation is shown in a practical example that was supplied by a customer (CTS Reeves). Part of the report is quoted verbatim:

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"...All testes were performed on 44.736 MHz, .250" diameter inverted Mesa blanks.

The blanks were ready for final "bumping to the production frequency window of 46.205 to 46.269 MHz. A 31 MHz, .320" diameter blank was used as a monitor. The monitor blank had been pre-etched into the linear etch range. The results of the test groups are as follows:

	Group #1	Group #2	Group #3	Group #4
Frequency In	44.665-.690	44.553-.579	44.628-.656	44.418-.445
Target	46.205	46.180	46.180	46.192
Frequency Out	46.247-.274	46.208-.248	46.188-.225	46.224-.254
Average	46.262	46.225	46.210	46.239
Adjusted Average	46.249	46.237	46.222	46.239

In previous evaluations we had determined that these blanks typically overshoot the frequency when monitored by the CEM. We adjusted the target frequency to compensate for this overshoot. In order to determine the repeatability of the CEM, the differences in target frequency must be removed from the average of each group (adjust average). You can then see that the CEM maintains a repeatability of 580 ppm.

This would enable us to "bump" the blanks into the final etch window in one step..."

Operation

A typical installation is shown here. The operator enters the Initial and Target Frequencies of the etch load, inserts both the etch load and monitor probe in the etchant, and presses the Start button. The CEM measures the initial monitor blank frequency and computes a monitor target frequency. At a pre-settable offset from target and at target, it provides an audio alarm and a TTL signal that can be used for automatic etch termination.

Ordering Information
A complete system includes: CEM-1000, Monitor Cartridge, Medium or High Frequency Cartridge, Support Plate.

Each item is priced separate, contact: Transat Corp. for more details.

Reference 1: "Automatic Etch Control Using the CEM-1000" Application note: Transat Corp.

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