Ultra High Frequency Resonators

William P. Hanson

Piezo Crystal Company 100 K Street, P.O. Box 619, Carlisle, PA 17013

The demand for high frequency fundamental and overtone chemically polished resonators is increas- ing. Manufacturing methods are becoming more so- phisticated and more reliable for these devices. A 100 MHz, 300 MHz, 400 MHz, or even 1000 hlHz resona- tor can be made by chemical polishing. However, set- ting such a resonator to within 25 ppm is a much more difficult task, especially with the small elec- trode diameters needed ( . 3 mm in some cases). AT, SC, IT, BT, and FC cut resonators can be chemically polished. The etched surface varies with cut. the quality of the raw material, the blank orientation, and the etching solutions. Some cuts etch better than oth- ers and consequently can be made at higher frequen- cies. Some of the things which affect the final fre- quency are: the blank frequency, the base plating process, the electrode material. the final frequency adjustment process, and sealing. Good processes and good raw materials can result in resonators set to fre- quency close enough for use in oscillators or filters.

Introduction

Etching has been used a s a frequency adjustment method for many years. Heising stated over 40 years ago. “Finished quartz surfaces are sometimes etched to re- move surface debris (fragments of quartz loosened by grinding, and grinding refuse embedded in microscopic irregularities], and to remove pre-determined small amounts of the surface for frequency adjustment.”’ Heis- ing used hydrofluoric acid and ammonium bifluoride as etching solutions, and they are still used successfully to- day.’ Frequency adjustment was done by placing the quartz blank in an etching tank held at a constant tem- perature, measuring the frequency, putting the blank back into the etching solution, and repeating until the crystal was on frequency.

A wet chemical etching process is very sensitive to crystalline defects. The deeper the etching progresses the more etch pits appear. Reed-Hill states, “One of the best and simplest ways of observing dislocations in crys- tals is through the use of an etching reagent which forms an etch pit in the surface of a crystal at each point where a dislocation intersects the surface.”? Etch pits and etch tunnels are the most common defects encountered when manufacturing ultra-high frequency quartz resonators.

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When natural quartz is used about 50% of the resonators have too many defects to be used. Most cultured quartz is unusable unless it is swept first. Sweeping (electrodiffu- sion) is a post-growth process designed to reduce the abundance of alkali-ions by A secondary ef- fect is the reduction of etch defects. Sweeping makes chemical polishing more time consuming and more ex- pensive, but reduces the number of etch defects to the level where blanks can be deeply e t ~ h e d . ~

Once a blank has been etched to frequency it must be plated, mounted and sealed. Aluminum, gold and silver can be used as baseplating materials. However, only alu- minum plated resonators achieve good electrical param- eters for overtone frequencies above 300 MHz. Baseplat- ing ultra-high frequency resonators is more difficult than lower frequency resonators for two basic reasons: smaller electrode sizes are needed, and final frequency adjust- ment is much more sensitive to small deviations in the electrode thickness. These are the problems that must be overcome in order to manufacture chemically polished high frequency resonators.

Etching Solutions

The etching solution used depends upon the orienta- tion of the quartz plate to be etched. AT cut resonators are etched well with saturated ammonium bifluoride. The doubly rotated cut family (FC. IT and SC cuts) etch well in solutions as reported by Vig, Brandmayer, and Filler.? The solutions contain ammonium fluoride (NH,F) and hydrofluoric acid (HF). Solutions of 4:l and 5 : l at 75OC work very well for FC, IT, and SC cut blanks.

All etching is done in teflon beakers and Teflon crystal holders. The resonators are constantly agitated. The so- lutions are maintained at temperature by placing the teflon beakers in a large bath of water. The water tank temperature is controlled by a thermostat and heater. The thermal cycling of the water is kept to less than 2’C.

All blanks are cleaned with alcohol, soap-solution, and well rinsed in deionized water before etching. The cleanliness of the blanks is very important to the etching quality. Contaminated etching solutions and dirty blanks are often the cause of poorly etched blanks.

Measuring the Blank Frequency

Before the blank can be etched to frequency a method of measuring the blank frequency accurately must be

0090-5607/85iOOO0-0306 $1.00 Cl 1985 IEEE

used. Blank frequencies are commonly measured by de- vices called air-gaps. An air-gap works by oscillating a quartz resonator between two metal electrodes that are separated by a small air gap. Higher frequency blanks need smaller electrodes to function well. Therefore, a modified air-gap with small electrodes was made for measuring high frequency blanks. The air-gap is de- signed so a variety of electrode sizes may be used, rang- ing from .020 inches to ,150 inches. The technique of measuring the frequency is also important. The fre- quency must be measured in the same place on the blank. The air-gap between the blank and the electrode must be the same each time. The frequency can be mea- sured with an oscillator or network analyzer. An oscilla- tor is preferred when available. The repeatability of the blank frequency measurements determines the degree to which the blanks can be grouped together.

Chemical Polishing Groups of Blanks to Frequency

Groups of 20 to 30 blanks are etched together. Before the first etch the blanks are inspected for scratches and chips. Scratches or chips in blanks will be propagated by the etching process; such blanks can not be used. Accu- rate frequency control during the etching process is de- pendent upon precise measurement of the etching rate. Vig, LeBus, and Filler2 reported differences between etching rates for surfaces already etched and those which have only been lapped. This effect was found to be im- portant and consequently all blanks are mass etched for 15 minutes before any frequency measurements are made. After the 15 minute mass etch the frequencies of the blanks are measured and the blank identity main- tained.

Figure 1 is a histogram of the spread in frequencies for a typical group. There were 2 3 blanks in the group. The absolute frequencies of the blanks in this stage are unim- portant because they will be grouped in frequency sev- eral times during the etching process.

After the frequencies have been measured the blanks are etched another 15 minutes. and then the frequencies are measured again. Figure 2 shows the computer print- out from this set of data. The etching rate for each blank and an average etching rate for the group are calculated. The average etching rate for the group is used during the following 4 hours, after which time another etching rate is measured.

The last set of numbers on the computer printout rep- resent the time each blank must be etched so that they are all the same frequency when they are removed. These times are used to keep the blank frequencies grouped to- gether during the etching process. The spread in fre- quency of a group of flawless blanks being etched to- gether follows a simple function.

1000 X F

1000 - (N X F) F,,,,,, - F,,,, = (- ) - F

F : Center frequency N : Spread in f-squared of group F,,,, : Upper limit of frequency F,,, : Lower limit of frequency

This function describes the spread in frequency for flawless blanks. The spread will never be less than this, it can only be greater. The frequency spread for the exam- ple group measured in f-squared started at .3 f-squared (316 KHz at 32.3 MHz). At the target frequency of 107.2 MHz, the same group of blanks with a .3 f-squared spread would vary in frequency by 3.5 ILIHz. Figure 3 shows the frequency spread of the blanks decreased to .03 f-squared at 107.2 MHz (346 KHz). The example group is a typical group; tighter grouping can be achieved when needed. The lower limit of the frequency spread is determined by the repeatability of the fre- quency measurements. The frequency measurement for blanks with etch defects is less repeatable than flawless blanks or blanks with fewer defects. Again, the limita- tion of the chemical polishing process can be traced back to the quality of the raw material. Poor etching can also result in poor frequency repeatability. but a quartz bar with defects will always result in poor frequency repeat- ability and poor resonators. Cultured quartz is unusable for chemical polishing unless it is swept first. The etch channel density must be decreased dramatically before unswept cultured quartz can be used for deep etching. If the etch channel density of premium Q pure Z cultured quartz can be made low enough for use in deep etching then more consistent electrical parameters and higher Q's can be expected from resonators manufactured from such quartz.

Frequency Limitations

Currently, the practical frequency limit of chemically polished resonators is near 400 MHz. The limitation is in reading the blank frequency just before baseplating. The frequency before baseplating must be measured accu- rately. If it is not, the plated blank will not be close enough to the final frequency to be adjusted on fre- quency. A better designed air-gap will solve this prob- lem. However, higher frequencies can be made without precise control of the frequency.

Base Plating

Base plating of ultra-high frequency resonators is more difficult than lower frequency resonators for two basic reasons. The spot sizes are smaller than conventional resonators, and final frequency adjustment is much more sensitive to deviations in electrode thickness because of the higher frequencies. High frequency fundamental res- onators up to 100 MHz have been plated with gold and silver. Aluminum is used for higher frequency funda- mentals and overtones above 250 MHz. Chemically milled masks are used in the baseplating process, pro- viding good electrode alignment and diameter control. Standard resonator design techniques may be used with confidence." Electrode diameters are determined by the customer specifications, as usual. Energy trapping has been used to reduce spurious content and maximize Q.

1985 ULTRASONICS SYMPOSIUM - 307

Final Frequency Adjustment Final frequency adjustment can be done by several

processes depending on the base plating material. Gold and silver baseplated resonators are processed in a cryo-plater. Aluminum resonators are adjusted with UV-OZONES or a liquid etching process or both. The fi- nal frequency adjustment process is very sensitive to changes in electrode thickness. A 100 MHz fundamental SC cut resonator is .018 mm thick (180,000 Angstroms). If the blank thickness is increased by 100 angstroms the resulting frequency change would be -55.56 KHz (-556 pprn), or -5.56 pprn / angstrom deposition. The frequency sensitivity per angstrom of deposition of alu- minum is nearly the same as that for quartz because the densities are nearly the same. Resonators plated with sil- ver and gold are more sensitive by the ratio of the densi- ties (factors of 4 and 7.3 respectively). At the current up- per frequency control limit of 400 MHz the blank is 45,000 angstroms thick. The frequency sensitivity to plating thickness is 22.3 ppm I' angstrom.

To set a 100 MHz fundamental SC to within 10 ppm the electrode thickness must be controlled to within 2 angstroms, if it is plated with aluminum. This extreme sensitivity requires frequency adjustment processes to be slow, or frequencies will be overshot. Added control is achieved by monitoring the frequency during the adjust- ment process. This can be done for all three types of plat- ing materials. The frequencies of gold or silver plated resonators are monitored in the cryo-plater by oscilla- tors. Aluminum plated resonators are monitored during the UV-OZONE exposure with an oscillator or a network analyzer. Figure 4 shows the frequency change of a 257.500 MHz 3rd overtone SC resonator during the fre- quency adjustment process in UV-OZONE. Aluminum plated resonators may also be adjusted with a liquid etching solution. The liquid etching solution increases the frequency while the UV-OZONE process decreases the frequency. Figure 5 shows the change in frequency using the liquid etching solution.

Frequency Shifts in Sealing Sealing of chemically polished resonators is done the

same way all other resonators are sealed. The blanks are thinner then conventional bulk wave devices and deform more easily. Some frequency shifts occur during sealing due to blank stress and vacuum. Frequency shifts up to 20 pprn are common when cold welding chemically pol- ished resonators above 100 MHz on the fundamental. The effects of the sealing process require compensation if tight set-on tolerances are to be met.

Measured Resonator Parameters The following table lists the electrical parameters and

frequency set-on for a variety of chemically polished res- onators.

FREQUENCY Q c1 Rs

70 MHz Fund AT 50,000 .0018 50

100 MHz Fund SC 40,000 ,0005 100

283 MHz Fund SC 20,000 ,0008 180

165 MHz Fund AT 32,000 .OD50 50

320 MHz 3rd SC 15,000 .0001 200 - __

1.

2.

3.

1.

2.

3.

4.

5.

6.

7.

8.

9.

Conclusions

Chemically polished resonators up to 400 MHz can be manufactured and set-on frequency close enough for use in oscillators or filters. The raw material used in making these resonators needs to be improved. The etch defect density must be decreased to improve yields and electrical parame- ters. Many steps in the chemical polishing process affect the final frequency. Good processing can keep those factors under control.

Bibliography

D. Heising, Quartz Crystals for Electrical Circuits, pp 193-194, Van Nostrand Company, 1946. J. R. Vig, J. W. LeBus and R. L. Filler. Chemically Pol- ished Quartz, Proc. 31st Annual Symposium on Fre- quency Control, pp 131-143, 1977. R. Reed-Hill, Physical Metallurgy Principles, Litton Educational Publishing, p 160, 1973. R. N. Brown, J . J . O'Conner, A. F. Armington, Sweep- ing and QMeasurements at Elevated Temperatures in Quartz, Rome Air Development Center, In-house re- port RADC-TR-79-175, May 1979. J. R. Vig, R. J. Brandmayr and R. L. Filler, Etching Studies on Singly and Doubly Rotated Quartz Plates. 33rd Annual Symposium on Frequency Control,

V. E. Bottom, Introduction to Quartz Crystal Unit De- sign, Van Nostrand Reinhold Company, 1982, pp 82- 232. J . R. Vig, Method of Adjusting the Frequency of Quartz Crystal Units, Patent Disclosure. Docket #3515, 1985. W. P. Hanson, Computer Controlled Quartz Electro- diffusion (Sweeping) with Real Time Data Collection, Prc. 38th Annual Symposium on Frequency Control,

M." P. Hanson, Chemically Polished High Frequency Resonators, Proc. 37th Annual Symposium on Fre- quency Control, pp 261-264, 1983.

pp 351-358, 1979.

pp 50-54, 1984.

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Figure 1. 4

AFTER MASS ETCH (15 MIN) NO %

5 - 21.:

Figure 2.

AFTER FINAL ETCH [TARGET= 107.200)

Figure 3.

F i . , ,Sf..! t . .

4

7

" H i "Hi MHZ " H I r lH I W H Z "HI

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MHZ W I Z M H I FIH I " H i MHZ MHZ MH2

m w i

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. . .

1985 ULTRASONICS SYMPOSIUM - 309

Figure 4.

257.7 MHz 3rd OT SC Cut

(8mm from bulb) UV-OZONE FREQUENCY ADJUSTMENT

257 .750

2 5 7.745

257. 7-10

2 57. 735

2.57.730

257 .725

257.720

257.715

257.710

257 .705

257.700

-1 t i

TIME IN HOURS

Figure 5.

FREQUENCY CHANGE OF ALUMINUM PLATED BLANKS IN AN ETCHING SOLlJTION

72HOO -

.. I ;2 ;no

7 2 7Idl

7 2 i4(I

71 7211 - - 4 , 2 ;Ill,

310 - 1985 ULTRASONICS SYMPOSIUM