Process for mass producing high frequency crystal resonators

A process for making a plurality of quartz thickness shear resonators with resonating means for use in high frequency oscillators with operating frequencies of 30 MHz or greater. The invention includes grinding a single crystal quartz wafer to achieve a highly uniform thickness, thus enhancing uniformity of the resonators produced. The invention also includes etching the quartz wafer to form a plurality of resonators, each having a support structure and a much thinner etched resonating membrane cantilevered at the support structure. In addition, the design of the support structure is such that a sloped edge occurs during membrane etching between the support structure and the membrane, thus facilitating the application of electrodes extending from the support structure to the membrane. Yet another aspect of the invention is to perform a fine-tune etching during the formation and tuning of the resonating membranes, thus further enhancing the uniformity of the resonator frequencies across the wafer.

BACKGROUND OF THE INVENTION 
This invention relates to crystal resonators and, more particularly, to a 
process for producing a plurality of high frequency crystal resonators of 
30 MHz or greater. 
Crystal resonators are used in a variety of timing dependent applications, 
such as in computers. Computers are capable of executing multiple tasks 
simultaneously. Yet such execution typically involves sharing buses, 
memory, and other common structures. Computers are therefore synchronized 
by a high frequency clock signal to maintain data integrity. Crystal 
resonators are used in computers to generate the clock signals for 
maintaining synchronous operations. 
The crystal resonator is part of an oscillating circuit, The oscillator 
circuit generally comprises a piezoelectric crystal, a housing for 
protecting the crystal, and an amplifier-feedback loop combination capable 
of sustaining oscillation. 
When a voltage is applied between certain faces of a piezoelectric crystal, 
a mechanical distortion is produced within the crystal. This phenomenon is 
known as the "piezoelectric effect". If the oscillator circuit provides an 
alternating current, the piezoelectric crystal is excited to a vibrating 
state at the frequency of the resonating circuit. When the oscillator 
circuit is energized, electrical noise will begin to excite the crystal at 
its natural resonant frequency. The crystal's output is then amplified and 
the amplified signal is fed back to the crystal. This causes the amplified 
signal to build up in strength at the resonating frequency of the crystal, 
until saturation of the circuit elements causes the overall loop gain in 
the circuit to fall to unity. This signal is fed to the output terminal of 
the oscillator. 
Although a variety of piezoelectric materials may be used for resonators, 
monocrystalline (single crystal) quartz offers certain advantages. Single 
crystal quartz has low internal mechanical loss when used as a vibrator. 
Another important feature of quartz is that its frequency of vibration is 
highly stable with changes in temperature and over long periods of time. 
A resonator is formed from single crystal quartz by first cutting the 
quartz into slabs, grinding the slabs to a desired thickness by a lapping 
process, and then polishing the slab surfaces. The choice of cut is 
usually dictated by the range of operating frequencies and the temperature 
range required for a particular application. Resonators with particular 
oblique cuts, such as AT, SC or BT, display negligible frequency variation 
with changes in temperature and operate at high frequencies. These 
resonators are generally referred to as thickness shear resonators, and 
are useful for making high frequency resonators on the order of 30 MHz or 
greater. The resonant frequency is approximately inversely proportional to 
the thickness of the wafer in the area of the vibration, so higher 
frequency devices require thinner wafers. 
Single crystal quartz must be ground down to a very thin membrane to enable 
high resonant frequencies. However, a thin membrane is a poor structure 
for attaching a resonator. It is therefore desirable to produce a 
resonator with both a vibrating membrane region and a thicker region, the 
latter region serving as a support structure for attachment purposes. 
Such a structure is obtained by grinding the crystal down to the thickness 
of the support structure, then etching the crystal to form the membrane 
portion. 
At least two problems arise in this process. First, the crystal must be 
ground and polished to a highly uniform surface topography to assure 
successful membrane etching. Second, the precise thickness of the 
resonating membrane requires high precision etching. It is desirable to be 
able to produce the resonator from a wafer of single crystal quartz. Until 
now, the grinding process has not provided a quartz crystal wafer with 
sufficient thickness uniformity suitable for the mass-production of 
crystal resonators from wafers. Second, the standard etching process lacks 
the precision required to mass-produce resonators on a wafer with 
consistent frequencies. The production of crystal resonators with both an 
etched membrane and a support structure consequently has necessitated 
grinding and etching each unit individually. This process is 
time-consuming and costly. 
One such prior art crystal resonating structure used in high frequency 
resonators is an "inverted mesa structure". "Inverted mesa structure" is a 
term of art referring to crystal resonators with a thin central membrane 
completely surrounded by a thicker support structure. Electrodes deposited 
on the membrane cause it to vibrate. 
Inverted mesa structures have at least one disadvantage in addition to high 
production cost. The oscillating wave traveling outward from the vibrating 
(electrode) region of the membrane must be diminished to a very low 
amplitude by the time it reaches the surrounding support structure. The 
membrane must therefore be large relative to the electrode area to avoid 
undesirable damping of the resonance. Additional area is needed for the 
thicker supporting region, placing a physical constraint on the minimum 
size of the resonator. 
SUMMARY OF THE INVENTION 
The invention is a process for mass-producing high frequency crystal 
resonators of 30 MHz or greater. The resonators utilize single crystal 
quartz resonating in a thickness shear mode, etched to form a thin 
membrane cantilevered from a thicker crystal support structure. 
In contrast to inverted mesa structures, the support structure of the 
present invention does not surround the etched membrane. Rather, it 
borders only one side of the membrane and provides a base from which the 
membrane projects. Consequently, a smaller support structure enables 
smaller resonator dimensions than are possible with inverted mesa 
structures having similar sized membranes. Another major advantage of the 
cantilever arrangement of the invention is that it enables mounting to be 
relatively free from mechanical stress. 
The method for making the present invention utilizes a high precision 
process similar to semiconductor chip fabrication, thus allowing the 
invention to be mass-produced while still achieving the precise dimensions 
required for high frequency applications. This method presents a distinct 
advantage over prior art, since single crystal quartz resonators do not 
have to be lapped and etched individually. In processing a plurality of 
resonators in a wafer, the cost per resonator unit is reduced. 
Briefly, a slab of quartz suitable for high frequency applications is 
lapped and polished to form a smooth wafer of uniform thickness. A 
sequence of metalizing and etching steps then define a plurality of 
resonator patterns on the wafer face, each having a peninsular resonating 
membrane cantilevered from a thicker support structure attached to the 
wafer. 
The membranes are then tuned to the correct resonating frequency and 
electrodes are applied. The resonators are tuned a second time, then 
broken out of the wafer and packaged in a housing. 
Other features and advantages of the invention either will become apparent 
or will be described in connection with the following, more detailed 
description of a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
The following relatively detailed description is provided to satisfy the 
patent statutes. However, it will be appreciated by those skilled in the 
art that various changes and modifications can be made without departing 
from the invention. The following description is exemplary, rather than 
exhaustive. 
FIG. 8 is a flow diagram of a preferred embodiment of the process. 
Reference to this flow diagram will be helpful in understanding the 
following. 
Lapping and Polishing Step 13 
A wafer of AT cut single crystal quartz is mounted to a smooth, highly 
uniform and flat surface such as glass. Grinding and polishing apparatus 
is used to lap and polish one face of the wafer. 
The wafer is then flipped over and the opposing face lapped and polished to 
provide the wafer with its specified thickness dimension, corresponding to 
the desired thickness of the bases of the resonators being produced. 
Metalize and Define Resonator Shapes 16 
In a first metalization step, opposing sides of the wafer are coated with a 
thin layer of a conductive metal. In a preferred embodiment, gold and a 
chromium adhesion layer are applied to the wafer by evaporization. Gold 
displays good conductivity and favorable masking characteristics, although 
other conductive metals may be substituted without changing the nature of 
the invention. The presence of chromium improves the adhesion of the gold 
to the quartz crystal (SiO.sub.2). The use of adhesion layers is well 
known and need not be disclosed in further detail. 
The metal layer on the wafer is then etched to define a plurality of 
resonator shapes. In a preferred embodiment, a pattern of resonator shapes 
such as those represented in FIG. 5 is masked onto the wafer and metal 
etched using standard photolithographic techniques. FIG. 2(a) depicts one 
of the resonator shapes 1, which are generally rectangular and are 
oriented so that their shorter sides are parallel to the x-axis of the 
crystal. Alternatively, the resonator shapes are disposed so that their 
longer sides are parallel to the x-axis, but the former orientation 
provides slightly better definition of the mounts and edges of the 
resonator. 
In the preferred embodiment, images of two thin gold contacts 4 and 4' are 
formed on both sides of each resonator on the region of the crystal that 
will eventually form the resonator support structure 3 by 
photolithographic techniques. 
Applying Additional Gold 
In one embodiment, a thicker layer of gold is plated onto the gold contacts 
at the posterior region of each resonator, forming two contact pads 5 and 
5'. The contact pads are applied prior to chemical etching and provide 
rigidity and strength at the resonator's point of attachment to the wafer. 
This point of attachment can be observed in FIG. 5. The strengthening 
function of the contact pads 5 and 5' may not be necessary if etching 
technology improves. It should be understood that conductive metals other 
than gold may be used, and may be applied by methods other than plating 
without departing from the spirit of the invention. 
Chemically Etching Shapes 18 and Membranes 19 
Next, the wafer is chemically etched through the crystal to further define 
the resonator shapes. A chemical etch of the metal layer is performed to 
provide the image of the membrane of each resonator. A second quartz etch 
is then performed to carve out the membranes 2. 
AT cut crystals have the tendency to form slanted angles during chemical 
etching. Where these angles are acute or right, continuous metalization 
from one level of the crystal to the next becomes difficult. It is 
therefore desirable to shape the aforementioned support structure so that 
at least one edge on each face between the membrane and support structure 
will always be obtuse. In a preferred embodiment, the support structure 3 
resulting from the chemical etching step 19 is U-shaped. In this U-shaped 
configuration, the crystal forms slopes 6 and 6' on the inside edges of 
the U-shaped support structure 3. These edges 6 and 6' each provide a 
gradual slope between the support structure 3 and the membrane 2, thus 
facilitating metalization later in the process. Shapes other than the 
U-shape may be used which result in at least one sloped edge on each side 
of the resonator between the support structure and the membrane, such as a 
rounded or V-shape. Chemical etching techniques are well-known and need 
not be described further. 
Probing Membrane Sampling 20 and Tune Etching 21 
After the membranes 2 are formed, the wafer is probed to obtain a sampling 
of membrane resonating frequencies across the wafer. In the preferred 
embodiment membranes are sampled with a probe arrangement to obtain a 
range of frequencies representative of the wafer. If the range of 
frequencies falls below the desired frequency, the wafer is further etched 
21 to reduce membrane thicknesses. 
Probe Etching Each Membrane 22 
Slight nonuniformities in wafer thickness may result in frequency 
variations across the wafer. To correct this variation, each resonator 
membrane is probed and its frequency measured. An etching time is then 
calculated for each resonator based upon the difference between its 
measured frequency and the desired frequency. The etching time corresponds 
to the time required to further etch the membrane to the desired 
frequency, and will depend on the etching solution used. A person skilled 
in the art would possess the knowledge to perform such a calculation. 
Fine-Tune Etching Membranes 23 
Once etching times have been calculated and stored for each resonator, a 
fine-tuning etch is performed. In the preferred embodiment, etching 
solution is first applied to the resonator associated with the longest 
etching time, next to the resonator associated with the second-longest 
etching time, and so forth until etching solution has been applied to all 
resonators in need of further etching. By this process, each resonator 
will be etched for a time proportional to its deviation from the desired 
frequency, resulting in greater uniformity of frequencies across the 
wafer. 
Applying Electrodes and contacts 24 
Once fine-tune etching is complete, resonating means are applied to each 
resonator. In the preferred embodiment, another coating of thin gold with 
a chromium adhesion layer is applied to the resonator extending from the 
U-shaped support structure 3 to the membrane 2 over the sloped edges 6 and 
6'. This gold-chromium layer forms a contact 13 coupling one of the pads 4 
on one side of the resonator to a corresponding gold pad 4' on the 
opposing side. Aluminum electrodes 7 and 7' are then applied to opposing 
sides of the resonator. They are applied overlapping the gold-chromium 
using standard masking and etching techniques well-known to artisans. 
Other means of applying the electrodes may be used, and other conductive 
materials may be substituted for the aluminum. Furthermore, it should be 
understood that resonating means other than a pair of electrodes may be 
used without changing the nature of the invention. For example, the 
resonator shown in FIG. 4 has two electrodes and 7' on one face, and a 
third electrode 7" on the opposing face. This type of electrode 
configuration would be used in monolithic crystal filter applications. 
Probing and Marking Membranes 25 
Once the resonating means have been applied, each crystal resonator is 
again probed to measure its resonating frequency with the resonating means 
in place. Resonators with frequencies and/or electrical parameters falling 
outside of an acceptable range are marked and later discarded. 
Breaking Out Resonators 26 and Mounting to Header 27 
The unmarked crystal resonators are broken out of the wafer and packaged as 
depicted in FIG. 6. First, each resonator is mounted to a header 8. Lead 
terminals 9 from the header are coupled to the resonating means 7 by 
conductive epoxy or other coupling means. 
Fine-Tuning Resonator 28 
A final frequency measurement is made for each mounted resonator and 
fine-tuning is performed by mass loading the resonator, an evaporated 
metal for example. 
Sealing Lid to Header 29 
After fine-tuning the crystal is then encapsulated by a lid sealed to the 
header. In a preferred embodiment, the crystal resonator is covered with a 
cylindrical cap 10 hermetically sealed to the header. The container 
provides protection to the crystal from moisture and contamination. 
Methods for mounting and sealing crystal oscillators are well-known in the 
art and need not be discussed in detail. 
Adding to Oscillator Circuit 30 
The packaged resonator is then incorporated into an oscillator circuit 
(FIG. 7), including an amplifier 11 to amplify the oscillating waveform 
generated by crystal, and a feedback circuit 12 for feeding a portion of 
the amplifier output back through the resonator and into the amplifier 
input until the amplifier amplitude has stabilized. 
As mentioned at the beginning of the detailed description, applicant is not 
limited to the specific embodiment(s) described above. Various changes and 
modifications can be made. The claims, their equivalents and their 
equivalent language define the scope of protection.