Method for manufacturing a frequency control device

A method of manufacturing is employed to produce a size-reduced frequency control device (300), according to the present invention. A temperature compensation circuit (304) and an unsealed piezoelectric element (302) are disposed on a substrate (306) such that the compensation circuit and the unsealed piezoelectric element are electrically connected. A hermetic seal (318) is established between a lid (308) and the substrate such that the unsealed piezoelectric element and at least a temperature sensitive portion of the temperature compensation circuit occupy a sealed environment (320). In this manner, dimensions (322, 324) of the frequency control device are reduced.

FIELD OF THE INVENTION 
The present invention relates generally to frequency control devices and, 
in particular, to a method for manufacturing frequency control devices. 
BACKGROUND OF THE INVENTION 
Frequency control devices are known to include temperature-compensated 
crystal oscillators (TCXO). A typical TCXO utilizes piezoelectric 
materials and temperature compensation circuitry to produce reliable 
oscillator output (e.g., high frequency waveforms) under varying 
environmental conditions. Such devices are commonly found in portable 
radio frequency (RF) communication equipment, such as cellular telephones. 
As consumer demand continually drives down the size of cellular 
telephones, the need for TCXO's having smaller dimensions becomes greater. 
FIG. 1 illustrates a cross-sectional block diagram of a prior art TCXO 
(100) including a sealed piezoelectric element (102), temperature 
compensation circuitry (104), a substrate (106), input/output pads (107), 
and a device lid (108). The sealed piezoelectric element (102) includes a 
piezoelectric crystal (110), conductive adhesive (112), a crystal package 
(114), and a crystal package lid (116). Furthermore, a hermetic seal (118) 
disposed between the crystal package (114) and crystal package lid (116) 
creates an isolated crystal environment (120). 
When a voltage is applied across the crystal (110), the crystal (110) 
resonates to produce the oscillator output. Also, the resonant frequency 
changes (i.e., drifts about a nominal frequency) responsive to changes in 
the temperature of the isolated crystal environment (120). A temperature 
sensing device (not shown) provides information to the temperature 
compensation circuitry (104) regarding the isolated crystal environment 
(120). As the temperature within the isolated crystal environment (120) 
fluctuates, the temperature compensation circuitry (104) modifies circuit 
parameters to ensure minimal frequency drift in the oscillator output. 
Obviously, the height (124) and width (122) of the TCXO (100) are partially 
dependent upon the sealed piezoelectric element (102). That is, the finite 
thicknesses of the crystal package (114) and the crystal package lid (116) 
add to the overall dimensions of the TCXO (100). Typical dimensions (in 
millimeters) for the TCXO (100) shown are 11.4.times.13.97.times.3.58. 
While these dimensions may appear to be quite small, the demand for 
smaller cellular telephones necessitates TCXO's having even smaller 
physical dimensions. Therefore, a need exists for frequency control 
devices which overcome prior art dimensional limitations, but which meet 
the same performance criteria of their larger counterparts.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Generally, the present invention provides a method for manufacturing a 
frequency control device. This is accomplished by placing a compensation 
circuit, having a temperature-sensitive portion, on a substrate. An 
unsealed piezoelectric element is mounted onto a portion of the substrate, 
such that electrical connection is provided between the compensation 
circuit and the piezoelectric element to produce an untuned frequency 
control device. After frequency tuning the untuned frequency control 
device, a hermetic seal is provided around at least the unsealed 
piezoelectric element and the temperature-sensitive portion of the 
compensation circuit. With such a method, dimensional limitations of prior 
art frequency control devices are overcome. 
The present invention can be more fully described with references to FIG.'s 
2 and 3. FIG. 2 illustrates a logic diagram which may be employed to 
manufacture a TCXO in accordance with the present invention. At step 201, 
a compensation circuit is placed on a substrate. The compensation circuit 
may be an integrated circuit (IC) or discrete components and include a 
temperature sensitive portion or a discrete temperature-sensitive device. 
In a preferred embodiment, the compensation circuit includes an IC 
containing a temperature sensitive portion and two chip capacitors, and 
the substrate is fabricated from a ceramic material such as co-fired 
alumina formed into a "bathtub" shape (i.e., a substantially flat base 
with at least four side walls). Additionally, a brazed metal ring is 
attached to the substrate at the top of the side walls. After placement of 
the IC and chip capacitors with silver-filled epoxy, the IC is wirebonded 
to metallized input/output connection pads provided on the substrate. 
At step 203, an unsealed piezoelectric element is compliantly mounted to a 
portion of the substrate. In a preferred embodiment, the unsealed 
piezoelectric element is any one of an AT-cut strip quartz blank, an 
AT-cut round quartz blank, or a GT-cut quartz blank having electrodes on 
opposite, substantially-parallel faces of the crystal, such that 
electro-mechanical connections can be made at either end of the crystal. 
In the case of a double-end mount, a compliant adhesive, such as 
silver-filled silicone or epoxy, is used to mount the crystal to prevent 
undue stresses on the crystal. 
Having mounted the crystal (203), the untuned frequency control device is 
frequency tuned (205) using the compensation circuit as an electrical 
load. This frequency tuning is achieved through the deposition of 
additional metal, sometimes referred to as mass loading, on the top 
electrode of the compliantly-mounted quartz crystal. Such tuning typically 
lowers the variance of the oscillator output to less than 18 parts per 
million from the nominal frequency. Note that in prior art frequency 
control devices, the sealed piezoelectric element (102) required the use 
of an artificial electrical load unrelated to the actual electrical load 
for tuning. Accordingly, the present invention provides an improved tuning 
method since the tuning can be performed under actual loading conditions. 
After frequency tuning, the unsealed piezoelectric element, and at least 
the temperature sensitive portion or discrete temperature-sensitive device 
are hermetically sealed (207). In a preferred embodiment, a parallel 
seamweld between a metal lid and the brazed metal ring attached to the 
substrate creates a hermetic seal such that the unsealed piezoelectric 
element and temperature compensation circuit (IC-based) reside within the 
sealed environment. 
At step 209, the sealed frequency control device is temperature compensated 
over a predetermined operating range. This is accomplished by operating 
the device over the predetermined range and storing frequency output 
versus temperature readings into, for example, an electrically-erasable, 
programmable read-only memory (EEPROM) included in the temperature 
compensation circuitry. An algorithm, stored as software in memory within 
the temperature compensation circuitry, calculates those capacitor values 
needed to compensate for frequency variations at each tested temperature. 
The resulting device is a TCXO, manufactured in accordance with the 
present invention. 
FIG. 3 illustrates a TCXO (300) manufactured in accordance with a preferred 
embodiment of the present invention. The TCXO (300) includes an unsealed 
piezoelectric element (302), a temperature compensation IC (304), a 
substrate (306), input/output pads (307), and a metal lid (308). A 
hermetic seal (318), established by seamwelding the metal lid (308) to the 
substrate (306) in an inert atmosphere, encloses the unsealed 
piezoelectric element (302), the temperature compensation IC (304), and 
chip capacitors (not shown) within the same environment (320). The 
temperature compensation IC (304) is mechanically secured to the substrate 
(306) using an epoxy (not shown). Furthermore, the temperature 
compensation IC (304) is electrically wirebonded to metallization on the 
substrate (306). Tungsten-filled feed-through holes are used to complete 
the connection of the compensation IC (304) to the input/output pads 
(307). 
The unsealed piezoelectric element (302), preferably comprising an AT-cut 
quartz crystal strip (310) having a top electrode (314) and a bottom 
electrode (316), is electrically and mechanically connected to metallized 
pads (not shown) on top of ledges formed in the substrate using compliant 
conductive adhesive layers (312,313). Note that the compliant conductive 
adhesive (313) is wrapped over the edge of the crystal strip (310) to 
establish contact with the top electrode (314). In this configuration, the 
crystal strip (310) and the compensation IC (304) are electrically 
coupled, such that the frequency tuning (205) and temperature compensation 
(209) can occur as described above. Through the elimination of the 
redundant crystal packaging (e.g., the crystal package (114) and the 
crystal package lid (116) of FIG. 1), the height (324), width (322), and 
depth (not shown) of the TCXO (300) are reduced. A TCXO (300) manufactured 
in accordance with the present invention can have dimensions (in 
millimeters) of 8.89.times.8.89.times.2.79, representing less than 39% 
total volume of prior art TCXO's (100). 
The present invention provides a method for manufacturing a frequency 
control device (e.g., a TCXO). With such a method, the double-packaging of 
piezoelectric crystals, found in prior art devices, is eliminated. This is 
accomplished by configuring the hermetically-sealed package such that the 
unsealed piezoelectric element and at least the temperature sensitive 
portion of the temperature compensation circuit are enclosed within the 
same environment. In this manner, frequency control devices having reduced 
dimensions can be manufactured.