Electric heating – Metal heating – By arc
Reexamination Certificate
2002-01-31
2003-03-18
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121600
Reexamination Certificate
active
06534743
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable
TECHNICAL FIELD
The present invention relates to laser trimming and, in particular, to laser trimming thick or thin film resistors with a uniform spot from a solid-state laser.
BACKGROUND OF THE INVENTION
Conventional laser systems are typically employed for processing targets such as electrically resistive or conductive films of passive electrical component structures, such as film resistors, inductors, or capacitors, in circuits formed on ceramic or other substrates. Laser processing to trim the resistance values of film resistors may include passive, functional, or activated laser trimming techniques such as described in detail in U.S. Pat. No. 5,685,995 of Sun et al.
The following background is presented herein only by way of example to thick film resistors.
FIG. 1
is an isometric view of a work piece
10
, such as a prior art thick-film resistor
10
a,
forming part of a hybrid integrated circuit device, and
FIG. 2
is a cross-sectional side elevation view depicting thick-film resistor
10
a
receiving a conventional laser output pulse
12
. With reference to
FIGS. 1 and 2
, a conventional thick-film resistor
10
a
typically comprises a thick film layer
14
of a ruthanate or ruthinium oxide material extending between and deposited on portions of the top surfaces of metallic contacts
16
. Layer
14
and metallic contacts
16
are supported upon a ceramic substrate
18
, such as alumina. Modern ruthinium-based thick film pastes have been optimized to be stable after laser trimming with a 1.047 micron (&mgr;m) Nd:YLF laser or a 1.064 &mgr;m Nd:YAG laser.
With particular reference to
FIG. 1
, the resistance value of resistor
10
a
is largely a function of the resistivity of the resistor material and its geometry, including length
22
, width
24
, and height
26
. Because they are difficult to screen to precise tolerances, thick-film resistors are intentionally screened to lower resistance than nomimal values and trimmed up to the desired values. Multiple resistors
10
a
having approximately the same resistance values are manufactured in relatively large batches and then subjected to trimming operations to remove incremental amounts of the resistor material until the resistance is increased to a desired value.
With particular reference to
FIG. 2
, one or more laser pulses
12
remove substantially the full height
26
of the resistor material within the spot dimensions
28
of laser output pulses
12
, and overlapping spot dimensions
28
form a kerf
30
. A simple or complex pattern can be trimmed through the resistor material of a resistor
10
a
to fine tune its resistance value. Laser pulses
12
are typically applied until resistor
10
a
meets a predetermined resistance value.
FIG. 3
is an isometric view of a portion of a prior art resistor
10
showing for convenience two common pattern trim paths
32
and
34
(separated by a broken line) between metal contacts
16
. “L-cut” path
32
depicts a typical laser-induced modification. In an L-cut path
32
, a first removal strip
36
of resistor material is removed in a direction perpendicular to a line between the contacts to make a coarse adjustment to the resistance value. Then an adjoining second removal strip
38
, perpendicular to the first removal strip
36
, may be removed to make a finer adjustment to the resistance value. A “serpentine cut” path
34
depicts another common type or laser adjustment. In a serpentine cut
34
, resistor material is removed along removal strips
40
to increase the length of film path
42
. Removal strips
40
are added until a desired resistance value is reached. Removal strips
36
,
38
, and
40
are typically the width of a single kerf
30
and represent the cumulative “nibbling” of a train of overlapping laser pulses
12
that remove nearly all of the resistor material within the prescribed patterns. Thus, when the trimming operation is completed, the kerfs
30
are “clean” with their bottoms being substantially free of resistor material such that the substrate
18
is completely exposed. Unfortunately, the formation of conventional clean kerfs
30
necessitates a slight laser impingement of the surface of substrate
18
.
As film resistors become smaller, such as in the newer 0402 and 0201 chip resistors, smaller spot sizes are needed. With the 1.047 &mgr;m and 1.064 &mgr;m laser wavelengths, obtaining smaller spot sizes while employing conventional optics and maintaining the standard working distance (needed to avoid ablation debris and to clear the probes) and adequate depth of field (ceramic, for example, is not flat) is an ever-increasing challenge. The desire for even more precise resistance values also drives the quest for tighter trim tolerances.
An article by Albin and Swenson, entitled “Laser Resistance Trimming from the Measurement Point of View,”
IEEE Transactions on Parts, Hybrids, and Packaging;
Vol. PHP-8, No. 2, June 1972, describes measurement issues and the advantages of using a solid-state laser for trimming thin film resistors.
Chapter 7 of an NEC instruction manual describes the challenges encountered when using an infrared (IR) Gaussian beam to trim resistors, particularly thick film resistors. Heat-affected zones (HAZ), cracks, and drift are some of the problems that are addressed.
An article by Swenson et al., entitled “Reducing Post Trim Drift of Thin Film Resistors by Optimizing YAG Laser Output Characteristics,”
IEEE Transactions on Components, Hybrids, and Manufacturing Technology;
December 1978, describes using green (532 nm) solid-state laser Gaussian output for trimming thin film resistors to reduce HAZ and post trim drift.
U.S. Pat. Nos. 5,569,398, 5,685,995, and 5,808,272 of Sun and Swenson describe the use of nonconventional laser wavelengths, such as 1.3 &mgr;m, to trim films or devices to avoid damage to the silicon substrate and/or reduce settling time during functional trimming.
International Publication No. WO 99/40591 of Sun and Swenson, published Aug. 12, 1999, introduces the concept of resistor trimming with an ultraviolet (UV) Gaussian laser output. With reference to
FIG. 4
, they employ the UV Gaussian laser output to ablate an area
44
of the surface of film resistors to maintain their surface area and conserve their high frequency response characteristics. By intentionally retaining a depth
46
of resistor film in the trimmed areas
44
, they avoid having to clean the kerf bottoms
48
and substantially eliminate the interaction between the laser output and the substrate
18
, thereby eliminating any problems that might be caused by such interaction. Unfortunately, surface ablation trimming is a relatively slow process because the laser parameters must be carefully attenuated and controlled to avoid complete removal of the resistor film.
Microcracking is another challenge associated with using a solid-state Gaussian laser beam for trimming resistors. Microcracks, which often occur in the center of a kerf
30
on the substrate, may extend into the resistor film causing potential drift problems. Microcracks can also cause a shift associated with the temperature coefficient of resistance (TCR). Such microcracking is more pronounced in the newer 0402 and 0201 chip resistors that are fabricated on thinner substrates
18
, with a typical height or thickness of about 100 to 200 &mgr;m, compared to those of traditional resistors. Microcracking in these thinner-substrate resistors can propagate and even result in catastrophic failure or physical breakage, particularly along the trim kerf
30
, of the resistor during subsequent handling. Microcracking can also create “preferred” break lines that are more pronounced than the desirable break prescribed break lines in snapstrates.
Improved resistor trimming techniques are, therefore, desirable.
SUMMARY OF THE INVENTION
An object of the invention is, therefore, to provide an improved system and/or method for solid-state laser trimming.
Another object of the invention is to provide spot sizes of less than 20 &mgr;m to trim
Harris Richard S.
Sun Yunlong
Swenson Edward J.
Electro Scientific Industries Inc.
Elve M. Alexandra
Stoel Rives LLP
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