Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – Passive components in ics
Reexamination Certificate
1999-07-21
2001-07-10
Loke, Steven (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
Passive components in ics
C257S380000
Reexamination Certificate
active
06259151
ABSTRACT:
FIELD OF INVENTION
The invention addresses the general problem of thin film microcircuit resistor fabrication and adjustment, and the specific problem of improving laser trimming to achieve circuit parametric values for such resistors.
DISCUSSION OF PRIOR ART
The technique of laser trimming to bring thin film resistors to their circuit parametric values is well-known in the art. See Elshabini-Riad and Barlow, sections 5.6.4 through 5.6.7, Thin Film Technology Handbook, McGraw-Hill (1998). Current practice requires the tight process control of the thickness and optical properties of layers of the semiconductor wafer that lie below the thin film resistor layer, in an effort to avoid irregularities caused by non-uniform laser energy beam interference.
Consider the structure of a typical precision resistor as shown in FIG.
1
. It has a silicon handle substrate
90
that is oxide bonded
80
to a silicon device substrate
70
. A thermal oxide
60
is grown on the device surface. Layers of deposited dielectric materials
50
cover the thermal oxide
60
. A precision trimmable resistor material
40
of NiCr or SiCr is deposited on the field dielectric layers
50
. Another oxide layer
30
is deposited on resistor layer
40
. The top oxide layer
30
is planarized and coated with a passivation nitride layer
20
. Incident laser light
10
passes through all the layers until it is finally reflected or absorbed by the handle substrate
90
. The reflections are shown by the arrows
55
,
65
,
75
and
85
. Note that the path
10
of the laser beam is altered as it passes through the different materials that form the device.
After the incident laser beam passes through the NiCr or SiCr resistor, the beam encounters a number of other layers. The layers and the interface of one of the layers with another layer have different indices of refraction. As such, the laser beam is deflected from its incident path in accordance with Snell's law: the ratio of the sine of the angle of incidence to the sine of the angle of refraction equal the ratio of the refractive indices of the materials. The laser beam is reflected by one or more of the lower layers onto the reverse surface of the resistor material. The reflected laser may be strong enough to chemically alter some of the resistor material and thereby alter the resistance in a manner not desired.
Thus, the quality of laser trim of precision thin film resistors can vary due to constructive and destructive interference of the laser's beam energy with layers above and below the resistor material. Trim quality is affected by optical properties and thickness of layers above and below the NiCr resistor. This effect has been confirmed both empirically and with computer simulation of optical effects. Consequently, many production wafer lots may be delayed due to poor laser trim. Laser trim quality is subjectively evaluated and some wafer lots are scrapped as a result of poor laser kerf quality, which can vary across a wafer or even across a single die. As a result of this problem, it may take an operator or an automatic laser trimming machine several cycles of operation, measurement and retrimming in order to trim the resistor to the precise resistance required by the integrated or thin film circuit.
U.S. Pat. No. 5,608,257 relates to laser cutting of a fusible link. It acknowledges the problem posed by a complex of underlying reflective and refractive layers. However it provides no solution to the problem posed by precision resistors.
SUMMARY
The invention inserts a refractory material which acts as an optical barrier below the thin film resistor layer, and a dielectric film separating the refractory material from the thin film resistor in order to preserve the resistor's electrical behavior in the circuit. These layers eliminate the interaction of the laser trim with the lower layers of the semiconductor wafer including the silicon and dielectric layers, eliminating the need to control tightly the thickness and optical properties of these layers.
The proposed invention requires the addition of a refractive layer, and a dielectric layer, both below the laser-trimmed resistor films. This innovation ensures quality trim by eliminating laser energy interaction with device silicon and bond oxide layers below the barrier refractive layer. Since such device silicon and bond oxide lower layers can no longer affect the local intensity of the laser energy, the uniformity of laser trim and kerf is improved.
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patent: 4682204 (1987-07-01), Shiozaki et al.
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patent: 4853758 (1989-08-01), Fischer
patent: 4935801 (1990-06-01), McClure et al.
patent: 5021867 (1991-06-01), Przybysz
patent: 5260597 (1993-11-01), Usuda et al.
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patent: 5554873 (1996-09-01), Erjeljac et al.
patent: 5608257 (1997-03-01), Lee et al.
patent: 6090678 (2000-07-01), Maghsoudnia
Tadanori Yamaguchi, Sudarsan Uppili, June S. Lee, Galen H. Kawamoto, Taner Dosluoglu and Shaun Simpkins, “Process and Device Characterization for a 30-GHz ft Submicrometer Double Poly-Si Bipolar Technology Using BF-2-Implanted Base with Rapid Thermal (contd) Process”, IEEE Transactions on Electron Devices, vol. 40, No. 8, Aug. 1993.
Intersil Corporation
Jaeckle Fleischmann & Mugel LLP
Loke Steven
Owens Douglas W.
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