Precision Laser Trimmable Resistors
By Kevin D. Christian, Technical Marketing Manager, ATC // AVX Thin Film Technologies

ATC // AVX Thin Film Technology offers unique design solutions not feasible using traditional passive resistors when space savings and tolerance are critical. By using photolithographic processing to produce laser ladder trimming structures, 0.05% tolerances or matching can be obtained. This is a notable departure from electrical sorting used in standard passive components to cull out tight tolerance product. Through the use of BGA (Ball Grid Array), LGA (Land Grid Array), and wire bond terminations, high-density input\outputs per device are possible. Key applications include medical, military, sensor, and optical applications.

Overview
Precision laser trimmable thin film resistors offer circuit designers options and capabilities not possible with traditional passive components. ATC // AVX Thin Film Technology has developed sputtered SiCr resistor material with 300-1500 ohms/sq. sheet resistance, allowing production of resistors up to 20MOhms rated for 1000V use with TCRs of ± 25ppm/ºC. Please see Table 1 and Figure 1 for typical resistor materials’ properties and designs. Laser trim ladders are employed to obtain reproducible and high yielding passive designs. When combined with transparent glass or quartz wafers, the end customer also has the option of laser trimming the ladder in circuit to obtain real time circuit tuning.

Laser Trimmable SiCr Resistors
Continuous electronic design demands of more functionality in a smaller space have necessitated higher sheet Rho than available with conventional NiCr or TaN resistor technology. SiCr material has traditionally provided sheet Rhos of 1000-1500 ohms per square with reasonable TCRs. Previously this material has had limited availability from commercial and captive foundries. ATC//AVX Thin Film Technology has developed a stable SiCr material with tunable sheet Rho from 300 to 1500 ohms per square and TCRs from -200 ppm to +/- 10 ppm. These resistors have been successfully produced on 150mm silicon, glass or quartz wafers in a custom designed sputter tool, yielding resistivity standard deviation of 3%.

Combined with laser trimming, precision resistors can be designed with 1% or better absolute resistance tolerance and 0.05% tracking between matched resistors. To achieve the desired high sheet resistance and low TCR processing requires optimized doping and annealing. To guarantee resistor stability, ladder structures can be designed so all trimmed rungs have to be completely cut, thus eliminating potential instabilities at the edges of that can be encountered using plunge cut designs. To maintain environmental stability of the finished products, double passivation over the resistor surface is typically employed; the first layers being silicon nitride with the second layer of BCB. Because of these passivation layers, the typical stability is better than 0.05% on a 1000 hour life-test, as shown in Figure 2.

As a design example of the ratio and tolerance possible, Figure 3 shows the front and back side view of a 1mm BGA pitch, 500K:5MOhms, +/-0.1% ratio tolerance SiCr resistor. These same design techniques can also be employed to construct precision resistor pairs or networks fabricated with matching tolerance to 0.01% and absolute tolerance to 0.05%. Utilizing transparent glass and quartz substrates with flip chip technology, resistors have been fabricated for use in active circuitry trimming from the backside through the wafer of the flipped die.

Figure 4 is a high magnification detailed view of the SiCr ladder structure and laser trim cut which allows stable reproducible electrical properties to be obtained in volume production. Note, as discussed earlier, the complete cut ladder design structure, guaranteeing optimum reliability for this medical implantable resistor.

Termination Styles and Technology
Termination systems of lead based or lead free BGA, and LGA are available. For BGA typical solder ball diameter is 0.3mm on a minimum 0.5mm pitch. The minimum ball diameter available is 0.15mm. The wire bondable termination options are Al or Au (minimum wire diameter 1.5µm) for top metal layer versions. Extensive development was devoted to optimizing the UBM (Under Bump Metallurgy). To decouple the mechanical stresses generated at the termination, an aluminum compliance layer is utilized. Critical to the termination pad design is that most of the surface of the adhesion layer is on the substrate, maximizing the mechanical stability. Mechanical stress generated by the UBM could induce cracking of the more brittle substrates like glass or quartz. To overcome this, a TiW(600Å)\Al(3µm) compliance layer between the UBM and the substrate is used. The entire UBM surface lays on the top of the compliance layer for optimal results.

Summary
ATC//AVX Thin Film Technology laser trimmable resistors offer unique design solutions not feasible using traditional passive components when space savings, signal integrity, and tolerance are critical. SiCr material offers resistors up to 20MOhms rated for 1000V use with TCRs of ± 25ppm/ºC. To achieve high I\O terminations, styles including ball grid array, land grid array, and gold and aluminum wire bond are offered. These technologies are currently employed in implantable medical, military communication, sensor and reference applications as well as commercial applications.

AVX
www.avx.com
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