Ultrastable Thin Film TiC Resistors for Space Applications Robert C. Cole1 and Gouri Radhakrishnan2 The Aerospace Corporation 1Parts, Materials and Processes Dept. Electronics and Sensors Division May 5, 2010 2 Materials Science Dept. Space Materials Laboratory

Topics of Discussion Thin Film Resistor Problems TiC thin films Fabrication Details Testing Summary

Thin Film Resistors - Overview Thin film resistors are widely used for high performance hybrid microcircuits. Integral part of the metal interconnect system. Offer greater design latitude to the circuit designer; can be laser trimmed with high precision and show good stability in hermetic packages. For RF circuit applications, thin film resistors offer a reduction in electrical circuit parasitics by reducing unwanted inductance and capacitance. Package size and weight can be significantly reduced using hybrid technology as compared to discrete resistor use on PC boards. This reduction in weight and increased performance are necessary for meeting today’s Space Systems Requirements.

Thin Film Resistors – Status and Problems The low TCR (Temperature Coefficient of Resistance) of Nichrome (<50 PPM) has made it the material of choice for hybrid microcircuits and resistor networks. Issues with Nichrome: Sensitivity to moisture under typical bias loads in circuit applications. Sensitivity to short duration high power transient voltage pulses. Worst case circuit analysis has predict that resistor failures from these high power pulses can result in system failures. Tantalum Nitride Moderate TCR ~ 100 PPM Good stability to moisture under bias loads Film properties degrade at temperatures above 125°C

Titanium Carbide Thin Films Poly-Si SiO2 (2) SiO2 (1) Poly-Si (0) Si3N4 Si Hybrid pulsed laser deposition and Si-surface-micromachining process for integrated TiC coatings in moving MEMS G. Radhakrishnan , R.E. Robertson, R.C. Cole, P.M. Adams Appl. Phys. A 77, 175–184 (2003) Poly-Si (1) TiC (2) N+ Poly-Si (0) SiO2 (1) Silicon Nitride TiC (1) Silicon substrate

Titanium Carbide Thin Films TiC has a very high melting point (3500 K), a high resistance to chemical attack and most importantly low sheet resistivity (100 /sq) Pulsed Laser Deposition (PLD) Titanium Carbide was incorporated into a standard Silicon/MEMS wafer processing flow. Excellent adhesion properties were observed to sapphire and silicon substrates. Readily etched by standard Reactive Ion Etch gases. The TCR of TiC was calculated to be 76 PPM on sapphire substrates.

Fabrication Process Flow

Fabrication cont’d Resistor Network Individual Thin-Film TiC Resistor Final Packaged Resistor Network

TiC Resistor Data Resistor # Pulse # Pulse Width (µs) Voltage (V) Resistance (kΩ) 4 1 5 250 1.7726 1.7735 10 6 1.7763 15 7 1.7823 20 8 1.8522 300 1.8011 1.8009 2 1.8 3 1.8048 1.8206 25 OPEN 200 1.84 1.8391 1.8384 1.8472 29 30

TiC Resistor Network: Post Pulsed Power Testing Molten Metal (Gold 5KÅ) Intact TiC Resistor

Pulse Power Testing on TiC Resistor Initial Resistance k After Pulse Power Testing 1 1.79 1.975 2 not there 3 ground 4 1.776 R4* 1.8425 6 1.802 R6* 1.843 8 1.824 1.8228 9 10 1.818 1.8168 * Re-wirebonded and measured

TiC Resistor Operation under cw Power

TiC Resistor Operation under cw Power Package 8 Resistor 21 Ri Rf . 1846.9 2271 Run # V I (mA) R P (mW) % Change 1 23.702 12.807 1850.707 303.5515 0.21 2 27.092 14.329 1890.711 388.2013 2.37 3 30.466 16.434 1853.84 500.6782 0.38 4 33.374 17.987 1855.451 600.2981 0.46 5 36.294 19.546 1856.851 709.4025 0.54 6 38.72 20.832 1858.679 806.615 0.64 7 41.195 22.139 1860.743 912.0161 0.75 8 43.137 23.182 1860.797 1000.002 9 45.603 24.478 1863.02 1116.27 0.87 10 48.063 25.77 1865.076 1238.584 0.98 11 49.054 26.291 1865.81 1289.679 1.02 12 51.064 27.33 1868.423 1395.579 1.17 13 53.04 28.362 1870.108 1504.32 1.26 14 55.04 29.398 1872.236 1618.066 1.37 15 56.55 30.17 1874.379 1706.114 1.49 16 58.6 31.213 1877.423 1829.082 1.65 17 60.19 31.986 1881.761 1925.237 1.89 18 61.8 32.769 1885.929 2025.124 2.11 19 62.97 33.277 1892.298 2095.453 2.46 20 64.87 34.061 1904.524 2209.537 3.12 21 67.03 34.577 1938.572 2317.696 4.96 22 68.18 34.427 1980.422 2347.233 7.23 23 70.33 34.474 2040.088 2424.556 10.46 24 72.85 34.332 2121.927 2501.086 14.89 25 75.62 33.3 2270.871 2518.146 22.96 Package #8 – Pin 21 – 400x

Summary Simple thin film resistor networks have been fabricated with Titanium Carbide as a new material. The excellent thermal and mechanical stability of TiC, together with TCR value make it a good candidate as an alternate thin film resistor material. Pulsed Laser Deposition (PLD) was combined with Si- wafer processing techniques for fabrication of TiC resistor network. Under pulsed power operation, TiC resistors remained intact, and testing was halted due to melting of Au wire bonds and metal interconnects. TiC resistors were capable of withstanding up to 2 W of cw power. Patent application submitted.

Potential Applications Future work RF attenuators Thin film hybrid circuitry High power applications Discrete resistors Perform bias humidity testing

Acknowledgements Aerospace Corporation Parts Materials and Process Department Mrs. Katie Feistel Dr. Wayne Martin Scott Nuccio Jeremy Young Electronic Engineering Subdivison Peter Carian Laboratory Operations Mrs. Barbara Hill