Engineering Electrical Interfaces to Silicon via Indium Solder

Abstract

This paper provides engineering models of a simple and robust approach for creating electrical connections to silicon using reduced temperature (<200°C substrate) soldering. This removes a significant hurdle to the fabrication of high performance, custom silicon piezoresistors. The approach focuses on reducing the resistance of diodes that are undergoing reverse bias behavior, commonly considered to be unacceptable for electrical connections. Reverse bias Schottky barrier analytical models based on quantum mechanical first principles are developed to explain how the behavior is affected by doping, soldering temperature, and geometry. This understanding is encapsulated within parametric models that enable rapid design and optimization of the electrical contacts to silicon. Using this model, one may design contacts for practical applications that do not require the conventional microfabrication processing or the high-temperature processing. Indium solder is found to be the best solder for this process, with ohmic contact resistances of ≈1 -cm 2 for 110 p-type wafers at 10 17 cm −3 doping.

Fabrication of Six Degrees-of-Freedom Hexflex Positioner With Integrated Strain Sensing Using Nonlithographically Based Microfabrication

“…The circuit bonding step covers the attachment of the sensors to the deposited electrical structures. This research focused on the use of tabletop indium soldering [38]. The requirements of high flexibility, small batch size, and low per-device cost led to the development of a combined indium soldering and conductive epoxy operation for generating contacts to the silicon.…”

“…The immobilized structure is placed on a hotplate to raise the surface temperature of the mechanical structure to %150 C. A soldering iron set to 290 C is wetted in pure indium and touched off against the contact pads of the piezoresistors, leaving behind indium contacts whose performance is described in Ref. [38]. Chemtronic (Kennesaw, GA) Circuitworks CW2400 two part epoxy is used to link the soldered contact pads to the electrical traces deposited onto the surface of the mechanical structure.…”

“…The overall I-V parameters for the gages are listed in Table 5, along with the standard deviations on the average values. The predicted values are drawn from models generated in previous work on semiconductor piezoresistor design [38,39]. The bonding parameter, b, describes the variation of the ohmic resistivity from a baseline best-case term associated with ideal operating conditions [38].…”

“…The Hexflex architecture is a six degrees-of-freedom (6DOF) planar flexural nanopositioning platform which is linked to ground via six flexural bearings in series, each of which provides a location of integrated strain sensing [2]. The sensors are designed via previously described work on semiconductor interfaces [38] and piezoresistor performance [39,40]. Integrating sensing was the main thrust of this work; however, full 6DOF actuation may be integrated into the design via three paddles on the central stage.…”

Ultrasonic powder consolidation of Sn/In nanosolder particles and their application to low temperature Cu-Cu joining

“…Most of the solder materials that meet the requirement of low melting temperatures are bismuth (Bi) or indium (In) based alloys [18][19][20][21][22][23][24][25][26]. Among the low melting temperature solders, 34Sn-43Pb-14Bi, 58Bi-42Sn, and 40Sn-40In-20Pb are the relatively popular ones [26].…”