Wafer-scale fabrication of infrared detectors based on tunneling displacement transducers

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Abstract

Miniaturized Golay cells based on tunneling displacement transducers were first produced at the Jet Propulsion Laboratory (JPL). The devices produced have performance characteristics comparable to the best thermal infrared detectors of comparable dimensions. This work describes a high yield wafer-scale process for fabrication of tunneling infrared detectors. The sensors produced have performance comparable to the best tunneling sensors made so far.

Introduction

All infrared detectors fall under one of two major categories [1]. Quantum infrared detectors work by the direct interaction between incident infrared radiation and the atomic lattice of the sensor material. Photoconductors are the most common type of quantum infrared detectors and can be made of many different materials such as silicon doped with arsenic. Generally, radiation incident on a photoconductor must be sufficiently high in energy so as to cause donor electrons associated with the dopant ions to be released into the conduction band thereby changing the resistance of the sensor material. The bandgap in these materials leads to a maximum cutoff wavelength for response, and causes a relationship between the longest detectable wavelength, the bandgap, and the operating temperature. In general, long-wavelength quantum infrared detectors must be operated at cryogenic temperatures to function at all.

The second type of infrared sensor is based on thermal principles. When infrared radiation is incident on a thermal infrared detector, it produces a temperature rise that changes some physical parameter that is calibrated and read off as sensor output. Pyroelectric detectors, the most common thermal infrared sensors, are made of noncentrosymmetric crystalline materials that undergo a change in electric polarization when subjected to a temperature rise.

Thermal infrared detectors are generally slower than quantum detectors since their response time is limited by the thermal mass of the sensor structure. Low temperature (less than 100 K) operation of some quantum infrared sensors extends the wavelength of detectable radiation but comes at a higher cost. Clearly, when cost is a limiting factor and the objective is to detect radiation in the mid- to long-wavelength region (3 μm < λ < 100 μm), thermal infrared detectors are the sensors of choice.

While care must be exercised when comparing detectors since the overall performance of a particular sensor depends on many factors such as sensitivity, bandwidth and linearity, the figure of merit that commands the most attention is resolution otherwise known as noise equivalent power (NEP). This is not surprising since in many applications the detection of very small signals is the primary objective. The Golay cell, invented by Marcel Golay in the late 1940s offers the best performance among thermal infrared detectors [2], [3]. In the Golay cell, infrared radiation is admitted into a gas chamber where its absorption causes thermal expansion of trapped air. The resulting pressure build up within the chamber causes deflection of a flexible mirror that is sensed by a complicated optical readout system. Despite some clear disadvantages such as high cost (more than US$ 5000) and relatively large size, the Golay cell is commercially available and is used where high performance is essential and quantum infrared detection is not an option.

Given its high performance, and with a view to overcoming some of its shortcomings, miniaturized micromachined Golay cells that utilize capacitive as well as tunneling displacement transducers have been developed [4], [5]. Initial work on micromachined tunneling Golay cells was done at the Jet Propulsion Laboratory (JPL) where devices were made in a low yield process that involved the hand assembly and gluing together of the sensor parts. This mode of fabrication produced devices with large variations in key operating parameters. For example, as would become clear in later sections of this paper, an important operating parameter of the tunneling sensor is the deflection voltage. Early tunneling infrared detectors had differences in deflection voltage between sensors made from the same wafer set of more than 300 V. This is because it is impossible to effectively and consistently standardize the distances between the parts of the sensor in a hand-assembled device.

This paper describes work done at Stanford University in collaboration with IC sensors that produced a high yield wafer-scale fabrication process with sensors that feature uniform operating parameters while maintaining the high performance of hand assembled early generation devices [6]. In addition to the tunneling infrared detector, micromachined tunneling accelerometers have been developed with micro-g resolution [7].

Section snippets

Description of micromachined tunneling infrared detector

The micromachachined tunneling infrared detector is a miniaturized bulk-micromachined form of the Golay cell. Sensors were made by aligning, bonding and dicing three separately processed 425 μm thick double polished silicon wafers. A cross-sectional view of the sensor is shown in Fig. 1. Infrared radiation is absorbed by a 50 Å thick platinum film evaporated on the inner side of the 1 μm thick upper nitride membrane. The top two parts come together to form gas cells with a square radiation

Electron tunneling as a displacement transducer

Electron tunneling between a pair of metal electrodes was first analyzed in detail by Simmons [8]. In this phenomenon, electrons traverse a thin barrier by a quantum mechanical process. Classically, there are no allowed energy states for these electrons in the barrier, but quantum mechanics allows a small but finite probability for electrons to cross the gap. One important feature of this process is that for electrode barrier widths smaller than 10 nm the tunneling probability increases

Thermal modeling

When exposed to infrared radiation, the absorber is heated, and this heat is conducted to the gas trapped within the cell. The gas undergoes a thermal expansion, leading to displacement of the membranes, which is detected by the tunneling transducer. From a thermal perspective, the transport from the membrane to the trapped air determines the thermal time constant and the response of the detector.

A lumped capacitance thermal model was developed for the sensor. The thermal resistance is:RT=LkA

Wafer-scale fabrication

The bottom (tip) wafers are coated with 0.5 μm silicon dioxide grown by wet oxide diffusion as shown in Fig. 5a. Photoresist is spun on the wafer and patterned to form the 10 μm square mask used during BOE (buffered oxide etch) etch step (Fig. 5b). The wafers are put in KOH solution at 70 °C and the exposed silicon around the oxide mask is etched down to a depth of 7 μm (Fig. 5c). The oxide is stripped of with BOE exposing silicon everywhere. The edges of the tip are made smooth by growing and

Assembly and packaging

After the processing of all three wafers is complete a thorough cleaning is performed, first in ammonium hydroxide/hydrogen peroxide solution and then for 30 min in oxygen plasma. The pre-assembly cleaning is a very important step since the metal electrodes must be very clean for tunneling and wafer bonding to occur. The next stage is the alignment of all three wafers; and this is done with an infrared microscope that has the capability of seeing through silicon. The three-wafer stack is placed

Sensor operation

To operate the sensor an offset voltage is applied to the deflection electrode, electrostatically deflecting the flexible membrane down towards the tip (Fig. 10). A small bias of 150 mV is applied to the membrane and the tip is grounded through a 10 MΩ resistor. As the membrane comes close to the tip tunneling current begins to flow. When the membrane is 10 Å away from the tip a tunneling current of 1.5 nA flows from tip through the resistor to ground. This circuit balances pressure force due to

Performance

The sensitivity data shown in Fig. 11 was measured using a bright light as a source of infrared radiation. To limit the wavelengths seen by the sensor to the infrared region, a plane silicon wafer served as an optical filter. The light was modulated in frequency through the use of a chopper. At each frequency, the ratio of the voltage output of the tunneling infrared detector and that of a reference (Intrel, model 101D) was multiplied by the known sensitivity of the reference sensor to arrive

Conclusion and future work

In this work, we have successfully developed a wafer-scale process for a miniaturized Golay cell based on tunneling displacement transducers. The sensors were fabricated with high yield (∼80%) with consistent and reliable performance. The performance of these devices is comparable to the best commercially available uncooled, broadband IR sensors. The roughly 20% of the sensors that failed to work had the following problems:

  • 1.

    Contamination of the tunneling surfaces by the water used during the

Acknowledgements

The authors would like to thank Gary Yama and the staff at IC sensors for help in the design and fabrication of the tunneling infrared detector. Special thanks to Jack Kisslinger and Warren Vidrine for help during testing of the sensors produced. The work described in this paper was supported by the Research Development Laboratory, Jet Propulsion Laboratory—Center for Space Microelectronics Technology and Raychem Corporation.

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