ME PhD Oral Examination - Shingo Yoneoka, Tuesday, May 31st, 1:00 pm

University PhD Dissertation Defense

 

"ALD Metal Microbolometer Arrays"

 

Shingo Yoneoka

Department of Mechanical Engineering

 

Advisor: Prof. Thomas W. Kenny

Co-advisor: Prof. Roger T. Howe

 

Tuesday, May 31st, 2011

1:00 pm

(Refreshments served at 12:45 pm)

 

Location: Packard Building, Room 202

http://ee.stanford.edu/directions.php?bld=packard

 

Abstract

 

A bolometer is a device that measures the energy of incident electromagnetic radiation using electrical resistance change. One of the important applications of the bolometer is thermal imaging, which detects radiation in the long-wavelength infrared region (8-14 um). Conventional micromachined bolometers consist of multiple functional layers that optimize their performance. Vanadium oxide (VOx) and amorphous silicon are commonly used as thermistor. Silicon dioxide and silicon nitride are often used as the supporting and absorption layers because of their small thermal conductivity. These functional layers can be replaced by a single metal layer to further improve the thermal properties and simplify the fabrication process. However, this requires a film thickness on the order of nanometers since the impedance of the film must approach that of free space in order to absorb the wavelength of interest.

 

In this talk, we present an uncooled infrared bolometer using a few-nanometer-thick platinum (Pt) film that is formed by atomic layer deposition (ALD). ALD is used to reliably deposit Pt films with less than 10-nm thickness. While Pt has relatively low TCR, it also has low 1/f noise, good linearity, and low hysteresis, making it a good temperature sensing material over all. Incorporating the U-shaped trenches, 50x50 um bolometer pixels and 25x25 um bolometer arrays made of ~12 nm ALD Pt/Al2O3 films are successfully fabricated. The aspect ratio of the freestanding structures fabricated in this process exceeds 4,000, which is much larger than the conventional MEMS devices. The developed process can provide unusual combination of electrical, thermal, and mechanical properties that will be useful for many applications. Having the fabrication technology for ALD-grown freestanding structures, the electrical and thermal conductivities of ALD Pt films of thickness 7.3, 9.8, and 12.1 nm are measured at 50-320K. Conductivity data for the 7.3-nm bridge are reduced by 77.8% (electrical) and 66.3% (thermal) compared to bulk values due to electron scattering at material and grain boundaries. The experimental Lorenz numbers of ALD Pt films exceed bulk values due to phonon conduction. Finally, the characterization results of the fabricated bolometer pixels and arrays are described. The thermal time constant of 50x50 um and 25x25 um bolometer pixels are 1.5 ms and 0.4 ms, respectively, which are about 10 times smaller than conventional VOx bolometers. The noise equivalent temperature difference of the bolometer is 112 mK assuming negligible 1/f noise. The presented bolometer is suitable for low-cost and high-speed thermal imaging applications.