Front Matter Template

Acknowledgements

I would like to thank my advisor, Professor Dean P. Neikirk, for giving me the opportunity to pursue this study under his guidance and support. Especially, without his insights and patience during this research, it is impossible to for me to finish this work. Whenever I was in trouble, he guided me to escape out of it. He is a mentor in my life as well. The author also appreciated the assistance of Professor John Mcdevitt, Jack Lee, A.J. Welch, and Ray Chen, the committee members of this dissertation. A special thanks goes to Dan Hammer for optical measurement of microbolometer in chapter 5 and for editing this dissertation. Through teamwork with him I learned what the attitude of scientist should be for the research. A special word of thanks goes to Dr. Jaeheon Han and Dr. Yuijung Yoon, both are professors in Korea, for their help and advices when I started the graduate study. Especially, rookie, Yunsuk Park edited my part of dissertation and provided enjoyable environment at boring period of study. The author also wishes to recognize the support and friendship of the Team Neikirk. Well, there are lots of people to say thanks in Microelectronic Research Center. I would like to thank to all. This research conducted for this dissertation was funded by MURI.

MICROMACHINED WAVELENGTH SELECTIVE MICROBOLOMETER SENSORS OPERATING AT ROOM TEMPERATURE

Publication No._____________

Seung-Jin Yoo, PhD

The University of Texas at Austin, 2000

Supervisor: Dean P. Neikirk

Micromachined wavelength selective microbolometers are fabricated, characterized and modeled using a transmission line equivalent circuit. The amount of power absorbed by the device is adjusted using interference effects, resulting in a device with wavelength depend response characteristics. Using micromachining, significant improvement in thermal performance is achieved by removing the substrate from the bolometer, supporting it with long and narrow suspension legs to increase the thermal impedance. Constructive interference produced by a mirror placed a quarter wavelength of the incoming infrared signal behind the microbolometers is used to enhance the absorption of the microbolometer. The model of power coupling is verified by optical measurement at several infrared wavelengths.

Table of Contents

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List of Tables............................................................................................................ ix

List of Figures........................................................................................................... xi

Chapter 1 Introduction........................................................................................ 1

Chapter 2 Review of Microbolometer................................................................ 5

2.1 Microbolometer Operation ....................................................................... 5

2.2 Detector Performance Characterization....................................................... 8

2.2.1 Responsivity ................................................................................. 8

2.2.2 Noise Equivalent Power (NEP) and Detectivity (D*)..................... 10

2.3 Performance Limitations .......................................................................... 11

2.3.1 Thermal impedance....................................................................... 12

2.3.2 Temperature coefficient of resistance............................................. 14

2.3.3 Bias current.................................................................................. 16

2.3.4 Coupling efficiency (P_absorbed/P_incident)............................................. 19

2.4 Overview of Silicon Micromachining Technique......................................... 20

2.4.1 Bulk micromachining techniques.................................................... 22

2.4.2 Surface micromachining techniques................................................ 23

2.5 Summary.................................................................................................. 29

Chapter 3 Modeling of Absorbed Power Coupling Efficiency......................... 30

3.1 Impedance matching thin-film absorber with mirror.................................... 30

3.2 Model of absorbed power coupling efficiency............................................ 35

3.2.1 Effect of parameter variation ....................................................... 45

3.3 Resolving multi-wavelength ambiguities...................................................... 49

3.4 Summary.................................................................................................. 52

Chapter 4 Micromachined Microbolometer Fabrication................................. 53

4.1 Chromium as microbolometer material....................................................... 53

4.2 The process for conventional micromachined microbolometer.................... 54

4.3 The process for Resonant dielectric cavity enhanced microbolometer......... 59

4.4 The process for Resonant air cavity enhanced microbolometer................... 63

4.5 Fabrication Issues..................................................................................... 66

4.6 Summary.................................................................................................. 71

Chapter 5 Measurement results of microbolometer........................................ 72

5.1 DC measurements for conventional microbolometer................................... 72

5.2 Experimental setup.................................................................................... 77

5.2.1 Optical and electrical setup........................................................... 78

5.2.2 Power and spot measurement....................................................... 82

5.3 Measurement results ............................................................................... 84

5.3.1 Modulation frequency response..................................................... 84

5.3.2 Irradiance, output Voltage, and responsivity measurement.............. 87

5.4 Noise measurements............................................................................... 101

5.5 Summary................................................................................................ 102

Chapter 6 Conclusion...................................................................................... 104

Bibliography .................................................................................................... 107

Vita .................................................................................................................. 115

List of Tables

Table 2.1:..... Bulk thermal conductivity (300K) of various materials used in microbolometers. These values are for bulk material; thin films may have different characteristics............................. 13

Table 2.2:..... Comparison of TCR of various materials............................................. 15

Table 2.3:..... Important mechanical properties of silicon crystal................................. 21

Table 3.1:..... The combination of dielectric layers that comprises the microbolometer structure for wavelength selection........................................................................................................... 43

Table 3.2:..... Summary of simulation on microbolometers with the thickness of 707 Å of silicon nitride, 5020 Å of silicon oxide, and 905 Å of silicon nitride with thickness variation of 5%......... 48

Table 5.1:..... The comparison thermal impedance for membrane of different geometry. 77

Table 5.2:..... The summarized optical result of HeNe infrared laser at 1.15 and 3.39 mm. 94

List of Figures

Figure 2.1:.... (a) Incoming radiation causes the instantaneous temperature of the sensing element to be T_s + ∆T. The element is connected via a conducting link through conductance G to the heat sink, which remains at temperature T_s.(b) bolometer geometry for a “long” rectangular device.. 7

Figure 2.2:.... Schematic view of the fabrication procedure for a planar diaphragm using bulk micromachining. (a) Deposition of silicon nitride, silicon dioxide, and silicon nitride by LPCVD. (b) Patterning and plasma etching on the backside of wafer. (c) Anisotropic etching using KOH at 110 °C 24

Figure 2.3:.... Schematic view of fabrication procedure for typical surface micromachining. (a) Deposit and pattern the sacrificial layer; (b) Overcoat to make mold. (c) Selectively remove sacrificial layer and release the structure (lost layer is usually either polysilicon or oxide)...................... 26

Figure 2.4:.... The micromachined inductor on a membrane made by metal wet etching process. 27

Figure 2.5:.... The micromachined storage wells for chemical sensing beads in an artificial tongue using bulk micromachining................................................................................... 28

Figure 3.1:.... Schematic view of matching a radiation absorber to free space; (A). Geometry (B) Equivalent Circuit........................................................................................................... 31

Figure 3.2:.... Schematic view of enhanced radiation absorption in a thin film with mirror placed [(odd integer)/4]* l behind absorbing layer. (a) Geometry and (b) Equivalent circuit........... 33

Figure 3.3:.... Simulation result of coupling efficiency (P_absorbed/P_incident) with matching impedance at free space (377 ohms), placing a reflecting short at one-quarter wavelength behind thin conductor. 34

Figure 3.4:.... Schematic diagram of transmission line equivalent circuit model of multi stack micromachined structure with bolometer layer and mirror coating layer.............................................. 37

Figure 3.5:.... Figure 3.5 Actual structure with cross sectional view, matching with transmission model shown in Fig.3.4........................................................................................................... 38

Figure 3.6:.... Simulation of the spectral response on multi stack micromachined structure with several air gaps. 43

Figure 3.7:.... The frequency domain representation of spectral response of absorbed power coupling efficiency............................................................................................................ 44

Figure 3.8:.... Effect of thickness variation on coupling efficiency of the incoming infrared signal. 47

Figure 3.9:.... Effect of thickness variation on coupling efficiency of incoming infrared signal in frequency domain............................................................................................................ 48

Figure 3.10:.. Effects on change of bolometer sheet resistance for coupling efficiency.. 49

Figure 3.11:.. Resolving multi-wavelength ambiguities using 4 color detector arrays.... 51

Figure 4.1:.... Figure 4.1 Schematic view of fabrication procedure: (a) deposition of silicon nitride, deposition and patterning of polysilicon, and deposition of top bolometer structure; (b)

(a)

Etching bolometer structure and bottom silicon nitride; (c) Sacrificial poly silicon etching and forming self-aligned structure; (d) Deposition of Cr as bolometer layer..................................................................... 55

Figure 4.2:.... Self aligned structure simulated by Anisotropic Crystalline Etching Simulation (ACES) tool [8]. The bolometer structure is not shown in this figure...................................... 58

Figure 4.3:.... Top view of the fabricated conventional microbolometer...................... 59

Figure 4.4:.... Schematic view of fabrication procedure: (a) formation of membrane; (b) Patterning the microbolometer using RIE; (c) forming self-aligned structure (d) Deposition of chromium on the top surface and deposition of gold as mirror layer......................................................................... 61

Figure 4.5:.... The top view of fabricated resonant dielectric cavity enhanced microbolometer (50 x 10 Optical Picture)............................................................................................................ 62

Figure 4.6:.... Schematic view of fabrication procedure: (a) formation of top and bottom membrane with forming sacrificial polysilicon layer; (b) Patterning the top membrane for microbolometer using RIE; (c) releasing the top and bottom membrane and forming the self-aligned structure (d) Deposition of chromium on the top surface and deposition of gold as mirror layer................................ 65

Figure 4.7:.... The crack of overetched legs caused by the residual tensile stress (3 mm width and 20 mm length legs)............................................................................................................. 68

Figure 4.8:.... Capillary forces related the surface status............................................ 69

Figure 4.9:.... Phase diagram illustrating two paths to pass from the liquid to vapor state without encountering a liquid vapor interface [14]............................................................................ 70

Figure 5.1:.... The dimensions of the conventional microbolometer fabricated using surface micromachining... 73

Figure 5.2:.... Resistance as a function of dissipated power for the conventional micromachined bolometer at various temperatures from 0.1 V to 1 V biased with step of 0.1V.................... 74

Figure 5.3:.... Resistance as a function of temperature for micromachined bolometer at 1 V bias 76

Figure 5.4:.... Optical setup used to irradiate the microbolometer. The visible and infrared helium-neon lasers are made collinear to allow alignment of the beam on the microbolometer active area. Detector position is controlled with a three-axis stage. M: Mirrors, BS: beam splitter, Det A and Det B: power meters, A1-4: iris apertures, S: Shutter, DFG: difference frequency generator.)................ 79

Figure 5.5:.... The probing set up on the XYZ stage.................................................. 65

Figure 5.6:.... Schematic of the circuit and instrumentation used to make an incoming infrared responsivity measurement: voltmeter (Keithley model 195A) for confirming the bias condition, and oscilloscope for finding the best alignment between the incident beams and detector active area............ 81

Figure 5.7:.... Measured output voltage of resonant dielectric cavity microbolometer without gold mirror by chopper modulating the incoming HeNe infrared Laser at 3.39 mm. Detector specification: active area 26 mm x 20 mm; 20 mm leg length, 3 mm leg width; 230 ohm/sqr. sheet resistance of chromium as bolometer layer; and no gold mirror coating on the backside of microbolometer, and lock in amplifier settings: sensitivity 10mV, time constant pre = 1 msec post = 0.1 msec............................ 85

Figure 5.8:.... Measured signal voltage (a) of resonant dielectric cavity with gold mirror and without gold mirror biased at 100 mA in response to IR power (b) at 3.39 mm. The chopping frequency is 1500 Hz... 89

Figure 5.9:.... Irradiance of incident infrared at 1.15 and 3.39 mm. irradiance_w/o and irradiance_with are the irradiance of resonant dielectric cavity microbolometer with having gold mirror and without having gold mirror, respectively........................................................................................ 90

Figure 5.10:.. Comparison of power coupling efficiency between simulated and measured data..... 92

Figure 5.11:.. The measured and calculated results of output voltage and responsivity of the microbolometer biased at 100 mA in response to1.15 and 3.39 mm light...................................... 93

Figure 5.12:.. Spectrum of output from ultrafast laser system at the wavelengths used in the experiments. (This is characterized and measured by Dan Hammer)..................................... 95

Figure 5.13:.. Averaged irradiance incident on the resonant dielectric cavity microbolometer from HeNe (1.15 and 3.39 mm) and ultrafast (2.75, 3.25, 4 and 4.5 mm) laser............................... 97

Figure 5.14:.. Output voltage of resonant dielectric cavity microbolometer in response to 1.15 and 3.39 mm monochromatic light and 2.75, 3.25, 4, and 4.5 mm broadband light..... 99

Figure 5.15:.. The responsivity comparison of resonant dielectric cavity microbolometer with gold mirror and without gold mirror....................................................................................... 100

Figure 5.16:.. Voltage noise as a function of frequency for microbolometer. The noise was measured over a bandwidth of 10% of the selected frequency. Resonant dielectric cavity enhanced microbolometer is biased at 100 mA................................................................................................... 102