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«A Dissertation Presented to The Academic Faculty by Jungchul Lee In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in ...»

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FABRICATION, CHARACTERIZATION, AND APPLICATION OF

MULTIFUNCTIONAL MICROCANTILEVER HEATERS

A Dissertation

Presented to

The Academic Faculty

by

Jungchul Lee

In Partial Fulfillment

of the Requirements for the Degree

of Doctor of Philosophy in the

George W. Woodruff School of Mechanical Engineering

Georgia Institute of Technology

May 2007

FABRICATION, CHARACTERIZATION, AND APPLICATION OF

MULTIFUNCTIONAL MICROCANTILEVER HEATERS

Approved by:

Dr. William P. King, Advisor Dr. Mark G. Allen George W. Woodruff School of School of Electrical & Computer Mechanical Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Yogendra Joshi Dr. Oliver Brand George W. Woodruff School of School of Electrical & Computer Mechanical Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Ari Glezer George W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Date Approved: March 28, 2007

ACKNOWLEDGEMENTS

First of all, I would like to thank my advisor Prof. Bill King for inspiring and encouraging me throughout my PhD studies. Without his support and advice, I would never have been able to have a chance to write this page now. Under his guidance, I could participate in various interesting research projects and was able to meet and collaborate with smart, diligent, and nice people. I would like to thank Prof. Marchenkov at GT Physics for helping me finish microcantilever measurements in vacuum. I am also very grateful to Dr. Graham for his help on Raman spectroscopy. They were very supportive to my work and let me have access to their laboratory day and night. I would like to thank to Dr. Thundat at ORNL for giving an idea to make a new type of microcantilever hotplate. When Bill and I visited ORNL, we had fruitful discussions which turned into a chapter of my thesis. Last fall, Bill arranged a meeting with Dr. Chui who has done early work on microcantilever heater devices. It was such a great chance to meet him since my research has been influenced by him and his collaborator at CSEM, Switzerland.

I am also very grateful to all committee members who have kindly accepted to review my dissertation. Prof. Glezer has been supportive for micro/nanojet research, Prof. Joshi and Prof. Allen always tried to give me an insight to look and think deeper, and Prof. Brand went through all of my manuscripts carefully. I was truly blessed to interact with the best academic committee. I wish to express my sincere thanks to Linda Perry, Terri Keita, and Glenda Johnson who have done great administrative work to support all ME students at GT.

–  –  –

Harry, Fabian, Marcus, Andrew Cannon, Shubham, Jessica, Andrew Garner, Fuzheng, KJ, Keunhan, and Tanya. They have always supported my work and life here at GT, made the atmosphere enjoyable at all times, treated me like family. My gratitude also goes to all Korean student association ME members at GT. Especially, Dr. Shin always helped me regarding any fabrication issues in the MiRC cleanroom. Keunhan and BJ were eager to discuss any interesting question. Dr. Sangil Lee, Dr. Hyunjin Lee, Hoy, Namsu, and Minseok were nice and kind to share their valuable time to relax our tired bodies and minds. Additional thanks go to Hanif, John, Nisarga and Kianoush. I have got great help from them for the nanojet project.

Finally, my best thanks and appreciation go to my family. My father, mother, sister have never stopped praying for my academic success and health. I am truly blessed by them. I wish to express my sincere thanks to my wife, JuHee. She has been sacrificing her health, time and career to support my PhD degree. During my PhD studies, we have been blessed by two babies, Kevin and Ryan. JuHee also took best care of them while supporting me. Her sacrifice may not be tangible but I could feel and see on every page in my dissertation. All my family has been supporting me with kindness and patience throughout my PhD studies. Now it is my turn to support and take care of my wife, my sons, and my parents.

–  –  –

Table 3.1 Electrical, thermal, and mechanical design requirements

Table 3.2 Summary of mechanical characterization: Spring constants, resonance frequencies and corresponding quality factors

Table 4.1 Coefficients used in simulation for boron and phosphorus [26]

Table 4.2 Coefficients and parameters for bulk mobility calculation [30].

Table 4.3 Summarized characterization results.

Table 7.1 Mechanical properties of the two piezoresistive cantilevers

–  –  –

Figure 1.1 The universe of AFM-based microscopy and local sensor techniques [20].

..... 2 Figure 1.2 Scanning electron micrograph of 64 × 64 array of microcantilever heaters from IBM [32]

Figure 1.3 Principles of (a) Dip pen nanolithography [33] and (b) Thermal dip pen nanolithography [17]

Figure 1.4 Principles of (a) Fountain pen nanolithography [35], (b) NADIS (nanoscale dispensing) [36], and (c) Eletrodeposition [37].





Figure 2.1 (a) Scanning electron microscope image of microcantilever heater (b) Infrared microscope image of the heater cantilever during steady electrical excitation.

The IR image is approximately 0.5 mm square. The doped silicon cantilever is fabricated in a ‘U’ shape such that it forms a continuous electrical path. The region near the cantilever free end is a highly resistive heater and the legs have lower electrical resistance such that they carry electricity. The IR image confirms substantial heating only near the free end of the cantilever.

Figure 2.2 Testing circuit used to characterize the cantilever.

There is a sense resistor connected to the cantilever in series to protect the cantilever at high power and to sense the current during pulse excitation. A high speed amplifier is configured when the cantilever is operated with a sense resistor having high resistance.

Figure 2.3 (a) Typical cantilever DC responses with various sensor resistors.

As the resistance of the resistor decreases, more power will be dissipated in the cantilever with a given excitation voltage. (b) Resistance sensitivity – ratio of the cantilever resistance change to the input voltage change – as a function of the excitation voltage for different sense resistors. The resistance sensitivity increases as the resistance value decreases such that a resistor having high resistance is preferred to protect the cantilever............... 23 ix Figure 2.4 (a) Current and cantilever resistance as functions of DC excitation voltage showing temperature-dependent resistivities and thermal runaway of doped silicon. (b) Current and resistance as functions of cantilever power.

The cantilever resistance is overall nonlinear but partially linear and TCR changes from positive to negative

Figure 2.5 (a) Two distinct responses of the cantilever resistance depending upon the high voltage level, Vhigh where square pulses of 1ms duration are applied to the cantilever with minimum voltage level of 0.

5V. (b), (c) Cantilever resistance and generated power in the cantilever increase monotonically and reach steady state when Vhigh is lower than VPeak. With higher Vhigh, the cantilever resistance and generated power increase, reach the maximum, and then decrease until the cantilever reaches steady state (d) The heated cantilever can reach approximately 560 °C within 16 μs.

Figure 2.6 Power spectrum density of the heated cantilever with 100 Hz sinusoidal excitation.

As AC voltage increases, more high order overtones are generated due to highly nonlinear cantilever resistance.

Figure 2.7 The cantilever AC responses are examined by frequency sweep of 5V-rms AC from 100 Hz to 3 MHz.

At a given frequency, steady resistance ( Rs ), resistance oscillation ( ΔR ), and reactance ( X ) of the cantilever are obtained using experimental data and phasor analysis and cantilever heater temperature is measured using Raman spectroscopy. There are two different regimes characterized by oscillation amplitude of the cantilever resistance and reactance.

Figure 2.8 Cantilever resistance and heater temperature as functions of the cantilever power, measured using Raman Spectroscopy, where the temperature is based on Stokes peak position.

Near room temperature, the cantilever resistance increases with temperature as carrier mobility decreases with increasing temperature. The cantilever electrical resistance drops steeply above 560 °C due to presence of thermally-generated intrinsic carriers.

x The heated cantilever has linear TCR of 5640 ppm/°C when the cantilever power is between 1.6 and 2.4 mW.

Figure 2.9 (a) Temperatures at 6 specific locations with respect to the cantilever power based on Stokes peak position.

Each location shows linearity of the temperature to the cantilever power. (b) Temperature distribution which exponentially decays from the heater to the legs at different excitation voltages. Temperature gradient also increases as the driving voltage increases.

Figure 2.10 (a) Peak resistances are shifted to the lower power as the substrate temperature increases.

(b) Linear relationship between the cantilever power at RPeak and the substrate temperature. Due to this linearity, the heated cantilever can act as a temperature sensor for the substrate................. 35 Figure 2.11 Stress profiles for (a) the top and (b) bottom side of the heated cantilever. A high level of intrinsic tensile stress is present in the system as seen under zero load. This tensile stress is then reduced with increasing voltage indicating compressive stresses that counteract this inherent portion. A contrary trend is seen on the bottom side of the cantilever as a much lower intrinsic tensile stress is seen.

Figure 2.12 Thermal noise of the cantilever is measured and Fourier transformed to obtain a power spectrum.

The power spectrum can be converted into an actual displacement of the cantilever by taking a force-displacement curve using AFM. The measured resonant frequency is 157.6 kHz at room temperature

Figure 2.13 The elastic behavior of the heated cantilever is characterized as a function of temperature by measuring the thermal spectra of the cantilever during heating.

(a) After initial heating, the resonant frequency increases during cooling. (b) Upon further heating, resonant frequency matches between heating and cooling curves.

Figure 2.14 As the cantilever temperature increases, the average harmonic displacement also increases, in accordance with both increasing thermal energy and decreasing spring constant

xi Figure 2.15 Spring constant calculated using various estimates for the temperature in the thermal energy. The temperatures TH, T0, and T correspond to the cantilever heater temperature, the ambient temperature, and the stress integral average temperature, respectively. The only estimates of the thermal energy of the cantilever that yield the expected reduction in spring constant with increasing temperature are those using a temperature much closer to room temperature than to the heater temperature, including the stress integral average temperature.

Figure 3.1 Six different designs for the cantilever type micro hotplate.

Each type has different shape and different doping area. The two regions of phosphorous doped silicon are intrinsic silicon and doped silicon.

Figure 3.2 Five major fabrication steps to produce the microcantilever hotplates.

.......... 57 Figure 3.3 SEM images of the fabricated devices.

The cantilevers were generally flat, indicating low intrinsic stress in the silicon device layer after processing. The traces of doped silicon can be seen in some of the SEMs.... 58 Figure 3.4 DC responses of microcantilever hotplates which are typical of heatercantilevers.

Figure 3.5 Transient electrical measurements that monitor the cantilever heating (τh) and cooling time constants (τc).

(a) Transient resistance response of a type A device during a square pulse operation. (b) Comparison of heating and cooling time constants of each device type extracted from exponential growth / decay fits

Figure 3.6 Fundamental resonance frequency of each device from thermomechanical noise spectra.

Figure 3.7 Temperature calibrations performed using IR microscopy.

During IR temperature mapping, each device was heated with moderate electrical power in order not to exceed a defined temperature range (300 °C). 50 measurements for each device were made, averaged, and contour-plotted..... 63 Figure 3.8 Histograms of the local temperatures in the heaters. Type A and C are highly populated at temperatures lower than average, however, other devices have the local temperature populated more at temperatures higher xii than average. Average temperatures and standard deviations for given electrical power are also included.

Figure 3.9 Maximum local temperature measured by the Stokes peak shift method using Raman spectroscopy

Figure 4.1 Range and straggle of boron (B) and phosphorus (P) as a function of implantation energy.

Inset shows graphical indication for Nmax, Rp, and ΔRp [26]

Figure 4.2 (a) Doping concentration of low doped phosphorus, high doped phosphorus, and medium doped boron after implantation and diffusion.

(b) Resistivity after implantation and post diffusion.

Figure 4.3 Flow chart to simulate doping concentration, resistivity, and resistance of doped silicon devices.

Figure 4.4 (a) Design of 1 × 4 array of microcantilever heaters with integrated piezoresistors and dimensions for an individual cantilever in micron.



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