Xiaoxu Kang

A common MEMS micro-bridge structure-based process was developed on 200 mm standard CMOS Al back end of line. Thick amorphous-Si (alpha-Si) film was developed and used as the sacrificial material with well-controlled film stress to improve the wafer warpage. Anchor/contact structure was formed by a trench first scheme with no metal or dielectric plug in it, and thin Ti/TiN bilayer barrier metal was used as an electrode layer to define the sensing material resistor with improved step coverage on the anchor sidewall. A wet etching process was introduced for electrode layer patterning with almost no damage to the sensing material. The micro-bridge structure-based bolometer device was fabricated and evaluated on this platform. Good electrical performance had been achieved, and after optimized stress engineering the range of the released micro-bridge surface height variation was controlled to less than 10%, which indicated that the process platform can well match the bolometer, micro-lens and related sensor requirements.

In this work, CMOS thin barrier metal Ti\TiN was used as electrode layer to define the sensing material resistor for MEMs/Sensor application. APM (ammonia peroxide mixture) based wet etch process was introduced in electrode layer patterning process with no process loss of Si based sensing material. It was found sidewall recess problem at bottom Ti layer after electrode layer wet etch process due to faster horizontal wet etch rate of Ti than that of TiN. Optimized process scheme were introduced to solve this problem, including Ti layer thinning, surface nitridation of Ti layer and optimized TiN layer instead of Ti\TiN bilayer. After process evaluation, best option was selected with dimension of electrode layer sidewall recess less than 20nm after about 50% overetch time, which can greatly improve the CD (critical dimension) uniformity control of sensing resistor.

Polysilicon is the commonly used material for resistor device in standard CMOS technology. Due to the process limitation, polysilicon resistor has the problem of high resistance variation and high temperature coefficient of resistance (TCR), which may cause performance degradation in analog circuit design. In this work, high resistivity TaN metal film was developed and integrated in thin TaN metal resistor structure, which was fabricated on 200mm wafer CMOS Cu-BEOL. The metal resistor performance was evaluated to check its potential value in CMOS resistor device application.

Graphene exhibits an extremely high in-plane thermal conductivity, which shows highly prospective value in thermal management and heat spreader application. In this work, graphene sheet was integrated into LED module as intermediate heat spreader with different schemes, and heat dissipation performance was measured and evaluated using standard InfraRed (IR) camera. The experimental data shows that introduction of graphene sheet can reduce chip temperature, which can potentially increase the luminous efficiency and performance of LED products.

In this study, a CMOS compatible capacitive humidity sensor structure was designed and fabricated on a 200 mm CMOS BEOL Line. A top Al interconnect layer was used as an electrode with a comb/serpent structure, and graphene oxide (GO) was used as sensing material. XRD analysis was done which shows that GO sensing material has a strong and sharp (002) peak at about 10.278°, whereas graphite has (002) peak at about 26°. Device level CV and IV curves were measured in mini-environments at different relative humidity (RH) level, and saturated salt solutions were used to build these mini-environments. To evaluate the potential value of GO material in humidity sensor applications, a prototype humidity sensor was designed and fabricated by integrating the sensor with a dedicated readout ASIC and display/calibration module. Measurements in different mini-environments show that the GO-based humidity sensor has higher sensitivity, faster recovery time and good linearity performance. Compared with a standard humidity sensor, the measured RH data of our prototype humidity sensor can match well that of the standard product.

Humidity sensor can found wide application in industry, automobile and consumer electronics. In this work, capacitive humidity sensor structure was designed and fabricated on 200mm CMOS BEOL Line. BEOL Al was used as electrode with comb/serpent structure, and graphene oxide (GO) was used as humidity sensing material. Result shows that GO based humidity sensor has high sensitivity, faster recovery time and good linearity performance. Based on the sensor structure, prototype humidity sensor was built by PCB-level integration with dedicated readout ASIC and display/calibration module, and the output data can well match that of standard humidity product.

In this work, TaN bottom electrode thermal sensing resistor for MEMs-based bolometer was designed and fabricated by 200 mm Cu -BEOL compatible process. Thermal sensing material was B -doped alpha- Si deposited by PECVD in situ doping process. PVD TaN film was used as the bottom electrode. Dedicated process on modified tool was introduced to achieve a good contact between the TaN and the sensing material. There are both CVD and Etch chambers installed on this modified tool. Wafer with bottom electrode pattern was pre-cleaned firstly by low-power Ar /CF4 gas to remove oxide and possible surface residue on TaN in the etch chamber. Then, the wafer was transferred to CVD chamber through transfer chamber in vacuum condition. With vacuum transfer condition under tight Q-time control, Ohmic contact can be achieved for the TaN bottom electrode and B -doped alpha- Si . Through the IV curve and TCR data, it can be seen that the bottom electrode device can well meet the MEMs-based bolometer requirements.

TaN \ Ta is the excellent material against copper diffusion, and is widely used as Cu diffusion barrier in Complementary Metal-Oxide-Semiconductor (CMOS) Back End of Line ( Cu -BEOL) process. Due to its good electrical and thermal property, TaN is also evaluated as electrode material for Micro-Electro-Mechanical Systems (MEMS)/sensor application. In this work, CMOS compatible MEMS based bolometer process with post-interconnect CMOS-MEMS single chip integration scheme was developed on 200 mm standard CMOS Cu BEOL, and TaN thin film was used as key electrode material in micro-bridge structure. The micro-bridge structure, with sensing resistor on surface of the micro-bridge, forms a resonant cavity for IR absorption. There are only several layers of thin film on the micro-bridge, and stress of the electrode layer play very important role to determine the performance of the micro-bridge. Additionally sheet resistance of TaN electrode should be controlled at around 377 Ohm/SQ to match the free space impedance, which is much higher than CMOS baseline process. Low stress and high resistivity TaN film was developed with thickness of 200 Å and compressive stress of about 900 MPa as best condition. From the electrical and physical data, the optimized TaN film process can well match this application.