H.T.M. Pham

Comparison of porous silicon, porous polysilicon and porous silicon carbide as materials for humidity sensing applications

Abstract This paper investigates the suitability of porous polysilicon and porous SiC as materials for sensing humidity. The investigation is a continuation of earlier work on porous single-crystalline silicon, where it was shown that this material was appropriate for humidity sensing, and could be easily integrated with standard Si processing. It was also shown that membrane structures enable the integration of a heating device to ‘reset’ the system. The best microstructure for humidity sensing was obtained for low-doped p-type silicon. The advantage of using polysilicon is that it is possible to tune its response (by doping) so that it has only a very small temperature coefficient of resistance. The idea is that a humidity sensor with a very small temperature dependence could be realised. The advantage of using SiC is that it offers the possibility of a humidity sensor that could withstand very harsh chemical environments.

Polyimide sacrificial layer and novel materials for post-processing surface micromachining

We present a low-temperature post-processing module, utilizing polyimide as a sacrificial layer and novel materials such as PECVD SiC and metals (sputtered aluminium and titanium) as structural layers. The use of spin-on polyimide allows an all-dry final release step overcoming stiction problems often encountered in wet sacrificial etching processes. The spinning and curing procedure has been tailored to the specific needs of the IC-compatible post-process module. For the patterning of the polyimide, thin films of aluminium, PECVD silicon oxide or silicon carbide are employed as a mask layer. Anisotropic etching of the mask film and of the polyimide layer is accomplished by RIE. After patterning the structural layer, sacrificial etching of the polyimide is done using an isotropic dry etch process in high-density oxygen plasma. An underetch rate of 4 μm min−1 is achieved. Compatibility with different structural materials is tested and test structures are designed and realized in a fully post-processing surface micromachining module.

Fabrication of a CMOS compatible pressure sensor for harsh environments

The fabrication and characteristics of CMOS compatible absolute pressure sensors for harsh environments are presented in this paper. The sensor which was fabricated using post-processing surface micromachining consists of 100 circular membranes with a total capacity of 14 pF. PECVD SiC was used due to its good mechanical properties, but since SiC has high resistivity, aluminium layers were used for electrodes. The stiction problems were avoided by using polyimide PI2610 as a sacrificial layer. The pressure sensors were fabricated and the change of capacitance over full pressure range, 5 bar, was 3.4 pF.

Robust Wafer-Level Thin-Film Encapsulation of Microstructures using Low Stress PECVD Silicon Carbide

This paper presents a new low-cost, CMOS-compatible and robust wafer-level encapsulation technique developed using a stress-optimised PECVD SiC as the capping and sealing material, imparting harsh environment capability. This technique has been applied for the fabrication and encapsulation of a wide variety of surface- and thin-SOI microstructures that included microcavities, RF switches and various accelerometers. Advantages of our technique are its versatility, smaller footprint, reduced chip thickness and process complexity, post-CMOS batch processing capability and added functionality due to the possibility of integrating additional electrodes for MEMS. Besides fabrication details, this work also discusses related design aspects for large-area MEMS and demonstrates the encapsulation results. Successfully encapsulation of device geometries as large as 955×827¿m2 has been achieved.

Micromachined nanofiltration modules for lab-on-a-chip applications

In this paper, we present a new concept of particle filtration modules for lab-on-a-chip (LOC) devices. The modules are designed as vertical walls that separate fluidic micro channels. In these walls, nano channels that connect the two adjacent micro channels are embedded. Fluid and small particles can penetrate the walls through the embedded nano channels, while particles larger than the nano channels size will be stopped. By keeping the fluid in the surface plane of the LOC, the module can be easily integrated with other LOC modules. To fabricate these modules, we use chemical vapor deposition to deposit nanometer thick sacrificial layers and embed them into the wall structure. Wet chemical enchants are used to remove the sacrificial layers and form the nano channels. This fabrication process can generate 100 nm−1 μm high nano channels with high accuracy and uniformity with well-established micromachining techniques. Two types of modules, surface micromachining design for more flexibility in the choice of substrate material and bulk micromachining design for higher porosity without increasing footprint, are fabricated and successfully tested.

Silicon carbide membrane relative humidity sensor with aluminium electrodes

We report a new RH sensor based on a porous SiC membrane and Al electrodes. The RESET function has also been successfully tested. Using SiC allows us to fabricate sensors that can withstand harsh environments, while using Al for the electrodes makes the fabrication process much more cleanroom friendly. SEM images were used to examine the fabricated devices. The response of our sensors to RH between 10% and 90% RH, as well as the effects on the response after being exposed to extreme environments such as the outlet of a car exhaust, an environmental oven operating at 85/spl deg/C & 85%RH for 24hours, and an ammonia atmosphere for /spl sim/5 days, is discussed. It is seen that our sensors survived the exposures, with only small decrease in sensitivity after ammonia exposure.

Relative humidity sensors based on porous polysilicon and porous silicon carbide

RH sensors have been made using porous polysilicon and porous SiC, both of which can also be made porous by electrochemical anodisation in HF, similarly to single crystal silicon. We show that polysilicon can be incorporated into a humidity sensor enabling a reduction (compared to single-crystal porous Si) of the effect of temperature on humidity measurements. We show that porous SiC can also be incorporated into a humidity sensor and that these sensors are able to withstand being subjected to harsh environments such as a high temperature and high RH chamber, and the outlet of a car exhaust.

Very thin SiC membranes for micromachined vacuum sensors

Very thin (20-200 nm) SiC membranes with controlled composition and thickness, low tensile stress, layer continuity and etching selectivity are fabricated using a specially developed LPCVD process. These characteristics are necessary for their application in vacuum sensors, especially for operation in harsh environment. The continuity, low stress (180 MPa) and etching selectivity in combination with the reduced thickness increase the sensor sensitivity. Full characterization of these membranes is performed to investigate their structural and mechanical properties and fine tune them according to the application specific requirements. The fabricated membranes have undergone a bulge test in a system that can generate up to 1bar pressure. The largest size tested (800 mum times 600 mum) can withstand a maximum pressure of 780 mbar, while the smaller membranes did not break even for the maximum pressure our test system supplies.

A new ammonia sensor based on a porous SiC membrane

Porous SiC has been found to be extremely sensitive to the presence of ammonia (NH/sub 3/) gas. We report the fabrication and preliminary characterisation of NH/sub 3/ sensors based on porous SiC and Al electrodes. The idea of the SiC is that it is a very durable material and that it should be good for sensors in harsh environments. Until now the only NH/sub 3/ sensors using SiC have been FET based, and the SiC was not porous. The SiC was deposited by PECVD on standard p-type single-crystal Si and was made porous by electrochemical etching in 73% HF and anodisation current-densities of 1-50 mA/cm/sup 2/. Because the etch-rate of Al in 73% HF is very low, we can use Al electrodes instead of Au. This also facilitates our sensor fabrication, as Al is more cleanroom friendly than Au. Preliminary data is given for our devices response to NH/sub 3/ in the range 0-10 ppm NH/sub 3/ in dry N/sub 2/ carrier gas.

Polyimide sacrificial layer for an all-dry post-process surface micromachining module

In this paper we present a novel post-process surface micromachining module that uses a commercial PI2610 polyimide as a sacrificial layer and PECVD SiC or SiN as structural layers. No wet etching is required thus avoiding stiction problems often encountered in wet sacrificial etching processes. A mask set containing cantilever beams, membranes, rotating structures, microswitches, etc. is applied to evaluate the potential of this process module. Silicon carbide micro-mechanical switches with different beam size are also designed and prepared using the three-mask process module suitable for appending to a CMOS fabrication sequence. A displacement /spl ges/15 /spl mu/m is achieved for 95 V and 235 V for the 1 /spl mu/m and 2 /spl mu/m wide silicon carbide beam, respectively.