Two microthermal shear stress sensors: surface micromachined and bulk-bonding micromachined

Abstract

We describe two fabricated microthermal shear stress sensors by antiadhesion surface technology and anodic bulk-bonding technology. Two sensors are based on thermal transfer principles with adiabatic structures. The thermal sensor element is a titanium—platinum alloy resistor sputtered on the top of a low pressure chemical vapor deposited (LPCVD) silicon nitride diaphragm with an adiabatic vacuum cavity underneath. The surface micromachined thermal shear stress sensor uses microbumps on the silicon substrate in the sacrificial layer technology to prevent the silicon nitride diaphragm's stiction to the substrate. Microbumps formed by isotropic silicon etching in HNA (the system HF, HNO3, and HC2H3O2) are arrayed in several points on the silicon substrate with distances of 147 µm in the (200×250)-µm2×1.5-µm vacuum cavity. This cavity is formed by LPCVD silicon nitride film sealing with 30-Pa vacuum degree. The anodic bulk-bonding micromachined thermal shear stress sensor uses bulk silicon substrate etching and anodic bonding to form the (200×250)-µm2×400-µm high aspect ratio cavity with 5×10−2 Pa vacuum degree. The titanium platinum alloy resistor, (5×150)-µm2×0.2 µm, sputtered on the top of the 1.5-µm-thick LPCVD silicon nitride diaphragm with this bonding chamber, has a temperature coefficient of resistance (TCR) value of 0.33%/°C. According to the comparison of the adiabatic characteristics among three cases—a titanium platinum alloy resistor located over the high aspect ratio 5×10−2 Pa vacuum cavity, over the 30-Pa vacuum cavity, and directly on top of the substrate—the first case has the best adiabatic characteristic: the titanium platinum alloy resistor located over the 5×10−2-Pa vacuum cavity has the maximum thermal resistance of 5362 °C/W. Besides the sensor sensitivity performances, it has a comparatively short time constant with value of 0.1 ms under the constant current (CC) mode driving circuit.