Magnetically Actuated Complementary Metal Oxide Semiconductor Resonant Cantilever Gas Sensor Systems

Abstract

In the present paper, an electromagnetically actuated resonant cantilever gas sensor system is presented that features piezoresistive readout by means of stress-sensitive MOS transistors. The monolithic gas sensor system includes a polymer-coated resonant cantilever and the necessary oscillation feedback circuitry, both monolithically integrated on the same chip. The fully differential feedback circuit allows for operating the device in self-oscillation with the cantilever constituting the frequency-determining element of the feedback loop. The combination of magnetic actuation and transistor-based readout entails little power dissipation on the cantilever and reduces the temperature increase in the sensitive polymer layer to less than 1 degrees C, whereas previous designs with thermally actuated cantilevers showed a temperature increase of up to 19 degrees C. The lower temperature of the sensitive polymer layer on the cantilever directly improves the sensitivity of the sensor system as the extent of analyte physisorption decreases with increasing temperature. The electromagnetic sensor design shows an almost 2 times larger gas sensitivity than the earlier design, which is thermally actuated and read out using p-diffused resistors. The gas sensor is fabricated using an industrial complementary metal oxide semiconductor (CMOS) process and post-CMOS micromachining.

Effect of Coating Viscoelasticity on Quality Factor and Limit of Detection of Microcantilever Chemical Sensors

“…According to (15), the sensitivity may be improved by increasing the thickness of the polymeric sensitive coating, provided that [1]. This analytical expression of the sensitivity has been verified experimentally in [7] and [22], which validate the fact that the mass effect is predominant and that the mass increase of the coating is small compared with the coating mass.…”

“…The geometric properties (5) and (6) are with respect to an equivalent fixed neutral axis, which is associated with the time-varying neutral axis of the hybrid beam. The position of this axis is given by the coordinate , which is measured from the top of the cross section (7) Using the complex flexural rigidity given by (4), the equation of motion of a harmonically excited hybrid beam, considering the only loss mechanism to be associated with the loss modulus of the sensitive layer, takes the well-known form (e.g., [16]) (8) where is the (complex and harmonically varying) transverse displacement, is the arbitrary distribution of the force amplitude, is the mass per unit length of the beam (including coating), and is the angular forcing frequency. Following standard procedures for solving (8) (e.g., [16]), an expression for the resonant frequency can be obtained (9) Mathematically, the expression for the quality factor is given by (10) where is the frequency bandwidth taken with 3-dB attenuation from maximum gain.…”

“…As a result, the microcantilever's mass and stiffness may be modified. The primary cause of the decrease of the resonant frequency is the mass variation [2]- [7]. Then, in a first-order approach, the stiffness variation can be neglected and only the mass variation is taken into account.…”

Monolithic CMOS multi-transducer gas sensor microsystem for organic and inorganic analytes

“…6 and Refs. [30,31]. The vibration of the magnetically excited cantilever is detected by a set of four stress-sensitive MOS transistors (two transistor gates parallel to the cantilever axis, which are severely deformed, and two gate regions perpendicular to the cantilever axis, which are hardly deformed) in a Wheatstone bridge configuration located at the cantilever base, which are biased in a linear region.…”

“…Compared to another widely used excitation method, namely thermal excitation, this method features the advantage of low power dissipation (1.3 mW), which leads to a significantly lower temperature (temperature increase only 1-2 • C above ambient temperature) on the cantilever as well as in the sensitive polymer layer. Since, the quantity of absorbed analyte in the polymer is inversely proportional to the temperature, a reduction of the power dissipation on the cantilever yields an enhanced chemical sensitivity of the device [31].…”