Novel piezoresistive e-nose sensor array cell

Transcription

1 4M2007 Conference on Multi-Material Micro Manufacture 3-5 October 2007 Borovets Bulgaria Novel piezoresistive e-nose sensor array cell V.Stavrov a, P.Vitanov b, E.Tomerov a, E.Goranova b, G.Stavreva a a Nano ToolShop Ltd., Microelectronica Industrial Zone, 2140 Botevgrad, Bulgaria, b Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko chaussee, blvd, 1784 Sofia, Bulgaria

2 Content Introduction Motivation Mass-measuring method Features of proposed MEMS cell Manufacturing technology Parameters Measurement results of MEMS cell Measurement set-up Measurement of MEMS cell bonded on PCB carrier Analyses of resonance frequency measurement Parallel measurement of MEMS cells Conclusions

3 Motivation Future of analytical and manufacturing methods based on micro-mechanical cantilevers, depends critically on the ability to implement parallel operation and fast signal processing. There are two main reasons: - high throughput requirement and - complexity (multidimensionality) of analyzed value. In order to get parallel function, any single device should be simultaneously: - recognizable, - autonomously actuated and - independently accessible for readout. Devices, fulfilling these requirements, are suffering from a substantial increase in complexity of both: layout and manufacturing technology. In present paper, we demonstrate a novel design of a MEMS (Micro-Electro-Mechanical Systems) cell, dedicated to e-nose applications, which solves above mentioned problems.

4 Mass-measuring method Fig. 1 A (а) (b) Fig. (1) - Relation between mass and resonance frequency. Resonance frequency and effective mass of the cantilever are correlated by equation (a). If the mass is changed, it causes resonance frequency change. In first order of approximation, the relation between these changes is given by equation (b). So, if one measures the resonance frequency shift precisely, the mass-change could be calculated, respectively. Fig. 2 Fig. (2). Detection of vibration amplitude by piezoresistor. Deflection of cantilever free end causes stress at it s base and stress changes piezoresistor value, proportionally to the amplitude. Thus detecting the frequency of maximum resistivity one can define amplitude resonance. Experimentally, it was found that resistivity change is very small, typically < 0.1%. Fig. 3 Fig. (3) Detection of small resistivity changes. Fine resistivity changes of one resistor could be measured by adding another three ones and connecting them in a Wheatstone bridge. Depending on the specific application, one or more bridge resistors are placed on cantilever, but rest to four should be added outside. Various designs with four, two or single piezoresistors integrated on the cantilever have been developed and studied, elsewhere.

5 Features of the proposed MEMS cell 1. It consists of four cantilevers of different length. Each of them has a single piezoresistor embedded at it s base. Fig. 4 Fig. 5 Fig All four resistors with same topology are connected in a Wheatstone bridge, together, as shown in Fig. (4). Four I/O pins are enough for power supply and output signal measurement. 3. Integrated bimorph thermo actuator. A bimorph thermo actuator is integrated on cantilevers, as it is shown on Fig. 5. In this particular case, it consists of four serial metal meanders with same topology. Thus, two pins are needed to supply the actuator. How the cell works? Having cantilevers with different resonance frequencies, at every particular vibration frequency, no more than one cantilever is in resonance and it s resistor value differs from the other ones. Thus, the output voltage of the bridge will be zero for all frequencies except, those around resonance frequencies of individual cantilevers. Our cell with four cantilevers of different length should have a resonance spectrum, similar to the one, shown in Fig. (6). Ones the resonance spectrum is measured, the individual cantilevers could be recognized/identified by their resonance frequency. Advantages of proposed new MEMS cell Simple design - just 6 I/O pins, with no performance sacrificed. Frequency recognition of the individual cantilevers is provided. Complete functional device sensor and actuator.

6 Manufacturing technology 1 Si N-type <100> SiO2 SiO2 Si 3 N 4 INITIAL OXYDATION 5 BACK-SIDE WET ETCH 2 BORON PATTERN/DOPING FRONT-SIDE DRY ETCH 3 Resistor RESISTOR PATTERN/DOPING 6 P + Al 4 Al Al Heater AL-METAL DEPO/PATERN Fig.7a 7 P + DIE SEPARATION Fig.7b The fabrication process (developed in Nano ToolShop), shown in Fig. 7a and Fig.7b, is based on the double side surface/bulk silicon micromachining. The raw Si wafers: DSP, n-type, <100>, TTV<2 µm, 4 6 Ω.cm.

7 Parameters Cantilever dimensions: Lengths - set in the range between 309.5µm and 320 µm Difference in lengths chosen 3.5 µm Pitch 126 µm (width 76 µm, space 50 µm) Thickness variation - wafer-to-wafer between 2.5 and 4.5 µm - die-to-die (on a wafer) up to 2 µm - within one cell ± 0.1 µm Alignment (front/back side) ± 5µm Electrical Parameters Piezoresisor resistance 1500 ± 250 Ω Thermo-actuator resistance 40 ± 4 Ω Fig. 8. Scanning electron micrograph of cantilever array with four cantilevers of different length

8 Measurement set-up The signal of PC controlled functional generator is supplied to thermo-actuator, shown in Fig. 9. Frequency sweep range of 30 to 75 KHz was used. Wheatstone bridge was supplied by 0.5V DC and its output signal was amplified X 500. Fig. 9 Schematic set-up for resonant frequency measurements RMS of amplified output signal vs. actuator s frequency was recorded.

9 Measurement of MEMS cell bonded on PCB carrier Fig.10а Fig.10b Fig. 10. Optical micrographs of fourcantilever PCB carrier during measuring Fig. 11. Resonance spectra of a cell X 4 cantilevers, obtained by using 6 pins In order the read-out the signal, the MEMS cell have been is bonded on a chip carrier made of PCB base material. Once spectra before and after sensor exposure are recorded, the frequency shift of each individual cantilever is corelated to the interaction it have been functionalized.

10 Simple analyses of resonance frequency measurement U out, V The amplified output signal U out to 63 khz is shown in Fig 12. for frequency range of 53 khz All four resonance peaks are clearly detected and the frequency gap is in an excellent agreement with predicted values. 0.2 The amplitudes at resonance frequencies of 54.46, 56.34, and khz, have been assigned to the four cantilevers. The second shortest cantilever (59.51kHz) is 63 nm thicker than others within the cell f, khz Fig. 12. Spectrum of resonance frequencies (in khz) of four-cantilevers array. Calculated sensitivity of resonance frequency vs. cantilevers thickness was estimated to 16 khz/µm.

11 Parallel measurement of MEMS cells Fig. 13. Parallel measurement of 16 (4 cellsx4) cantilevers device obtained by using just 6 pins If proper cell design and electronics are used, the MEMS cell could have independent behavior within a complex measuring system.

12 Conclusions Conclusions - A novel MEMS cell based on array of four silicon cantilevers and actuator was designed and fabricated. - The cantilevers lengths were chosen to provide resonance frequency recognition and to avoid mechanical cross talk. - Four pins for power supply and deflection sensing and two pins for integrated thermo actuator are used. Four-cantilever cell becomes a completely functional autonomous device. Functionalized cell operates as an e-nose for characterisation and identification of different gaseous analytes, which is the object of further investigations. Acknowledgements This work was a team work. It was possible because of the financial support of Bulgarian National Innovation Fund, Project IF-02-20/2005. The authors would like to thank all colleagues from Nano ToolShop for processing the cantilever arrays. The contribution of MBE and Surface Analyses lab of ISSP headed by Dr. G. Minchev in dramatically improved testing set-up and in understanding the nature behind the results, have to be highlighted separately.

13 Conclusions THANK YOU FOR YOUR ATTENTION!