Novel piezoresistive e-NOSE sensor array cell

Novel piezoresistive e-NOSE sensor array cell a

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V.Stavrov , P.Vitanov , E.Tomerov , E.Goranova , G.Stavreva a

Nano ToolShop Ltd., Microelectronica Industrial Zone, 2140 Botevgrad, Bulgaria Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72”Tzarigradsko chaussee”, blvd, 1784 Sofia, Bulgaria

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Abstract Future of analytical and manufacturing methods based on micro-mechanical cantilevers, depends critically on the ability to implement parallel operation and fast signal processing [1]. There are two mean 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 designed for e-NOSE applications, using results of previous works [2,3], which solves above mentioned problems. The cell consists of four integrated cantilevers, each having a separate piezoresistor. Additionally, the cantilevers are designed to be different in length and thus having different resonance frequencies. Thus, individual cantilevers are frequency recognizable/addressable. Samples of self-actuated piezoresistive cantilever sensor have been fabricated on n-type, silicon, applying combined surface and bulk micromachining techniques. The cantilever dimensions were chosen to provide approx. 1.8 kHz resonance frequency gap between neighbor individual sensors. The new micro-machined cell is suitable for chemical and biological recognition as a micro-balance. Keywords: cantilever, cantilever array, e-nose, piezo-resistive detection

1. Introduction Thin Si cantilevers have proved themselves as a very sensitive sensor in various Scanning Probe Microscopy techniques (AFM, MFM, etc.) and have been applied in a number of new analytical methods. Specifically, over the last decade some very impressive results have been obtained in chemical and bio-chemical analyses. These sensors are in fact microbalances made of silicon beam coated with specific analyte-sensitive layer. When the cantilever along with the deposited active layer is vibrating, its resonance frequency f is given by

f = 1/2π (k/m) , ½

(1)

where k is the spring constant and m is the effective mass of the system of the cantilever with active layer. If mass m is changed because of the molecular adsorption by Δm (Δm>0), the resonance frequency will be shifted by Δf and both values will be correlated by

Δm = - 2 m Δf/f, (Δf < 0 )

(2)

Using resonance frequency shift method, sensitivities of sub-picogram range have been reported [4, 5]. Although a laser beam cantilever deflection detection was preferred technique in most of studies [6], a number of works, exploiting piezoresistive self-sensing of single cantilevers, have been reported as well [3,7]. Piezoresistive sensors are using a Wheatstone bridge for detection of fine resistivity changes. Depending on the specific application, one or several of bridge resistors are placed on cantilever, but rest of them is either located on dummy levers [7], or standard external resistors were

used. Various designs with four [1,3], two [4] or single [7] piezoresistors integrated on the cantilever have been developed and studied. Cantilever arrays are good candidate for analyses of complex analytes like gas mixtures, explosives and toxic odors [6]. Because of the efforts of many researchers this issue has been thoroughly studied and formalized. However, until recently, these devices have found very limited application. Heavy, bulky equipment and complicated control systems are some of the reasons for this. As a part of e-NOSE (Nanotechnology Olfactory SEnsor) application project, a novel piezoresistive cell was developed and manufactured. It consists of four cantilevers of different length and each of them is having a single piezoresistor for displacement detection. Cantilevers of different length and resonance frequency allow for identification of the individual cantilevers during measurements and minimizes mechanical cross talk. If molecules from the environment adsorb on the cantilever surface, the mass changes involved can be detected by recording the shift in resonance frequency. In order to get an autonomously operating device, bimorph thermo-actuator was integrated on the cantilever. 2. Features of e-NOSE cell design The e-NOSE cell proposed, provides a simple design along with an integrated thermo-actuator, frequencybased “address-recognition”. Given that all four cantilevers are produced simultaneously and in close proximity to each other (in this particular design a 50 µm gap was provided), their thicknesses are very similar. Hence, the only parameter providing predictable frequency gap is the cantilever length. Preliminary

experiments show that the frequency shift caused by adsorption is typically bellow 300 Hz in the range of interest, thus a gap of 1kHz in resonance frequencies of any two neighboring cantilevers is enough to provide non-overlapping resonance curves after an exposure to analyte vapor.

2.1. Piezoresistive Sensor-cell layout Taking into account the above mentioned criteria, 3.5µm difference in length was chosen for the particular design. Thus, cantilevers lengths were set in the range between 309.5µm and 320 µm long.

designed to provide resonance frequency measurement in various conditions. Personal Computer (PC) controlled functional generator signal was supplied either to piezoor thermo-actuator. Frequency sweep range of 30 to 75 KHz was used for present measurements. Wheatstone bridge was supplied by 0.5V DC and its output signal was amplified by a factor of 500. Amplified signal was integrated and this value vs. actuator frequency was recorded. Thermo Actuator Amplifier Bridge supply

3. Manufacturing technology

PC ADC

Cantilever array have been micro-fabricated on double-side polished n-type, , Si wafers having Total Thickness Variation (TTV) less than 1 µm. A standard CMOS compatible process with combined wet KOH (for back side)/dry (for front side) etching techniques were used. The cantilever dimensions: length l = 309.5µm÷320µm, width w = 76 µm and spacing s = 50 µm are defined by photolithography. Since there was no etch-stop layer available, the thickness t of the membrane varied between 2.5 and 4.5 µm from waferto-wafer and latest co-responded to resonancefrequency in the range of approximately 35 to 75 kHz for operation in dynamic mode. Despite this, because of the good etching process uniformity and properly selected raw wafers (TTV