A surface micromachining process for the development of a medium-infrared tuneable Fabry–Perot interferometer

Sensors and Actuators A 113 (2004) 39–47

A surface micromachining process for the development of a medium-infrared tuneable Fabry–Perot interferometer Carlos Calaza a , L. Fonseca b,∗ , M. Moreno a , S. Marco a , C. Cané b , I. Gracia b a

Departament d’Electrònica, Instrumentation and Communication Systems (SIC), Universitat de Barcelona, Marti i Franqués 1, 08028 Barcelona, Spain b Centro Nacional de Microelectrónica (IMB-CSIC), Campus UAB, 08193 Barcelona, Spain Received 20 September 2002; received in revised form 22 December 2003; accepted 12 January 2004 Available online 28 March 2004

Abstract Electrostatically driven moving micromirrors are proposed in this work for the fabrication of a tuneable Fabry–Perot interferometer. The device is intended for a gas sensing application, and operates in the medium infrared spectral range. In order to achieve high filter selectivity and to reduce the loss of transmitted light it is of utmost importance that the mirrors are kept parallel after release and during device actuation. Surface micromachined polysilicon mirrors have been selected for this application. This technology provides an accurate control of the air gap between mirrors. The internal homogeneous stress and stress gradient states of the polysilicon films have a great influence on mirror deflection upon release. The micromachining process developed to obtain flat-surface micromirrors with an accurate air gap, by reducing the stress-induced polysilicon deflection and building thick oxide sacrificial layers is described in this work. © 2004 Elsevier B.V. All rights reserved. Keywords: Surface micromachining; Fabry–Perot interferometer; Internal stress and stress gradient

1. Introduction There is an increasing demand in the market for low cost and long-term stable gas detectors to be used in the detection of air pollutants and toxic or explosive gases. Widespread systems are suitable only for a limited range of undemanding applications, due to its lack of accuracy, stability and immunity to false alarms and poisoning. Optical gas detectors, based on the selective absorption of IR radiation by the molecules of different gases, offer important advantages compared to sensors based on other principles, especially concerning to selectivity and long-term stability [1]. Infrared radiation absorption is a favourable sensing principle for multicompound analysis, because most of pollutant gases have strong absorption bands at characteristic wavelengths in the medium infrared (MIR) spectral region. As the absorption level increases with gas concentration (Lambert–Beer’s law), the pollutants can be identified and their concentrations measured from the transmittance spectra, with high selectivity levels. ∗ Corresponding author. Tel.: +34-93-594-77-00; fax: +34-93-580-14-96. E-mail address: [email protected] (L. Fonseca).

0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.01.047

Conventional non-dispersive infrared (NDIR) systems consist of an IR source, a gas absorption chamber, an IR detector and an optical filter which selects the wavelength region corresponding to a particular absorption band. Traditionally optical measurements are made by using NDIR instruments based on the single channel, single wavelength (SCSW), single channel, dual wavelength (SCDW) or dual channel, dual wavelength (DCDW) methods [2]. The SCSW method suffers seriously from long-term instability due to the lack of an optical reference. The SCDW method is much more stable because source intensity variations or degradations in optical components are compensated with a reference signal, provided by a second optical filter located in an absorption-free wavelength region. But SCDW suffers from the need of a mechanical device for switching the wavelengths. This device is typically a motor-driven chopper wheel that requires regular maintenance and cannot be truly miniaturised. The DCDW method makes use of two detectors, one for the measurement and other for the reference channel. The two detectors must be identical in all aspects; otherwise problems with such factors as temperature variations may arise. The problem with optical devices has been their high cost, especially in low-end applications. However, as the produced instrument volumes grow, the

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Fig. 2. Structure of the Fabry–Perot interferometer. Fig. 1. Scheme of the optical set-up proposed for the gas analyser.

price competitiveness of optical sensors can be improved by integration and miniaturisation. To improve its performance and price competitiveness of NDIR systems it is a promising concept to apply this method in a miniaturised gas analysis detector, by using silicon based MOEMS [3–5]. The micromachined Fabry–Perot interferometer (MFPI) based sensor proposed in this work can overcome many of the disadvantages of the traditional methods. In this instrument the MFPI is tuned so that its pass band coincides with the absorption band of the measured gas; a detector records the strength of the signal getting through the measurement chamber. The pass band of the MFPI is then shifted to either side of the absorption band; the detected signal constitutes the reference signal. The ratio of these two signals indicates the degree of light absorption, and so the gas concentration. Fig. 1 shows the optical set-up proposed for the NDIR system. IR radiation produced by a thermal emitter will be focused onto the absorption area of the IR detector with the tuneable MFPI, through a gas-filled absorption chamber. Although the number and complexity of elements can be greatly reduced, it is not possible to decrease the length of the absorption chamber, by reason of Beer’s law. Detection of low molecular gases in the ppm range requires optical paths in the centimetre range. Our research interest has been focused on the integration into a single device of the MFPI together with a thermoelectric infrared detector, to obtain an active detector module with spectral selectivity that can be used as an infrared spectrometer. The aim of this work is to evaluate a surface micromachining process that has been customised for the development of the micromachined Fabry–Perot interferometer.

2. Interferometer structure The proposed arrangement for the Fabry–Perot interferometer is shown schematically in Fig. 2. It consists of a vertically integrated optical resonator structure, formed by a (simple or composed) dielectric moving micromirror suspended by cantilever beams over a second identical parallel mirror, which is attached to the silicon substrate.

In such resonant cavities the transmittance of certain wavelengths (T(λ)) is favoured while the transmittance of others is punished as a function of the mirror optical separation, nd, where d is the distance between the mirrors and n the index of refraction of the media between them. (1 − R1 )(1 − R2 ) T(λ) = √ √ (1 − R1 R2 )2 + 4 R1 R2 sin2 2π(nd/λ) Tmax (λi ) =

(1 − R1 )(1 − R2 ) , √ (1 − R1 R2 )2 for λi =

Tmin (λi ) =

4nd , i = 2, 4, 6, . . . i

(1 − R1 )(1 − R2 ) , √ (1 + R1 R2 )2

4nd , i = 1, 3, 5, . . . i From the above transmission expressions corresponding to an ideal Fabry–Perot structure (without absorption losses in the mirrors) it is clearly seen that the maximum transmittance will be optimum when the reflectivity of both mirrors is identical (R1 = R2 ). To obtain additionally a good contrast between the maximum and minimum transmittance the reflectivity of both mirrors should be as close to one as possible (R1 = R2 ∼ = 1) (see Fig. 3). The antireflection coating (oxide layer) included between the bottom dielectric mirror and the silicon substrate in the proposed structure has been intended to make as similar as possible the reflectivity of both mirrors, increasing the transmittance of the MFPI structure. The wavelength tuning of the structure is achieved by changing the air gap distance between mirrors, d, by electrostatic actuation. Apart from determining the filtered wavelengths, the dimension of the gap used in the resonant cavity has also a huge influence in other features of the Fabry–Perot transmittance: the free spectral range and the width of the transmitted peaks (see Fig. 3). Due to the periodicity of the resonant optical response, a FPI structure presents a series of transmission peaks located at wavelengths equal to 2nd/i, where i = 1, 2, 3 . . . is usually called transmission order. The free spectral range (λ), defined as the distance between two adjacent peaks in the spectral transmittance, and the full width at half maximum (FWMH) of the corresponding transmitted peak at a given gap distance, d, and for a given transmission for λi =

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1,0

i=4 i=3

41

i=1

i=2

Transmittance

0,8

0,6

FWMH (λo) 0,4

0,2

0,0

∆λ o 0,5

1,0

1,5

λo

2,0

2,5

3,0

Wavelength, µm Fig. 3. Transmittance for an ideal Fabry–Perot structure, with R1 = R2 = 0.95, n = 1 and d = 1 ␮m.

order, i, are described by:

 1 1 1 λ0 = 2nd − = 2nd , i i+1 i(1 + i) i = 1, 2, 3 . . . FWMH(λ0 ) =

λ0 (1 − R) iπR1/2

For a gas detection application this peak width should be comparable to the width of the gas spectral line, and the free spectral range should be large enough to encompass the peak positions of the gases to be discriminated. Going back to the mirror requirements, mirrors with matched (and high) reflectivity are required for optimal performance. When dealing with dielectric mirrors, made up of alternating layers of thin films with different optical properties the maximum reflectivity of such mirror is obtained using layers with a λ/4n optical thickness, and it is finally limited by the contrast of index of refraction of the mirror material, n, and the surrounding medium, n0 :
R=

n2 − n0 n2 + n 0

2

2.1. Gas detection application: interferometer optical requirements As discussed above, the distribution of the absorption bands of the target compounds that have to be covered in the gas detection application determines the optical requirements that must be assured by the MFPI. In the planned specific case of a simultaneous CO, CO2 and CH4 detector, the MFPI device must operate in the medium infrared wavelength range (λ = 3.2–4.7 ␮m) in order to be tuned with

the major absorption bands of these three gases (λCH4 = 3.30 ␮m, λCO2 = 4.26 ␮m, λCO = 4.67 ␮m). A fourth position where no absorption is present (λφ = 3.95 ␮m) is also useful in that range in order to assure the baseline of the device operation as it was discussed in Section 1. With these figures in mind, and taking into account the periodicity of the Fabry–Perot interferometer transmittance, it is necessary to obtain a tuneable optical filter with a free spectral range higher than the 1.37 ␮m separation of the CO and CH4 absorption bands to achieve an optimal selectivity between gases. This requirement makes necessary the use of the first order transmission peak of the MFPI, that is associated with a minimum mirror spacing d = λgas /2. Apart from these application specific requirements, there are several additional technological aspects that may possibly have an effect on the final behaviour of the tuneable MFPI, mostly by reducing the spectral resolution that can be obtained with this device. It is important that the active area of the micromirrors remains flat and parallel during the MFPI operation in order to physically have a well defined distance between them and, therefore, a well defined transmitted wavelength. This will prevent the broadening of the MFPI transmission peaks and thus avoid the loss of spectral efficiency. In order to minimise transmission losses it is important that the micromirrors present a low absorption level in the infrared spectral region, and also a good smoothness (compared to the micron sized wavelengths involved) to avoid scattering. The micromirror active area should cover at least the absorbing area of the infrared detector that is going to measure the transmitted radiation. In our case the infrared detector is a micromachined thermopile, and the characteristic dimension of its absorbing area is in 150–300 ␮m range. Summing up, the proper MFPI for the proposed application should have an initial gap of around 2.4 ␮m; it should

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be actuated till a minimum gap of approximately 1.7 ␮m, which defines a movable range of 0.7 ␮m that is slightly less than the 30% of the initial gap (the stability limit of electrostatic actuated structures due to pull in voltages); its micromirrors should be transparent to the infrared and they must be smooth and kept flat, with variations of only tens of nanometers, in areas with a characteristic dimension of 150 ␮m. The polysilicon-based surface micromachining technology is especially suitable for the fabrication of planar structures such as the flat and parallel micromirrors used in a MFPI. It provides an accurate control of the air gap distance d, which results from a final etch process that removes a sacrificial SiO2 layer deposited between mirrors. This process releases the movable structures, which have been previously defined by patterning the polysilicon structural layers. Polysilicon is a good candidate to fabricate the upper movable mirror. The good structural properties of polysilicon allow the fabrication of free standing structures rather planar, its high index of refraction makes it a good dielectric mirror material, and the possibility of making it electrically active by implanting it enables the electrostatic actuation. Provided that the required doping level for electrical actuation and optimisation of structural stresses is low enough to avoid free carrier infrared absorption, polysilicon is a good candidate to fabricate infrared micromirrors. In this way, the mirror fabrication will be simple, no discrete assembly will be required and the actuators and mirrors can be integrated in the same process. They will have low driving voltages, small power consumption, and high actuator density [6]. The main objective of this work has been the development of a tailored micromachining process to fabricate a MFPI being able to meet with the application specific requirements, i.e. to obtain a movable flat-surface micromirror with an accurate initial air gap and capable of be tuned in the required range. This has been mainly achieved by modifying a standard surface micromachining process to reduce the stress-induced deflection in the polysilicon layers, and to allow the build of thick oxide sacrificial layers. The other features, with a less important influence in the final system behaviour, have not been considered as design objectives during this technology tailoring process, and have just been characterised to confirm that they have tolerable values. The final metrological characteristics of such a MFPI based NDIR spectrometer, fulfilling these optical requirements, have been assessed through analytical system modelling and data processing [7]. The results obtained have shown that a good resolution can be obtained in the detection of the different gases provided that adequate signal processing techniques are used.

3. Fabrication process The thickness of the structural and the sacrificial layers is set to non-usual values for surface micromachining in

mechanical applications, due to the application optical constraints discussed above. Other constraints of technological nature, mainly related to the method employed to avoid the sticking of the mirrors during the release process, based on a vaporous HF etchant solution carried by a N2 flux, have been also taken into consideration. Our technical effort for the development of a surface micromachining technology suitable for the fabrication of the MFPI has been focused on two main processing aspects: • The reduction of the polysilicon intrinsic stress based on a polysilicon doping and annealing sequence, to avoid the stress-induced deflection of the micromirrors. • The growth of deposited sacrificial oxides, with an accurate thickness and without optically active remains after release, based on the thermal oxidation of pre-deposited polysilicon layers. 3.1. Stress-induced deflection Fabry–Perot micromirrors are made from polysilicon and Si3 N4 thin films, which have internal stresses due to their fabrication process. These internal stress and stress gradients would manifest when the sacrificial layer is removed. After release the internal stress relaxes and the mirror is deformed, affecting negatively to the performance of the interferometer. This deflection causes increasing difficulties as the thickness of the sacrificial and structural layers is reduced. Therefore, the performance of the Fabry–Perot interferometer will be critically dependent on the residual stress and stress gradient states in the structural layers [8]. To obtain structural materials enabling the fabrication of large area planar structures the polysilicon and Si3 N4 deposition and subsequent annealing parameters were tailored, to achieve films ensuring a tensile residual stress and a minimum stress gradient. Our own previous established micromachining technology, tailored to inertial micromechanical applications, relied on structural (LPCVD) polysilicon layers 2 ␮m thick. Due to the optical constraints stated above our efforts were devoted to scale down the structural polysilicon preserving a low tensile stress and a tolerable stress gradient, thus enabling the fabrication of large area planar structures. To reduce the internal stress level of the films involved (0.35 ␮m polysilicon and 0.6 ␮m nitride layers), they are doped and/or amorphized by ion implantation and annealed at high temperatures. Films with various doping levels, and annealing times and temperatures have been investigated. The internal tensile stress was evaluated with a surface micromachined test structure, consisting of a bridge attached to a ring in a diameter perpendicular to the clamping points of the structure, and the internal stress gradient was evaluated with test structures consisting of cantilever beams, by measuring the stress-induced tip deflection [9]. It has been found that long annealing times and high doping levels act favourably on internal stress and stress gradient reduction. An internal tensile stress of 31±5 MPa and a pos-

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Fig. 4. Optical interferometric measurement of mirror deflection.

itive internal stress gradient of 2.4 ± 0.9 MPa/␮m have been measured for the polysilicon structural layer selected for the Fabry–Perot fabrication (values not far apart from their thicker counterparts). These stress related parameters were measured on dedicated test structures, while the deflection of test membranes were directly measured, with a 273 nm precision, using an optical microscope operating in interferometric mode, based on the Mirau method (Fig. 4). The total deflection generated by the stress relaxation in a hexagonal membrane with an equivalent square area of 300 ␮m × 300 ␮m is lower than 273 nm. Only one interferometric circle can be observed in the hexagonal membranes when they are illuminated with a 546 nm wavelength light source. The presence of even a low stress gives rise to different trade-offs (planarity/mirror area, effective optical area/total area). If a large surface has to be covered a cellular design is a must. The unit cell design provides another opportunity to alleviate the stress influence on the planarity of the structure. For a given value of stress and stress gradient, the extent of the stress-induced deformation will depend on the actual details of the movable mirror: size and shape of the membrane and number, size and shape of its support arms. Cell designs that convey the greatest part of the mechanical deformation to the support arms (after release and during electrostatic actuation), keeping the membrane area as flat as possible, will be therefore favoured. A detailed FEM analysis (deflection characteristics and pull in behaviour) of the unit cell has been undertaken to predict the exact performance in optical use of different mirror structures [10] using the stress values measured for the selected structural layers. Other important aspect in the reliability of the MFPI structure is the influence of the support compliance in the movable mirror displacement. The polysilicon support arms of the movable mirrors are anchored to the silicon substrate. Hence, the thin polysilicon films in the structural layers must surmount a deep sacrificial layer step. The FEM analysis of this support predicts an accumulation of stress in the extreme of the arm near to the anchor, which may result in

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a fracture of the structures. To avoid this problem a second alternative based on the chemical–mechanical polishing (CMP) method was tested for the supports. After the patterning of the oxide sacrificial layer a thick polysilicon layer filling the anchor sites is deposited on the wafer. The wafer is mounted in a polishing head, which rotates against a polishing pad. Polishing slurry is added at the same time. In the polishing process the surface topology of the object is reduced by a combination of mechanical friction and chemical etching. The material at the highest position experiences the largest mechanical friction and is etched faster than material at lower positions. This difference in etch rate leads to a planarisation of the wafer. After this CMP process the polysilicon is completely removed on top of the sacrificial oxide while the anchor regions remain filled with columns of polysilicon. When the polysilicon structural layers of the movable mirror are deposited they remain anchored to these polysilicon columns, avoiding the complex topography of the sacrificial layer (Fig. 7). Multilayer mirrors (polysilicon/Si3 N4 /polysilicon) and single layer polysilicon mirrors have been tested for the MFPI. This second option decreases the difficulty of the fabrication process, reduces the total number of layers deposited, and results in more planar mirrors (lower stress gradients). However, it produces mirrors with lower reflectivity, increasing the FWMH of the MFPI transmission band as shown in Fig. 6, and reducing the maximum transmittance level, and the rejection degree in the suppression band. This less selective response of the bare single layer mirrors would be less convenient in a conventional NDIR measurement, but it has been shown that a broader filter can provide better resolution in quantitative terms than a selective filter, if it is used as an infrared spectrometer, i.e. it is tuned by applying different control voltages, so that its pass band can scan a certain wavelength range [7]. 3.2. Sacrificial layer The gap between mirrors after release must be in the order of 2.5 ␮m to fulfil the optical requirements. This non-usual large spacing requires a sacrificial SiO2 layer with a thickness that is much higher than in other common applications of silicon micromachining. This layer cannot be obtained by thermal oxidation techniques applied on the silicon substrate because the bottom mirror is already there, but it could be generated using a deposition process. However, none of the options we had at hand (doped and undoped APCVD deposited oxides and undoped PECVD deposited oxides) yielded universal good results. Some of them could not be deposited to the 2.5 ␮m thickness required (undoped APCVD) due to a high compressive stress, which produces the layer fracture, and others (doped APCVD and undoped PECVD) would leave an optically active residue after the vapour-HF release with absorption peaks in the 3–5 ␮m region. The method developed to obtain a versatile thick oxide that could overcome this problem consisted on

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the thermal oxidation of a pre-deposited polysilicon layer of appropriate thickness (45% of the desired oxide thickness). A thin Si3 N4 layer was previously deposited between the mirror and the gap oxide to avoid the oxidation of the underlying polysilicon layers of the fixed mirror. As discussed previously the optical performance of the dielectric mirrors that build up the Fabry–Perot structure will depend on the roughness of the structural layers. To obtain a high mirror reflectivity in the optical operating range used in the application it must be guaranteed that the mirror surfaces have a roughness lower than the optical wavelength. The roughness of the layer used for the bottom mirror will be low enough, as it is directly deposited on the silicon wafer surface. But on the other hand, the roughness of the top mirror will be strongly influenced by the texture obtained in the sacrificial oxide layer. To reduce the roughness of the top mirror layer the oxidised polysilicon sacrificial layer starts with the deposition of a LPCVD polysilicon layer at low temperature and high pressure (580 ◦ C, 46,66 Pa). These deposition parameters have been selected to attain a polysilicon amorphous structure, which presents a lower roughness level than the poly-crystalline one. The sacrificial SiO2 layer was finally obtained by a thermal oxidation process, in which the roughness level of the layer surface is nonetheless increased due the re-crystallisation of the polysilicon structure. Fig. 5 shows an atomic force microscope (AFM) picture of the surface obtained for the SiO2 sacrificial layer, and the underlying Si3 N4 layer used as a stop barrier for the oxidation process. As can be seen, the roughness level of the SiO2 layer is higher than the one of the Si3 N4 layer, but it is low enough to obtain a high reflectivity in the medium infrared spectral range. Effectively, the AFM roughness measurement performed with this layer provides an r.m.s. roughness value of 12.9 nm, value that is

Fig. 5. An AFM micrograph of the surface of the sacrificial SiO2 layer (left and top parts of the image), obtained from the oxidation of a pre-deposited polysilicon layer.

Fig. 6. FTIR measurement of the optical transmittance of a fixed Fabry–Perot interferometer fabricated for three different wavelengths corresponding to the CH4 , CO and reference wavelengths. The solid line curves correspond to interferometers implemented with single layer mirrors, the dotted line curve corresponds to an optical simulation of the behaviour of the CH4 single layer structure, and the dash-dotted line plot corresponds to an interferometer implemented with multilayer mirrors tuned to the reference wavelength.

one order of magnitude higher than the one measured for the original polysilicon layer. Therefore, there are no geometrical features in the mirror surfaces with sizes comparable to the wavelengths to be used in the application. On the other hand, a slightly rough surface could be even preferable to a completely smooth one to prevent permanent sticking of the top mirror to the bottom one during release or actuation. Fig. 6 shows the transmittance spectra measured for a fixed Fabry–Perot interferometer. This device uses the same dielectric mirrors as the tuneable device, and a fixed oxide gap instead of the variable air gap previously described. This gap has been obtained with the same technique used to obtain the sacrificial oxide layers, and therefore, it has the same roughness characteristics. This structure allows the evaluation of the effects of the surface roughness and infrared transparency of the doped polysilicon micromirrors in the interferometer transmittance, independently of the effects of the stress-induced mirror curvature. Transmittance levels as high as 85% have been measured with such fixed structures, value which fits very well with the results obtained from the simulation of the optical transmittance of the multilayer structure, using perfectly smooth surfaces and transparent layers. The simulation results can also be seen in Fig. 6 for one of the structures. The close match between the measured and simulated transmittance evidence how close to the optical ideal behaviour are the real layers. It can be then concluded that the doping level of the polysilicon needed for stress optimisation did not produce any opacity of the mir-

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ror in the infrared due to free carrier absorption, and that the residual roughness of the upper mirror did not pose any problem either. 3.3. Release methods A vapour-HF etching of the sacrificial layer has been tested for the release of the Fabry–Perot movable mirror. The tendency of the released structures to stick to the substrate in a wet medium can be overcome by using a vapour-HF release approach. In our case, a N2 flux was bubbled through a 49% HF solution, and brought onto the wafers, which were kept at 32 ◦ C during this process. Also, a direct 49% HF wet release can be attempted with success by altering a bit the usual technological sequence and introducing some minor modifications in the structure, without involving more complex anti-adhesion techniques [11].

Fig. 7. A SEM micrograph of a vapour-HF released Fabry–Perot interferometer structure. Detailed view of the support structures with (a) and without (b) the polysilicon CMP anchor.

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Figs. 7 and 8 show details of the so far described Fabry–Perot structures after the vapour-HF release process. As can be seen, the polysilicon movable structures remain almost plane and parallel to the fixed mirror after release. This ‘dry’ release process has shown a very good yield, allowing us to develop large area Fabry–Perot devices by using cellular designs (see Fig. 8). This cellular approach is a must if a large area has to be covered. Hexagonal unit cells (104 to 4 × 104 ␮m2 ) with parallel side arms have been chosen and optimised through FEM analysis.

4. Future work: actuation and integration After release, the movement of the interferometer could be shown in the brute mode by electrically collapsing the structure (Fig. 9). This movement proved to be reversible

Fig. 8. A SEM micrograph of a vapour-HF released Fabry–Perot interferometer structure. View of the cellular design based on hexagonal cells with single layer mirrors (a) and multilayer mirrors (b).

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Fig. 9. An optical photograph of an actuated Fabry–Perot interferometer. Unbiased (a) and biased to collapse for contrast (b).

Fig. 10. A couple of Fabry–Perot interferometers mounted on top of a CMOS IR detector (bulk micromachined thermopiles) wafer with a flip-chip technique. The bright spots in the field are the solder bumps that are going to receive the facedown interferometer dices.

and the same device could be cycled several times. A detailed study of a more controlled actuation of the device is underway. Even though all the designs have been successfully fabricated and released, a complete characterisation will be needed to evaluate the influence of different design features in device operation. The final goal of this Fabry–Perot interferometer is to be integrated with an IR detector, to obtain a spectrally selective IR detector module that can be tuned in the absorption bands of different gases. Flip-chip technology has been selected for this integration due to its advantages. The flip-chip process provides self-alignment of the two optical components, without involving more complex mechanical positioning processes. Additionally, flip-chip metal bumps provide electrical connection between the chips, allowing the placement of all the connection pads in the same plane. The flip-chip assembling does not involve any aggressive thermal or mechanical process that could damage the micromachined structures that have been previously released. This assembling technique has been successfully tested using fixed Fabry–Perot interferometers and CMOS compati-

ble thermopiles as IR detectors based on a dielectric membrane obtained with bulk micromachining (Fig. 10).

5. Conclusions A surface micromachining technology has been successfully tailored to enable the fabrication of a medium infrared Fabry–Perot interferometer. Several processing steps have been customised in order to allow the employment of structural and sacrificial layers with ‘non-conventional’ thickness, set by the application optical constraints. Micromirrors with different designs (size, shape, structural layers, and anchor types) have been developed using these processing steps. These structures were made free with a good yield using the vapour-HF release method, obtaining a complete set of released micromirrors. Reversible movement of the top mirror could be proved, although an electrode redesign is underway to better convey the bias to the structure. A flip-chip technology approach has been proposed for the integration of the interferometer with a suitable IR detector. First attempts

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on this direction have been made and the results have been encouraging.

Acknowledgements This work has been financed by the Spanish CICYT project nos. TIC-98-0987-C03-03 and DPI-2001-3213C02-01.

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Biographies Carlos Calaza was born in Mondoñedo, Spain, on 1974. He received his BS degree in physics from the University of Santiago de Compostela, Spain, in 1996, the degree in Electronic Engineering from the University of Barcelona, Spain, in 2000, and his PhD from the Department of Electronics, University of Barcelona, in 2003. From 1999 he has been working in the field of silicon MOEMS for infrared gas sensing applications. His research interests include the design, modelling and test of MEMS, and the development of pixel and system architectures for IR imagers. Luis Fonseca was born in Barcelona, Spain, on 17 February 1966. He received his BS and PhD degrees in physics from the Autonomous University of Barcelona in 1988 and 1992, respectively. In 1989 he joined the National Centre of Microelectronics as a post-graduate student, working till 1992 on the growth and characterisation of thin dielectric films for VLSI and ULSI applications. After this first research period he has worked as a process engineer, currently leading the diffusion and deposition areas of the CNM production facilities. This technological background has allowed him to undertake research tasks in the general field of MEMS fabrication, and lately in gas sensing applications. Mauricio Moreno was born in Barcelona, Spain. He received the degree in physics in 1989 from the University of Barcelona (UB), and the PhD Degree in sciences in 1995 from the Polytechnic University of Catalonia (UPC), Spain. He has been an associate professor in the Electronics Department, UB, since 1997. He is involved in the design and test of infrared thermopile detectors and the test of integrated optical devices in silicon technology for DWDM applications. Other fields of interest include arrays of CMOS integrated photodetectors for imaging, rangefinders, and CMOS optical receivers for fibre optical communications. Santiago Marco is associate professor at the Department d’Electronica of Universitat de Barcelona since 1995. He received the degree in Physics from the Universitat de Barcelona in 1988. In 1993, he received his PhD (honor award) degree from the Departament de F´ısica Aplicada i Electrònica, Universitat de Barcelona, for the development of a novel silicon sensor for in-vivo measurements of the blood pressure. In 1994, he was visiting professor at the Universita di Roma ‘Tor Vergata’ working in Data Processing for Artificial Olfaction. He has published about 40 papers in scientific journals and books, as well as more than 80 conference papers. His current research interests are two-fold: chemical instrumentation based on intelligent signal processing and microsystem modelling. Carles Cané received the BSc degree in Telecommunications Engineering in 1986 and the PhD degree in 1989 from the Universitat Politècnica de Catalunya in Barcelona, Spain. Since 1990 he is permanent researcher at the National Microelectronics Centre (CNM) in Barcelona. He is currently working in the fields of sensors and microsystems and their compatibility with standard CMOS technologies. Isabel Gràcia received the PhD degree in physics in 1993 from the Autonomous University of Barcelona, Spain, working on chemical sensors. She joined the National Microelectronics Centre (CNM) working on photolithography; she is currently in the Microsystems department working in the gas sensing field.