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A Portfolio of SEM microphotographs


various MEMS devices

made during


P.R. Apte apte@tifr.res.in

S.G. Lokhre lokhre@tifr.res.in

R. Pinto rpinto@tifr.res.in

Fax: +91-22-215 2110

Phone: +91-22-215 2971 ext. 2314

Tata Institute of Fundamental Research

Solid State Electronics Group

Department of Condensed Matter Physics

Homi Bhabha Road, Colaba, Mumbai - 400 005


Filename: E:\portfolio\portfolio4.doc

Date: 15-May-2000

MEMS activity at TIFR:  
micro-parts : (1995-96) :

We use Electron Beam Lithography (EBL) setup to obtain micron sized minimum feature masks and patterns for fabricating silicon micromechanical parts having dimensions of 1 um to 10mm. The microparts are made of silicon dioxide, silicon nitride, P+ doped silicon, and other novel and compatible materials (0.2 to 1um thick). These are suspended over a pyramidal cavity obtained by a crystallographic <111>-stop silicon etchants like KOH+Isopropanol+Water (KOH etchant) and Ethylene Diamine Pyrocatechol etchants (EDP etchant).

Specific devices and corresponding applications are as listed below,

As a reed vibrator. Bending by thermal radiation and resistive heating. Deflection by electrostatic or electromagnetic fields. Many applications are mirrors, AFM tips, microprobes for biological probing. (Fig. 1.) Two main applications are, (Fig. 2.)
    1. as a thermally isolated platform at the center of the cross. This is useful in microcalorimetry
    2. with a centrally attached mass it can be used as a 3-D accelerometer/gyroscope in space applications.
Micro movements in and out of cavity. These movements can be obtained by thermal or point loads at the center of the spiral. It acts as a very low stiffness spring. (Fig. 3.) Very high Q ( in excess of 40,000) can be obtained by double suspension scheme. These can be used to measure viscosity of gases/fluids in the resonant mode and as vibration sensors in space vehicles and large antenna structures. (Fig. 4.) Diaphragms have been first used for pressure sensors. Recently, several applications have emerged using diaphragm as the active part of a 'pump' to deliver micro quantities of fluids like ink, drugs etc. We have made very thin diaphragms for their use as targets in high-energy ion beam experiments.   These devices require etching full thickness of wafer from the backside of the silicon wafer..  

Fig. 1. Cantilever
Permission was sought and given for use in the following book
"Electronic materials and devices", D. K. Ferry & J. P. Bird, Academic Press, San Diego (2001)

Fig. 2. Cross-beam

Fig 3. Spiral

                                                                                                                                                        10 micron bar
Fig. 4. Balanced Resonator

A MEMS magnetic sensor: (1996-98):

We made our first sensor using the unbalanced resonator structure. The unbalanced structure made it possible to create a torque, which acts on the torsion wire to produce angular deflection. The torque arm was deposited with Cobalt and magnetic field was applied from below. The deflection characteristics were measured by slowly increasing the magnetic field and using a calibrated probe to measure this field and using microscope to measure the deflection of the torque-arm on our device. The fig. 5a shows the SEM micrograph of the device and Fig. 5b shows the measured device characteristics. The linearity is excellent.

A paper titled "micro-resonator modified as magnetic sensor" has been accepted for presentation at the SPIE-2000 symposium on "micromachining and microfabrication" to be held at Santa Clara, CA during 18-21 Sept 2000.

                                                                                                                                                     10 micron bar

Fig. 5a. An unbalanced torsion resonator  with cobalt (magnetic) film
deposited on the deflecting arm

Fig. 5b. Deflection characteristics for an unbalanced torsion magnetic sensor

Current Controlled Curled Tips : (1998-99)

Cantilevers are normally used either in bending or vibrating mode. Cantilevers traditionally have provided movement in a direction transverse to the length of the cantilever arm. But here we describe a novel actuator, which gives movement in the same direction as that of the length of the element. The cantilever is made from a silicon dioxide patterned in the shape of an acute angled ">" shape. Current sensitive actuators were planned as applications for such cantilevers. This is achieved by a curling up action of a bimetal like ‘metal on silicon dioxide’ straight element due to large differences in the coefficients of linear expansions and the high temperature which is obtained during the metal deposition. Such bending applications can be used to make current sensitive or temperature sensitive contacts. The cantilevers can be bend forward or backward where it will make contact with immovable metallized contacts. Cantilever-based actuators can then be integrated with associated signal conditioning electronics for a full-fledged device.

Micron-sized current control tips were designed by Design-CAD and chrome masks made by EBL system from NPGS. We have successfully fabricated current-controlled SiO2 microtips by using micromachining technique. Curling of tip takes place due to bimetallic stresses during fabrication. Etching the cavities for progressively more time give larger bending effects as seen in fig.6 (for 45, 60, 90 and 120 min etching). Uncurling is obtained by passing control currents of the order of 1 to 15 mA. Fusing currents of 22 mA (in air) were observed.

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Already appeared in the

Proc. Of SPIE, "Indo-Russian Workshop on Micromechanical Systems"

volume 3903, p223-230, (1999)

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                         45 minutes etching                                         60 minutes etching
                                                                                            10 micron bar  
                        90 minutes etching                                         120 minutes etching

Fig. 6. Curled tips


AutoRanging Torsion Sensors : (1999-2000)

Autoranging torsion sensors were conceived as a result of a conscious effort to make the sensor 'do things by itself' (this is a very important innovative concept in TRIZ - A Russian acronym for Theory of Inventive Problem Solving). We designed and fabricated unbalanced torsion (angular deflection) devices (shown in Fig. 7) that have
(1) 1st-STOP (say at 5 degrees as angle of deflection)
      (2) 2nd STOP (say at 10 degrees as angle of deflection)

      (3) both 1st and 2nd STOPs

      (4) no STOPs but the torque-arm itself having a STOP action

    (say at 30 degree angle of deflection)

The lengths of 1st STOP, 2nd STOP, torque-arm and width of torsion wire affect the angular deflection and the linearity of the characteristics. We have taken each of these dimensions of 3 values - smaller, nominal and larger. The numbers of possible (factorial) combinations are 3X3X3X3 = 81. Taguchi method uses orthogonal array (called L-9 array) for 9 experiments for determining the best-of-81 experiments. The L-9 array experiment has been laid down on the mask and fig. 8 shows the SEM micrograph of the same. (device #8 is broken). Results are still to be published.

NOTE: The SEM images have been grabbed with 256 X 256, 512 X 512 and 1024 X 1024 pixel.
            The lower resolution shows saw like edges/lines.  
  The higher resolution shows better images.
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  With 1st STOP                                                 With 2nd STOP
                                                                                             100 micron bar                         With both STOPs                                         Without any STOPs
10 micron bar                                                                10 micron bar
Fig. 7. Autoranging Torsion Sensor (with 2 stops for Piece-wise linear characteristics)

            100 micron bar
Fig. 8. Array of 9 torsion sensors as a part of Taguchi's L-9 experiment

Tunnel-Tips : (1999-2000)

Our most recent activity is "Tunnel-tips". The objective is to obtain a 'pyramidal tip' of silicon that is pointing up and is very close to a metal pad that covers it from top. Normally, the metal pad is used for etching a tip below it and is 'sacrificed' when the tip actually gets formed (the pad has no supports and hence falls off). We attempt here to design a pad in such a way that it allows tip to be etched but does not fall off. We have more or less succeeded in this effort, as shown in Fig. 9, 10 and 11.

Fig. 9 shows various cross-bridge patterns for metal pads (and tips forming below these after a particular time of etching. Fig. 10 shows cantilever type patterns for pads. Fig. 10 middle-left device (with broken pad) shows a very good quality silicon tip. Fig. 11 shows a close-up view of a single device that has the tip just being formed below the pad. Such a device can be used for measuring the tunneling currents. The work is yet to be published.

                                                                                                                100 micron bar

Fig. 9. Tunnel tips (cross-bridge type, various sizes, shapes)

                                                                                                               100 micron bar

Fig. 10. Tunnel tips (Cantilever type, various sizes, and shapes)

                                                            A broken cantilever tip (middle-left) clearly shows formation of tip  

                                                                                                    10 micron bar

Fig. 11. A close-up of Tunnel tip (Cantilever type)

                                                                     A well formed tip can be seen below the center


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