VLSI Technology and MEMS

Prakash R. Apte

Solid State Electronics Group

Tata Institute of Fundamental Research

Colaba, Mumbai-400 005

 

e-mail : apte@tifr.res.in

 


Abstract

Very Large Scale Integration (VLSI) of electronics components on a silicon chip is now a mature technology. Continuing development of electronics systems of increasing complexities in silicon and communication systems in fiber optics has brought us to the era of Information Technology. MicroElectroMechanical Systems (MEMS) are micrometer scale mechanical devices that are being used in growing number of application such as communications, computers, entertainment etc.  MEMS bring together the fabrication and manufacturing technologies of both VLSI and precision engineering fields.

 

While it is true that the precision mechanical components, micro-sensors and micro-actuators, on their own merit justify the development effort towards micromechanics, it should no longer be regarded in isolation from microelectronics and microoptics. Eventually, these fields would be integrated and emerge as a viable technology of the future.

 

Today, Ink-jet printer heads and hard disk sensing heads are being mass-produced in high volumes and hence at low costs.  Texas Instruments, USA and Sony, Japan would soon bring a variety of optical display products that use micromechanical parts as mirrors and gratings. Micro-fluidics is slowly catching up with medical applications like lab-on-a-chip and DNA arrays. Importantly, the future of MEMS will be in terra-bit data storage devices on one hand and in distributed structural control in avionics on the other.

 

 

 

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This paper was presented at the VSDT-2000 at IITB at Mumbai during 9-11 Jan 2001

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1.  VLSI Technology and Moore’s Law

The 1st Integrated Circuit was made in 1958 by Jack Kilby of Texas Instruments, Texas USA and  Robert Noyce of Fairchild Semiconductors, California USA. The complexity of the circuits, in terms of the number of transistors per chip, has grown at a fast pace.  This was first studied by Gordon Moore and he found that the ‘number of transistors doubled every 18 months’. This is known as Moore’s Law. There are many aspects of this fast growth and these are described briefly below for year 1995 – 2005,

·   component count    - 1995    Pentium   (5 million) per chip                - 2005          ?       (1 billion)

·   critical dimensions  - 1995       1 micron

                                 -  2005      .1 micron  (limit?)

·   chip size                 -  Today     centimeter

2.  Silicon as a Mechanical Material

Silicon was considered as a very fragile and brittle material and required very careful edge polishing and handling. However, as the technology matured it was found that the whole process could be automated using even pneumatic transport and handling at several stages. But the real impact was only felt when Kurt E. Petersen wrote a paper titled ‘Silicon as a mechanical material’ in Proc. IEEE (May 1982). He stated that silicon has excellent mechanical properties which he summarised as below,

·         Young’s modulus, Strength   -      ~ steel

·         no creep (perfectly elastic!)   -

·         Large piezo-resistivity  -  converts mech to elec

 

Several interesting properties of silicon and IC technology were to be rediscovered for later applications,

 

·         Optical finish            -  optical components

·         freely suspended silicon -  RF components

3.  micromechanical structures in sensors, actuators and systems

Some of the early devices using silicon as a mechanical material i.e. cantilevers and diaphragms and their bending and deflections were pressure sensors and accelerometers. Slowly more mechanical structures found interesting effects that could lead to micro-sensors. A summary is given below,

 

3.1 Sensors : on freely suspended microparts

 

·         Piezoresistivity :– change in resistivity with stress-strain

·         Capacitive sensors :– use deflection of cantilevers/diaphragms with respect to a fixed electrode.

·         Temperature sensor :-  P-N junction with forward or reverse bias or thermocouple voltage between hot and cold junctions

·         Magnetic sensor :- magnetic films on cantilevers/diaphragms

·         Visible or UV or Radiation or Particle sensors :- photoconductor or e-h pair generation

·         IR Radiation sensors :- Heat absorbed on a micro component (very low heat capacity) results in a change in temperature which is then measured.

 

When mechanical structures were used for sensing, they were used in passive mode. There was need to actuate them for getting further control on their properties. This led to the advent of micro-actuators. A brief summary of actuators is given below,

 

3.2  Actuators : to move freely suspended microparts by generating force or torque

 

·         Electrostatic :– Apply potential between cantilever/diaphragm with respect to a rigid electrode to generate a force that results in a deflection

·         Electromagnetic :– Current carrying conductor, in a magnetic field, experiences a force that results in deflection

·         Piezoelectric :-  Applied electric potential across a piezoelectric material results in mechanical movement

·         Magnetostrictive :- Applied magnetic field causes a change in dimension

·         Thermal :- change in temperature causes change in dimension/area/volume or a bimetallic action results in bending and deflection

 

Apart from the effort to integrate sensors and actuators, a new set of possibilities opened up as more mechanical functions could be performed using available structures like, cantilevers, diaphragms, etc. More and more fully mechanical functions were realised and a summary is given below,

 

3.3 (mechanical) Systems : to perform mechanical functions

 

·         Linear, Reciprocating or  In-plane movement in Links and mechanisms :-

A planar comb-drive on silicon provides a linear movement (using electrostatic force of attraction). This is similar to the plunger movement in a magnetic coil

·         Angular movement (restricted) :– Torsion devices, angle less than 90 degrees

A torsion microdevice can give angular deflection by torque generated by electrostatic or electromagnetic fields. One end is however held firmly to the rigid silicon substrate.

·         Angular or Rotary movement (free):– Gears and motors, part or full rotations

A comb drive can generate linear motion that is converted to angular motion by a rack-and-pinion arrangement. However, two comb drives, mutually at right angles, are required to engage and disengage the rack from the pinion, so that continuous motion in one direction can be imparted to the gears/wheels.

·         Z-axis movement :-  Pivoted mirrors with angular deflection (some part projecting outside a cavity in silicon and other inside the cavity). Gratings made of freely suspended sub-micron dimension digitised finger structure can be moved towards the silicon substrate to give controlled constructive or destructive interference of reflected light.

·         3-D movement :- Scanning in x-y plane and sensing tip movement along z-axis

Most modern angstrom-level imaging devices like Scanning Tunnelling Microscope (STM) and Atomic Force Microscope have this requirement.

 

 

4. Synergy between VLSI Technology and MEMS

 

4.1 VLSI Technology  :-  A choice of compatible materials

 

The VLSI technology was advanced by the time real interest in MEMS picked up in the 80’s. A variety of materials  used in VLSI were of immense importance to the MEMS designers because these were already available in the technology. The MEMS technology could, therefore, include these in its own processes. Many other aspects were also useful in MEMS, these  are briefly described below,

 

a)       Silicon dioxide :

i)        stable mechanical properties

ii)       amorphous and isotropic

iii)     under compressive stress

 

b)      Silicon Nitride :

i)        stable mechanical properties

ii)       amorphous and isotropic

iii)     (high temperature) ceramic

iv)     under tensile stress

 

c)      Poly-Silicon :

i)        Thick layers 5 microns or more

ii)       Electrically conducting (as electrodes)

 

d)      Aluminium :

i)        Electrical conductor

ii)       High optical reflectivity

 

4.2 VLSI Technology   :-   Lithography, Deposition, Selective etching

 

a)      Lithography :

i)        Optical/UV/Deep UV (down to .6 mm)

ii)       Electron beam (for mask making and direct wafer writing)

iii)     X-ray (down to .1 mm)

 

b)      Deposition  :

i)        Evaporation, DC or RF sputtering

ii)       Chemical Vapor Deposition (CVD)

iii)     Plating, molding etc.

 

c)      Selective Etching :

i)        Wet chemical etching  (isotropic, large undercutting)

ii)       Dry chemical etching  (anisotropic, minimal undercutting)

(a)    Reactive ion etching (RIE)

(b)    Deep RIE for trench etching

 

4.3 VLSI Technology and Micromachining Technologies :

 

a)      Bulk micromachining :

i)        structures are ‘carved’ in silicon

ii)       silicon etched from front, back or both

iii)     microparts hinged at one or more points

 

b)      Surface micromachining :

 

i)        Microstructures are deposited, patterned or molded on silicon which is used only as a mechanical support

ii)       Free microstructures are obtained by first including sacrificial layers and then removing them

iii)     Pattern ‘structural layers’ before removing the sacrificial layers

 

c)      LIGA and other plating/molding methods

 

i)        Microstructures are plated or molded on silicon (only as a mechanical support)

ii)       Free microstructures are obtained by first including sacrificial layers and then removing them

iii)     structures are first patterned in very thick photoresists (~ 30 mm ) or polymers (~200 mm)  exposed and developed after synchrotron radiation exposures. This is followed by thick (nickel/copper) plating

iv)     Generally, the thickness is much larger than the width, and hence these are called high-aspect ration structures. These deflect/bend easily in lateral dimensions (along x- or y-axis) but are rather stiff in transverse dimension (along z-axis).

 

4.4 VLSI Technology and Scaling down of critical dimensions :

 

The main result of scaling down is that the mechanical devices will become smaller and smaller as a natural consequence of linking of MEMS technologies to VLSI technology.

 

Small mechanical components imply,

a)      Less area, mass/weight

b)      Large number of mechanical components in a chip (Moore’s law!?) :

c)      Ever increasing natural frequency of vibration (natural limit of useful operation) :

 

The resulting applications will be,

i)        Miniaturised microscopes/spectroscopes

ii)       Miniaturised instruments through assembly of micro-components

iii)     Faster refresh in display applications

iv)     In a array form, it will be capable of parallel processing of acoustic and optical signals 

5.  Success Stories

There are a huge number of potentially exciting and profitable applications that are being worked on all over the world and in all leading academic and R&D laboratories. Only a few real successes have been mentioned below.

 

5.1  Pressure sensors and Accelerometers :

 

These are micro-miniaturised devices that are already being used in commercial applications. Pressure sensors in manufacturing industry and accelerometers are used automobiles.

 

5.2  Ink-Jet Printer heads :

 

With availability of precision nozzles in silicon micromachining, the first ink-jet printer head was visualised. It based on the principle that current pulse in a small heating resistor will instantly vaporise the ink and the resulting shock will eject a tiny drop of ink from the nozzle. This faced slow pile up of dried ink on the heater and thus slowly degrading the vaporising capability. The later versions, instead, based the ink ejection on a diaphragm pump. V-grooves in silicon provide the canals for ink to flow into a linear array of nozzles arranged much the same as in a dot-matrix printer head of earlier era. A 3 color printer cartridge has the 3 arrays of nozzles connected via the V-groove canals to the color ink reservoirs. For each nozzle there is a diaphragm pump operated by piezoelectric effect.

 

The ink-jet printer head is the real success story of the MEMS device. It is estimated that ink-jet printers alone will cross $2 billion mark in a few years!

 

5.3  Hard Disk heads :

 

Hard disks are presently based on magnetic storage. The sensing heads have to be in close proximity to the high-speed rotating disk. Silicon sensing heads with the excellent tribological properties of various materials used therein allow the sensing heads to move in proximity contact over the disk surface.  This is also one of the successful professional application of silicon MEMS device. 

 

As the future of data storage depends much on the progress of  MEMS based STM-like devices with Read/Write capability at angstrom-level. Terra-bit storage devices have been predicted.

 

5.4 Digital Micro-mirror Devices (DMD) and Digital Light Processing (DLP)

 (by Texas Instruments)  :

 

Texas Instruments have already announced a digital light processing chip that will revolutionise the image projection systems as used in TV and cinema. It is based on MEMS pivoted mirrors at 1000 X 1000 pixel locations. The mirrors are tilted by electrostatic signals and the strong laser light beam that illuminates the DLP chip then reflects the light to the specified pixel location on the screen or is reflected away from screen (into a black area).  The fig.1. shows the exploded view of the micromirror assembly in the DLP chip.

 

Figure 1  An exploded view of the micromirror assembly in the DLP chip (taken from Texas Instrument web-site)

 

5.5 Grating Light Valve (by Silicon Light Machines) :

 

Silicon Light Machines have found a novel use for a optical grating made in silicon nitride.  Instead of using the grating for selecting the wave length of an incident light beam (by Bragg diffraction), the alternate nitride grating fingers were pulled down by  l/4  (by using electrostatic potential) thus giving a destructive interference and thus acting like a ‘light valve’ – turning light ‘on’ and ‘off’.

 

For potentials that deflect partially, the interference is partial and thus the chip is capable of ‘grey’ levels – fully bright to fully dark.  With ‘grating light valves’ at each pixel position and illuminating it by a laser with appropriate lens, an image can be projected on a screen. Thus the GLV’s have the potential of replacing monitors and TV screens in near future. The fig.2. shows a simplified picture of the GLV (taken from the Silicon Light Machines’ web site).

Figure 2   A 'Grating Light Valve' (from the web-site of Silicon Light Machines)

6.  SCOPE  and  FUTURE

The present scenario of MEMS is warming up to a variety of immediate and future applications. However, it is not possible say anything definite about whether the applications fall under the current scope of activities or lies far in the future!

 

A brief discussion on various possibilities for MEMS applications is given below,  

 

6.1 Existing MEMS - Pressure sensors and Accelerometers :

 

Two prominent MEMS devices are already in the market. The pressure sensors and the accelerometers have already been entered volume production and have thus increased their market share.

 

6.2  Military Applications :

 

Today, most of the military applications use a few sensors that are made smart by electronics. In future, guidance systems will have many smart sensors with collective intelligence.

 

6.3 Optical Switches :

 

Fiber optics communication can use many of the MEMS devices for switching light from one fiber to another. The cellular technology can also use MEMS Radio Frequency devices in switching applications. In future, MEMS based variable gratings would find use in selecting wavelengths in the ‘wavelength multiplexing’ systems in fiber optics communication.

 

6.4 Optical Benches and Optical Aligners :

 

Today, silicon is used as a micro-optic bench for aligning various components like laser diodes, micro-mirrors, micro-lenses, fibers etc. This makes the systems compact, mass producible and lower costs. In future, silicon will provide the basic guiding rails for fibers and mounting locations for other optics devices that are individually made smart – sense, actuation and control functions are integrated.

 

6.5  Parts handling in manufacturing (robotics) :  

 

In today’s robots, microsensors sense dimensions, orientations and sort into separate bins. In future, micro-robots will recognise pattern and shapes, will pick up and correct orientation and guide parts through obstacles etc.

 

6.6 Pumps, Valves and micro-fluidics (pneumatics and hydraulics applications)

 

Today, Ink-jet printer head is the big success of the micro-pumps combined with precision micro-nozzles. In near future, we expect drug delivery systems consisting of smart micro-sensors and micro-actuators to be implanted near an organ. Recent developments in ‘lab-on-a-chip’, microarrays and DNA chips have ushered us into an era where medicine will be tailor-made for an individual for maximising its effectiveness at a very affordable price.

 

6.7 Inertial navigation and other space applications:

 

Today, the 3-axis accelerometer or gyroscope forms the basic measuring component and the control/actuation of thrusters is performed by electronics and computers. In future, micro-satellites, that are just a few kilograms in weight, will have MEMS based devices for gyroscopes and micro-thrusters.

 

A UC Berkeley project aims at spraying micro-sensors (called ‘micro-dust’) in atmosphere. The ‘micro-dust’ will collect and send environmental data by telemetry to base station. This may be vital before entering a ‘hostile’ or ‘chemically hazardous’ zone. It may have many civil or military applications.

 

6.8 Terra-bit data storage :

 

Today’s data storage is based mostly on effects that are magnetic, optic or combination of the two. These require a disk to rotate at high speeds with a sensing head in close proximity to the rotating surface. The future data storage envisages a scanning type device based on MEMS technology with ‘no rotating parts’.  An array of Micron sized beams, carrying Scanning Tunnelling Microscope (STM) tips will scan over a centimetre area and can store over terra-bit data.

 

 

6.9 Structural control in avionics :

 

Today, prototype wings are being made with thousands of cantilevers and air-eddy sensors. The wind tunnel tests have indicated that the cantilevers, when raised perpendicular to the surface, give similar results as those given by large moving surfaces on a wing – like the flaps, slats and ailerons. This demonstrates that in future the ‘smart skin’ wings will allow quicker turning, stabilization against turbulence and better fuel efficiency by reducing the drag on the wing.

7.  DISCUSSION

The MEMS technology has evolved from the initial start as a subset of the VLSI technology. It maximised the advantages of using the basic processes from the IC technology that were meant for microelectronics devices. Naturally, very good physical sensors were made in silicon. With more mechanical structures becoming available after the advent of surface micromachining, the MEMS technology has now pushed ahead on its own. However, the ideal goal of integrating both micromechanical as well as microelectronics components  on the same chip keeps these two technologies together.

 

8.  REFERENCES

 

[1] Kurt E. Petersen, “Silicon as a mechanical material”, Proc. IEEE, Vol. 70, No.5, May 1982, pp. 420-457,

 

[2]  James B. Angell, Stephen B. Terry and Phillip W. Barth, “Silicon Micromechanical Devices”,  Sci. Am., Vol. 248, No. 4, 1983, pp. 36-47,

 

[3]  Gary Stix, “Trends in micromechanics : Micron Machinations”, Sci. Am., 1992. pp. 107-117

 

[4]  K.J. Gabriel, “Engineering microscopic machines”, Sci. Am., 1995, pp. 118-121,

 

[5] web-site of Texas Instrument for DMD and DLP chip,                 http://www.dlp.com/

 

[6] web-site of Silicon Light machines for GLV technology,       http://www.siliconlight.com/