Lecture 3: MEMS applications (continue) RF MEMS

Eng. Nabil H. Khalil 9/3/2006 1

Lecture 3: MEMS applications (continue)

RF MEMS RF (radio frequency) are the means by which almost all wireless communication takes place - TV, radio,

cellular phone, cordless phone or two-way radio(any frequency within the electromagnetic spectrum

associated with radio wave propagation). RF MEMS are micro systems for radio frequency and millimeter

wave applications.

MEMS technology can be used to implement high quality switches, varactors, inductors, resonators, filters

and phase shifters. Among the broad range of applications MEMS technology gives a unique possibility to

implement micromechanical resonators and filters with high performance regarding selectivity and Q-

factors. When combining these mechanical structures with microelectronics, central parts in wireless

systems, RF systems (Radio Frequency systems) can be implemented.

Examples can be various types of oscillators, VCOs (Voltage Controlled Oscillators), mixers and sharp

filters. The MEMS structures can thereby replace traditional costly and large off-chip discrete components

by making possible integrated solutions that can be batch processed.


With the integration of such components, the performance of communication circuits will improve, while the

total circuit area, power consumption and cost will be reduced. In addition, the mechanical switch is a key

component with huge potential in various microwave circuits.

Vibrating MEMS resonators and filters that have been implemented so far are based on mechanical

vibrations in lateral or vertical directions on silicon wafers. Different types of beams, comb structures and

disks can be used. High frequency circuits will benefit considerably from the advent of the RF MEMS

technology. Table 1 shows some of the applications area of RF MEMS.

Application area

Frequency range

Utility

Satellite

communication systems

12-35 GHz

Switching Networks Switched filter banks

Phase shifter for multi-beam

Wireless communication

systems

0.8-6 GHz

Switched filter banks for portable units Switched filter banks for base stations

Transmit/Receive switches Antenna diversity switches

Instrumentation systems

0.01-50GHz

High performance switches Programmable attenuators

Phase shifter for test benches

Table1: Applications area of RF MEMS


Advantages of RF MEMS

1. Performance: Ultra-low RF loss which beats any available electronics technology for switching or tuning

of RF signals. Essentially no DC power consumption, so these systems are perfect for battery and low power

consumption applications.

2. Size: the microminiaturize size with unmatched performance make these systems able to work at very

high frequencies (> 50 GHz), with high tunability that supports reduction in number of passive components

and combines numerous switched parts into one tunable chip.

3. Cost: Low cost, much less expensive than competing exotic semiconductor technologies. These systems

able to be combined with other electronics for “system-on-a-chip”.

RF MEMS market


Why RF MEMS? Passive components such as inductors and tunable capacitors can be improved significantly compared to

their integrated counterparts if they are made using MEMS technology. These passive components have

very large area (about 80% of the mobile phone board), and their behavior is not competitive.





RF filters for mobile phone Instead of using a single tunable filter to select one of several channels over a large frequency range, a

massively parallel bank of switch-able, micromechanical filters can be utilized, in which desired frequency

bands can be switched in as needed. An example of these filters is the surface acoustic wave filter, which can

work from 10 MHz to 3 GHz.

MEMS replaceable transceiver passive components


RF switch

Switches: mechanical motion of a Micromachined structure that will toggle the RF signal between ON and

OFF.


Advantages of RF switches: 1) Better RF performances 2) Low power consumption 3) High-Q, Low Loss 4) Simplicity, Linearity, Tunability 5) Low cost manufacturing 6) Can be designed for any frequency

Disadvantages:

1) Relatively new technology 2) More complicated 3) Packaging is large and expensive

Structure: • The switch itself is made of Gold. (Red area) • The dielectric layer is made of Silicon Nitride. (Green area) • The Substrate used is LCP (Liquid Crystal Polymer).


DDiiooddee.. vvssMMEEMMSS sswwiittcchheess --RRFF


MEMS in Optical Networks An important new application for MEMS devices is in fiber optic networks. At the micron level, MEMS-

based switches route light from one fiber to another. Such an approach enables a truly photonic (completely

light-based) network of voice and data traffic, since switching no longer requires conversion of light signals

into digital electronic signals and then back to optical. This is important because switching using optical-

electrical-optical (OEO) conversion can often cause substantial bottlenecks, preventing the realization of

truly broadband networks. But MEMS and micromachined devices can be used as more than switches in the

optical network. Additional applications include active sources, tunable filters, variable optical attenuators,

and gain equalization and dispersion compensation devices.

The result is an end-to-end photonic network which is more reliable and cost effective, and which has

minimal performance drop-off. However the development of an all-optical network has been complex and

challenging due to the integration of optics, mechanics and electronics.

MEMS-based switches must be extremely reliable to meet the standards and requirements of optical

telecommunications networks – they must remain in precise position over millions of operations, and they

must be designed to meet stringent environmental specifications involving temperature and vibration.

However, there is a high degree of confidence that mechanical MEMS devices can meet these requirements,

as similar devices based on the same manufacturing processes have proven to be exceedingly robust in the

automotive, military and aerospace industries. A typical example of an optical network application is add-

drop multiplexing, especially in metropolitan area networks. Though non-MEMS-based devices may range


from 32 or 64 ports to 1000 ports, a number of companies are looking at lower port, scalable solutions which

offer immediate manufacturability with high yields, robustness and cost-effective batch process technology.

Both the metropolitan and access areas of the network have high volume requirements and may be the best

opportunity to capitalize on the optical network while proving the value of MEMS.

Optical Switches


MEMS Electronics Applications

Inkjet printers The trend in inkjet printer technology is to increase printing quality and speed by:

- reducing droplet size

- increasing the number of channels per head

- Increasing ejection rates

- reducing problems such as cross-talk between channels


Projection displays - Projection display based on Digital Micromirror Device (DMD).

- DMD element 16 µm square.

- An array of 2000 x 1000 mirrors.

- DMD switch reflects light in one of two directions (“0” or “1”) either into or out of the projection Lens.

Micromirrors


Digital cameras -In some new digital cameras, a MEMS tilt sensor is used to monitor how the camera is held when a photo is

taken.

-Software makes sure that photos are displayed right side up on built-in screens, no matter how they were

originally taken.

-Smaller accuracy needed than e.g. in the projector application

Wearable technology a) Wrist-top systems that measure in real time speed, pace, distance, heart rate, altimeter, barometer,

compass, GPS.


b) Smart shoe: Cushion control in Adidas 1 running shoe:

-Includes a sensor, a microprocessor, and an electric motor

-Adjust the cushioning of the shoe taking into account runner's weight and the terrain.

-The shoe's cushioning is reassessed every four steps and adjusted while the foot is in the air.

-To make the adjustment, the motor spins at 6,000 revolutions per minute,

-The shoe's system is powered by a battery that lasts about 100 hours and is replaceable.


c) Smart shirt system:

-Measures heart rate, respiration rate, ECG, motion, position and temperature.

-Base fabric: cotton, polyester, blends, i.e. typical textile fibers.

-Smart Shirt includes an optical fiber woven throughout the actual fabric.

-Developed "Interconnection Technology” facilitates plugging in sensors

-A data bus (conductive fibers) is integrated into the structure to transmit information from

sensors to a microprocessor

-Power wires integrated


Silicon properties and microfabrication

Why silicon? Semiconductors may occur in AMORPHOUS, POLYCRYSTALLINE or CRYSTALLINE forms. • In the AMORPHOUS state there is little or no evidence for long-range crystalline order. • POLYCRYSTALLINE materials consist of small CRYSTALLITES that are embedded in amorphous regions of material. • In the CRYSTALLINE state the atoms are ordered into a well-defined lattice that extends over very long distances.

CRYSTALLINE STATE POLYCRYSTALLINE STATE AMORPHOUS STATE

Figure 1: Semiconductors different crystal structures


• The great advantage of semiconductor materials is that it is possible to vary their conductivity over a much

wider range than is possible in either metals or insulators.

• This is achieved by the controlled addition of a small number of impurities known as dopants. The

number of dopants that is added to the semiconductor is typically a very small fraction of the total number

of atoms in the semiconductor. For microelectronic applications crystalline material of high purity is

required. Impurities must be kept to a very low level (one impurity atom per 109 semiconductor atoms) to

avoid unintentionally doping the semiconductor.

• Micromachining has been demonstrated in a variety of materials including glasses, ceramics, polymers,

metals, and various other alloys.

• Why, then, is silicon so strongly associated with MEMS? The main reasons are given here:

1. Well understood and controllable electrical properties;

2. Availability of existing design tools;

3. Economical to produce single crystal substrates;

4. Vast knowledge of the material exists;

5. Its desirable mechanical properties.


• Other crystalline semiconductors including germanium (Ge) and gallium arsenide (GaAs) are used as

substrate materials due to similar inherent features, but silicon is distinguished from other semiconductors

in that it can be readily oxidized to form a chemically inert and electrically insulating surface layer of SiO2

on exposure to steam.

• The homogeneous crystal structure of silicon gives it the electrical properties needed in microelectronic

circuits, but in this form silicon also has desirable mechanical properties.

• Silicon forms the same type of crystal structure as diamond, and although the interatomic bonds are much

weaker, it is harder than most metals. In addition, it is surprisingly resistant to mechanical stress, having a

higher elastic limit than steel in both tension and compression.

• Single crystal silicon also remains strong under repeated cycles of tension and compression.

• The crystalline orientation of silicon is important in the fabrication of MEMS devices because some of the

etchants used attack the crystal at different rates in different directions (Figure 2).


Figure 2: Low crystallographic index planes of silicon

• Silicon is dominant as a substrate for MEMS but research and development is ongoing with other non-

semiconductor substrate materials including metals, glasses, quartz, crystalline insulators, ceramics and

polymers.

• The ability to integrate circuitry directly onto the substrate is currently the underlying issue with today’s

MEMS substrate materials; hence the success of silicon.

• For high purity and excellent fabrication, the preparation of materials must be under stringent CLEAN-

ROOM conditions.


• CLEAN ROOMS USE FILTRATION TO REMOVE DUST PARTICLES PRESENT IN AIR AND ARE

RATED ACCORDING TO THE NUMBER OF DUST PARTICLES PER CUBIC METER OF AIR.

• EACH CUBIC METER OF ORDINARY ROOM AIR HAS SEVERAL MILLION PARTICLES OF A

SIZE OF 0.1 MICRONS!

Figure 3: Clean rooms


• Single crystal semiconductor ingots are typically prepared using a process known as the CZOCHRALSKI

method.

* The starting point in this process is a small piece of crystalline silicon which acts as a seed that is dipped

into a crucible containing molten silicon.

* Single-crystal silicon is then pulled from the silicon melt while the crucible is slowly rotated and this

results in the formation of an ingot of crystalline silicon.

Figure 4: Si wafers fabrication process


First Transistor, 1947 First Integrated Circuit, 1958 Bardeen, Brattain and Shockley Jack Kilby Nobel Prize, 1956 Nobel Prize, 2000

Intel 4004-2.3x103 transistors Intel Pentium III-5x106 transistors 1970 Today

Figure 5: Development in electronics


Moore’s Law Semiconductor devices shrink to the nano-scale

• Current technology becomes increasingly difficult. • If current trend continues, it will reach molecular scale in two decades.


Basic processes in silicon microfabrication Layer techniques The purpose of the layer techniques is to produce and pattern layers of materials in the micro range on a

substrate surface, this includes three different methods:

1. Thin film techniques This method is used for the generation of functional, structuring and sacrificial layers with a thickness of a

few nm to few µm, using deposition process. We have three types of deposition processes:

A) Thermal deposition (oxidation of Si): Silicon dioxide is grown on silicon wafers, accelerated by elevated temperature and exposure of an oxidizing

agent (O2 in dry oxidation and H20 steam in wet oxidation). This is done in a furnace at temperatures in the

range from 750°C to 1200°C. Silicon dioxide considered as a chemically resistant and an electrical insulator

material, which is used for masking purposes, isolation purposes and sacrificial layer in silicon

micromachining.


B) Physical layer deposition

Used for the deposition of electrical conductors on a substrate, like Aluminum, chromium, nickel, platinum,

gold and silver. Two types of physical layer deposition are Vapor deposition (evaporation), in which

Material to be deposited heated up in a vacuum chamber, and then evaporated material condenses on the

cold substrate.

Figure 6: Evaporation


The second type is Sputtering process, in which cathode is made of the metal to be deposited, then an inert

gas ions bombard the cathode, the loosed metal atoms condense on the substrate.

Figure 7: Sputtering

C) Chemical layer deposition

Widely used method to deposit polysilicon, silicon nitride and phosphor silicate glass. It is used for

generating structural and sacrificial layers in surface micromachining, giving durable layers and exact

deposition thickness. In the Chemical Vapor Deposition (CVD), the substrate heated (1250 C) and placed in

a gas flow, this gas contains the metal to be deposited.


Three most common types of CVD process are low-pressure CVD (LPCVD), where the step coverage

(conformality), uniformity, and the composition and stress of the deposited layer are determined by the

gases used and the operating temperature and pressure. The second type is Plasma Enhanced CVD

(PECVD), in which radio frequency (RF) power is used to generate plasma to transfer energy to the reactant

gases, the layer properties are affected additionally by the RF power density, frequency, and duty cycle at

which the reactor is operated. The third type is Atmospheric Pressure CVD (APCVD), in which the

deposition is mass transport limited; the design of the reactor is significant.

Figure 8: Chemical Vapor Deposition


2. Deposition from a liquid phase

This type of layer techniques includes two types, first, the galvanic techniques, where the substrate

immersed into an electrolyte, this electrolyte contains the metal to be deposited on a conductive substrate,

and then voltage is applied. This type can deposit various metals, and has high aspect ratio. It is used in

LIGA.

Second type is the spin-coating method, here, liquid coating material deposited on a substrate spun at

constant speed, then hardened in a drying process. This type used for photoresist coating.

3. Thick film techniques

Screen printing is one of the oldest forms of graphic art reproduction and involves the deposition of an ink

(or paste) onto a base material (or substrate) through the use of a finely woven screen having an etched

pattern of the desired geometry. The term “thick-film” can often be misinterpreted, so it is worth noting

that it does not necessarily relate to the actual thickness of the film itself. The typical range of thicknesses for

thick-film layers, however, is between 0.1 and 100 µm. The process is commonly used for the production of

graphics and text onto items such as T-shirts, mugs, and pencils, and it is very similar to that used for

microelectronic thick-films.


The degree of sophistication for the latter is, however, significantly higher in order to provide high quality,

reproducible films for use in electronic systems. Such circuits typically comprised semiconductor devices,

monolithic integrated circuits, discrete passive components, and the thick films themselves. A typical mask,

or screen, is made of a finely woven mesh of stainless steel, nylon, or polyester, which is mounted under

tension on a metal frame and coated with a UV-sensitive emulsion. The desired pattern is exposed onto the

screen photographically, leaving open areas through which a paste can be deposited. The pastes comprise a

finely divided powder (typically 5-µm average particle size), a glass frit, and an organic carrier that gives the

ink the appropriate viscosity for screen printing. Typically, thick-film pastes are resistive, conductive, or

dielectric in nature and are deposited onto substrates such as alumina or insulated stainless steels. Silicon

has also been used as a base material to make devices such as micropumps.


Masking For discrete devices, or small-scale-to-medium-scale ICs (typically up to 1000 components per chip), a large

composite layout of the mask set is first drawn. This layout is a hundred to a few thousand times the final

size. The composite layout is then broken into mask levels that correspond to the IC process sequence such

as isolation region on one level, the metallization region on another, and so on. The choice of the mask

material, just like radiation, depends on the desired resolution. For feature sizes of 5 mµ or larger, masks are

made from glass plates covered with a soft surface material such as emulsion. For smaller sizes, masks are

made from low-expansion glass covered with a hard surface material such as chromium or iron oxide.

Design rules are developed for each separate fabrication sequence. They include the minimum feature size,

but essentially they tell you how much overlap you need to leave between two mask layers to ensure that the

two features will be coincident when fabricated. They also ensure that there is a suitable gap between two

features that should not touch when fabricated. Design rules depend upon the expected alignment error that

will be introduced during fabrication. This will be comparatively large when doing double sided alignment.

If we take an arbitrary value of +/- 1um alignment error, for example, and we wish to etch a via hole over a

metal pad, then it would be wise to leave at least an under lap of at least 2um (try and work in multiples of

the minimum feature size, if possible) to ensure the final structure is fabricated as desired. (Figure (9)).


Figure 9: Masking


Doping

Doping atoms are introduced to a silicon substrate in a defined way so that n or p conducting layers are

formed. Doping is useful for determining the electrical properties, Improve mechanical properties such as

wear and corrosion. Also it is used to create etching stop barriers. We have two types of doping:

1) Diffusion Diffusion is done in a furnace, doping atoms from a surrounding gas at elevated temperatures in the range

800°C to 1200°C. Silicon dioxide can be used to create a two-dimensional spatially distributed pattern of

doped silicon. Doping profile only on the surface. This method is easy to construct, and not expensive, but

that effects the accuracy of the process.


Figure 10: Diffusion process


2) Ion implantation

Ion implantation is done by firing energetic ions directly into the silicon substrate; the ions can penetrate up

to few micrometers. After implantation, the silicon wafers have to undergo a thermal treatment, first, to

anneal damage to the crystal caused by the impact of the energetic ions, and to move the dopant atoms into

substitutional sites in the silicon crystal where they become electrically active. This technique gives better

results than diffusion, but it is expensive.

Figure 11: Ion implantation process

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Lecture 3: MEMS applications (continue) RF MEMS