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Micromechanical System for System-on-Chip Connectivity

INTRODUCTION

MEMS technology has enabled us to realize advanced micro devices by using processes similar to VLSI technology. When MEMS devices are combined with other technologies new generation of innovative technology will b created. This will offer outstanding functionality. Such technologies will have wide scale applications in fields ranging from automotive, aerodynamics, hydrodynamics, bio-medical and so forth. The main challenge is to integrate all these potentially non-compatible technologies into a single working microsystem that will offer outstanding functionality. The use of MEMS technology for permanent, semi permanent or temporary interconnection of non-compatible technologies like CMOS, BJT, GaAs, SiGe, and so forth into a System-on-Chip environment can be described using an example application. It is a hearing instrument in which an array of acoustical sensors is used to provide dynamic directional sensitivity that can minimize background noise and reverberation thereby increasing speech intelligibility for the user. The micro array can provide dynamically variable directional sensitivity by employing suitable beam forming and tracking algorithms while implanted completely inside the ear canal.

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Micromechanical System for System-on-Chip Connectivity

MEMS ACCOUSTICAL SENSOR ARRAY FOR A HEARING INSTRUMENTIn this application an array of capacitive type sensors are used in a hearing instrument to provide dynamic directional sensitivity and speaker tracking and can be completely implanted in the ear canal. The directional sensitivity is obtained by the method of beam forming. The microphone array is developed using MEMS technology and which can be used to form beam to provide directional sensitivity.

BEAM FORMING USING MICROPHONE ARRAYThe microphone array consists of nine capacitor type microphones arranged in a 3*3 array and utilizes the classical phased array technique for beam forming. In this technique, the relative delay or advance in signal reception is eliminated by applying a delay or advance is that the signal out puts from different microphones can be added to form a beam as shown in figure 1.

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Micromechanical System for System-on-Chip Connectivity

Figure 1. Beam pattern of a transducer array: normal beam

It is also possible to steer the direction of the beam by providing additional delay factor that is equal to the negative of the relative delay to the out put of each microphone in the array when a signal arrives from that direction. Figure 2. illustrates the beam steering concept.

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Micromechanical System for System-on-Chip Connectivity

Figure 2. Beam pattern of a transducer array: steered beam

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Micromechanical System for System-on-Chip Connectivity

Similarly, it is possible to form multiple beams out of the single array employing different delay factors and use such beams to scan the direction of the potential speaker. This scanning beam can easily realized by continuously steering the beam from top to bottom or from left to right by dynamically changing the steering delay using digital filters. An algorithm will detect a speech signal above some threshold level and will steer the main beam towards that direction. The block diagram for such a system is shown in figure 3.

MEMS Acoustical Array Module

MEMS Socket Interface

Analog CMOS Signal Conditioning

CMOS A/D Converter

Digital Beamforming & Beam Steering Engine

Digital Signal Processing

Array Control Interface

SoC PCI Bus

Figure 3: Block Diagram of Hearing Aid Instrument

To avoid spatial aliasing at all steering angles the spacing d between the microphones of the array is required to be

D < c/

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Micromechanical System for System-on-Chip Connectivity

= c/2f = /2] Where is the wavelength of the incident acoustical signal and f is the frequency in Hz. c is the velocity. If the sensor array is to be inserted inside the ear canal, the spacing between the microphones will be much smaller than the required. This constraint can be overcome by introducing additional delay factor to compensate for the difference in delay due to the required spacing d and the delay due to physical microphone spacing.

MEMS MICROPACKAGING SOLUTION

The MEMS technology can be used to create necessary structures for die level integration of MEMS devices or components and CMOS or non-CMOS, like BJT, GaAs, and Silicon-germanium devices. The basic structure of the proposed mechanism is a socket submodule (figure 4) that holds a die or device. The required no of submodules can be stacked vertically or horizontally to realize a completely system in a micropackage.

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Figure 4a. 3D model of socket submodule

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Figure 4b. top view of socket submodule

Connectivity between submodules is achieved by means of microbus card (figure 6.) constructed with heat deformed, gold coated polysilicon cantilever microspring contacts and platinum coated microrails fabricated inside an interconnection channel that is presented in each socketDept. of AEI 8 MESCE Kuttippuram

Micromechanical System for System-on-Chip Connectivity

submodule. An illustration of the micropackaging system is shown in figure 5.

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Figure 5a. Top view of MEMS micropackage

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Figure 5a. MEMS micropackaging system: cross section through AA

Microorganisms and moisture inside the ear canal may contaminate the microsensor array. This can be helped by the submodule type sensor array, which can be removed easily for cleaning or replacement. The submodules are connected by means of a MEMS microbus with gold coated polysilicon cantilever microspring contacts and platinum coated microrails fabricated inside an interconnection channel that is presented in each socket submodule. Figure 6 shows the 3D model of microbus

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Figure 6. 3D model of MEMS microbus card

DIE TESTING CONFIGURATION

The concept of socket submodules and connectivity can also be used in a die testing platform. The establishment of temporary connectivity for testing a die without exposing the die to otherwise harmful energy sources or contaminations during the test cycles is a major technological challenge. The MEMS submodule can be reconfigured to establish temporary connectivity for die testing with out exposing the die to any

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contamination while carrying out necessary test procedures. Figure 7 illustrates the die testing configuration using MEMS socket type structures.

Figure 7. MEMS die testing configuration

In this set up, two different type of MEMS sockets are used: a fixed one connected permanently to a Tester-on-Chip (ToC),which is a die testing SoC using an enabling goldto-gold thermo sonic bonding technology and a removable socket that acts a die specific carrier. The contact springs on both sides of the removable socket undergo deformation due to a compression mass on the top of the die and generate the necessary contact force. The removable MEMS socket can be redesigned to connect a die that is larger than the ToC. This makes the system a flexible one. The major designDept. of AEI 13 MESCE Kuttippuram

Micromechanical System for System-on-Chip Connectivity

objectives of contact spring mechanism is to develop a proper contact force, low-contact resistance, small area, and short contact path while having the ability to tolerate some torsional misalignment. Another important requirement is to maintain the contact surface that will remain reasonably flat even under torsional deformation to realize a higher contact area. Based on these constraints designs two of contact springs are given in figure 8.

Figure 8. Two types of micro spring contacts

ADVANTAGES AND DISADVANTAGES

ADVANTAGES High efficiency Cost effective Flexible High accuracy precision14 MESCE Kuttippuram

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Micromechanical System for System-on-Chip Connectivity

DIS ADVANTAGES Complex design Complex fabrication procedures

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CONCLUSION

MEMS technology offers wide range application in fields like biomedical, aerodynamics, thermodynamics and telecommunication and so forth. MEMS technology can be used to fabricate both application specific devices and the associated micropackaging system that will allow for the integration of devices or circuits, made with non compatible technologies, with a SoC environment. The MEMS technology allows permanent, semi permanent and temporary connectivity. The integration of MEMS to present technology will give way to cutting edge technology that will give outstanding functionality and far reaching eff

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