Micro electro mechanical systems and devices

Details Micro electro mechanical systems and devices Micro electro mechanical systems and devices

2001-03-23

2004-03-16

Wilson, Allan R. (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Physical deformation

C257S415000

Reexamination Certificate

active

06707121

ABSTRACT:

FIELD OF INVENTION
The present invention is related to methods and structures for improving mechanical strength in Micro Electro Mechanical Systems (MEMS). These methods and structures are used to produce MEMS devices with improved mechanical characteristics.
BACKGROUND OF THE INVENTION
Micro Electro Mechanical Systems (MEMS), also known as Microsystems or Micro Machined Systems, use the process technology as developed in semiconductor processing to obtain devices with the desired mechanical properties. In MEMS technology, these mechanical devices on microscale are embedded in micro-electronic circuitry. MEMS devices serve as an intermediate between the non-electrical and the electrical world or as transducers between physical quantities (e.g., transforming radiation or pressure into current). Contrary to semiconductor devices, mechanical properties such as weight, vibration, etc. are crucial for MEMS devices. The characteristics of Micro Electro Mechanical Systems (MEMS), e.g. accelerometers, thermal sensors, membrane-devices etc., are thus determined by both their transducing and mechanical properties. The strong interaction between the transducing and mechanical properties of these systems often imposes restrictions on meeting simultaneously both types of specifications.
Examples of Micro Electro Mechanical Systems devices are thermal sensors. Thermal sensors measure a temperature rise resulting from the deposition of an amount of energy within a thermally insulated piece of material, this insulated piece being the sensing element. The energy can result from a variety of interactions between the environment and the sensing element, e.g. X-ray or infrared flux absorption, reacting molecules. A “Method of fabrication of an infrared radiation detector and more particularly an infrared sensitive bolometer” is disclosed in U.S. Pat. No. 6,194,722 and is entitled “Method Of Fabrication Of An Infrared Radiation Detector And Infrared Detector Device.” U.S. Pat. No. 6,194,722 is hereby incorporated by reference in its entirety.
Thermal analysis shows that the change in temperature is proportional to the thermal insulation of the device layer, the energy deposited in this layer being in general a given quantity. The better the thermal insulation of the sensing element, the more sensitive the device becomes. The energy, deposited in the sensing element, will not or to a less degree, leak away to the environment thanks to the better insulation. A larger change in temperature is thus obtained for a given amount of incident energy. Maximum thermal insulation can be obtained by suspending the sensing element on thin narrow and long beams of a thermally insulating material, so that the sensing layer is only connected to the rest of the device by these suspending beams. Between the sensing element and underlying layer there is a gap providing better thermal insulation of the sensing element.
The very last process step, being the removal of the sacrificial layer between the sensing element and the underlying substrate thereby creating the aforementioned gap, is critical and difficult. This removal is generally done by a wet etch. Due to a general problem in surface micromachining, called sticking, the removal of such a sacrificial layer causes a lot of yield loss. During the wet etch of the sacrificial layer, the surface tension of droplets under the sensing element pulls this sensing element towards the substrate. Once the sensing element reaches the substrate, strong bonding forces between this sensing element and the substrate along with the mechanical weakness of the supporting beams prevent the sensing element from returning to its original position. Once the sensing element touches the substrate, the device has a large thermal conductance to the substrate and is useless as a bolometer. Making the supporting beams thinner will improve the thermal insulation of the sensing element but turns out to make the problem of sticking more severe, resulting in yield loss during processing and packaging of such devices.
In “Mechanical performance of an integrated microgimbal/microactuator for disk drives,” Proc. of the Transducers '99 Conference, June 1999, Sendai, Japan, p. 1002-1005 (1999) by L. Muller, J. M. Noworolsky, R. T. Howe and A. P. Pisano, a specific structure is presented to improve the mechanical strength of parts of an assembled device, i.e., a device for reading data stored in a disk drive. However, this structure is obtained at the expense of additional, time consuming, process steps. These process steps cannot be done in the course of producing the gimbal, but require separate processing, followed by assembly. The proposed process uses thick layers having large dimensions up to 75 micrometer in this so-called high-aspect-ratio fabrication technology. Only the torsional stiffness of the supporting bars is improved, no other mechanical or physical characteristics of the device are involved by the processing of the microgimbal.
The article “Silicon micro
anomechanical device fabrication based on focused ion beam etching surface modification and KOH ethcing,” in Microelectronic engineering 35 (1997) p 401-404, by J. Brugger et al. discloses the improvement of the mechanical stability of freestanding nanomechanical elements by increasing the moment of inertia of the supporing elements. The technique disclosed is very complex and time consuming and does not allow waferscale processing.
SUMMARY OF THE INVENTION
One aspect the present invention aims to improve the mechanical stability of the MEMS devices. The improved mechanical strength can lead to higher production and packaging yield. It can also improve the operation of MEMS devices that have moving elements. By introducing this method, the thickness of layers can be adapted to meet other technological specifications and still have sufficient mechanical strength. The improved mechanical strength can lead to higher production and packaging yield. The improvement in mechanical strength with respect to the device properties can be expressed in terms of a figure of merit M. In the finished MEMS device, layers may indeed not be fully supported by an underlying layer and hence lack mechanical strength.
Another aim of the present invention is to provide a device for sensing electromagnetic radiation with improved mechanical characteristics and device performance.
Another aim of the present invention is to obtain an infrared sensor with improved mechanical characteristics and device performance. An advantage of this device is that the thickness of the layer, e.g. the sensing layer, can be adapted to have the maximum temperature sensitivity thanks to a minimal thermal conductance possible for a given layout or material choice. Another advantage of such a device is that the time constant of this thermal sensor is minimized to such a level that the device can be applied in fast thermal sensor camera applications. The improved mechanical strength allows using a minimal layout of the devices so the thermal sensor can be used in an array-type of circuitry. The improvement in mechanical strength with respect to the device properties can be expressed in terms of a figure of merit M. In the present application, the term “layer” should be understood as a stack of at least one layer. U- or I-shaped layer means that a part of the cross section of this layer has a U- or I-shaped profile. Generally, a structure in a MEMS device which provides a three-dimensional shape to a part of a MEMS device will be called a “microstructure” and insofar as it improves the mechanical properties of the device, e.g. its rigidity, it will be called a “rigidizing microstructure.” In one aspect of the invention, MEMS devices with improved mechanical characteristics are provided.
The aims of the invention are solved by devices and methods defined in the attached claims.
In one embodiment of the invention, the resistance of layers in MEMS devices to bending, lateral or torsion forces, having a static or dynamic nature, is increased by providing these layers with one or

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