Micromachined capacitive component with high stability

Details Micromachined capacitive component with high stability Micromachined capacitive component with high stability

2002-07-16

2004-09-07

Le, Huyen (Department: 2643)

Electrical audio signal processing systems and devices

Electro-acoustic audio transducer

Microphone capsule only

C381S191000

Reexamination Certificate

active

06788795

ABSTRACT:

This invention relates to a micromachined capacitive electrical component in general. In particular the invention relates to a capacitive transducer such as a condenser microphone. Such micromachined components or systems are often referred to as Micro Electro-Mechanical Systems (MEMS).
BACKGROUND OF THE INVENTION
A capacitive transducer such as a condenser microphone typically has a thin diaphragm that is arranged in close proximity to a back plate defining an air gap therebetween. The thin diaphragm is constrained at its edges, so that it is able to deflect when sound pressure is acting on it. Together the diaphragm and back plate form an electric capacitor, where the capacitance changes when sound pressure deflects the diaphragm. In use, the capacitor will be electrically charged using a DC bias voltage. When the capacitance of the microphone varies due to a varying sound pressure, an AC voltage proportional to the sound pressure will be superimposed on the DC voltage. The AC voltage is used as output signal of the microphone.
The sensitivity of the microphone, ie the ratio of the output AC voltage to the input sound pressure acting on the microphone, increases with the applied DC bias voltage. Consequently, in order to obtain a highly stable sensitivity without drift in time, the DC voltage across the air gap between the diaphragm and the back plate must be very stable. Note that a highly stable sensitivity is a requirement for any critical application of microphones, such as for example microphones for sound level measurement or other technical or scientific purpose.
The DC voltage is applied from an external voltage source via a bias resistor. The bias resistance must be so high that it ensures a virtually constant charge on the microphone, even when the capacitance changes due to sound pressure acting on the diaphragm. The value of this bias resistor is typically 1 to 10 G&OHgr;. When the leakage resistance of the microphone is infinitely high, the voltage across the microphone equals the applied DC voltage. If however, the leakage resistance of the microphone is not infinitely high, the applied DC voltage is divided between the bias resistor and the leakage resistance of the microphone, and consequently, the sensitivity of the microphone decreases. Therefore, a usual and practical requirement for a highly stable microphone is that the leakage resistance must be at least 1000 times higher than the resistance of the bias resistor, even under severe environmental conditions, as for instance in conditions of high humidity and high temperature.
Another cause of a change in the voltage across the air gap between the diaphragm and the back plate is the presence of additional charges in the air gap, ie charges not related to an applied polarization voltage. This behavior is well known, and utilized in electret microphones, where an electric charge is intentionally stored in an insulator layer in the air gap, so an electrical field is present in the air gap of the microphone without the need for an external voltage supply. However, in condenser microphones that are polarized by an external voltage source, charge storage is undesirable, since it changes the DC voltage across the air gap, thus causing changes in sensitivity. Storage of charge in the air gap of the microphone requires the presence of an insulating layer in the air gap. So in a highly stable condenser microphone the presence of insulating layers between the diaphragm electrode and the back plate electrode is undesirable.
Summarizing, the construction of a condenser microphone with a highly stable sensitivity over time, requires:
1. A leakage resistance that is at least 1000 times the bias resistor value, even under severe environmental conditions
2. No insulating layers in the air gap between the diaphragm electrode and the back plate electrode
From traditional measurement condenser microphones for industrial and scientific purposes it is known that the leakage resistance is determined by the leakage current across the surface of an insulator disc that separates the electrical contacts of the connector of the microphone. Likewise, in micromachined condenser microphones, the leakage resistance is determined by leakage current across the surface of the insulating material that separates the diaphragm electrode and the back plate electrode. The leakage resistance increases if the shortest distance that the leakage current has to travel across the insulator is increased. In traditional measurement condenser microphones, the shortest distance is of the order of millimeters. In some of the micromachined condenser microphones that are presented in literature, the shortest distance comes down to the thickness of an insulator layer that is of the order of 1 &mgr;m! This is for example the case in designs, where both the back plate and the diaphragm are made of monocrystalline or polycrystalline silicon, where a silicon dioxide spacer layer with a thickness between 1 and 3 &mgr;m has to provide the electrical insulation between diaphragm and back plate. Examples of such constructions are presented in the publication entitled “A silicon condenser microphone using bond and etch-back technology” by J. Bergqvist and F. Rudolf in the journal Sensors and Actuator A, 45 (1994) 115-124, and “Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon back plate” by A. Torkkeli et al. in the journal Sensors and Actuator, 85 (2000) 116-123 (corresponding to U.S. Pat. No. 6,178,249, Hietanen et al.), and in U.S. Pat. No. 5,452,268 “Acoustic transducer with improved low frequency response”. That type of construction cannot be expected to have a leakage resistance that is at least 1000 times the bias resistor value, especially under conditions of high humidity and temperature. Consequently, that type of microphone would be suitable only for uncritical low-end applications, but is definitely not suited for any critical application that requires the sensitivity to be stable over time.
Another microphone construction that may ensure a high leakage resistance between the diaphragm electrode and the back plate electrode is presented in the publication “A new condenser microphone with a p
+
silicon membrane”, by T. Bourouina et al. in the journal Sensors and Actuators A, 31 (1992) 149-152. That microphone is made by bonding a silicon part, containing an etched diaphragm, onto a glass substrate, that contains the back plate electrode. The shortest distance between the diaphragm electrode and the back plate electrode is now considerably larger than the air gap thickness, so a higher leakage resistance can be expected. However, a disadvantage of using chips made of bonded silicon- and glass substrates is the thermal mismatch between the two materials. Although glass types exist (e.g. Pyrex 7740) that are developed with the purpose of providing properties matching those of silicon, they never exactly match the thermal expansion coefficient over the complete operating range of the transducer (typically −30° C. to +150° C.). The difference in thermal expansion coefficient causes a thermal stress in the diaphragm, which gives increased temperature sensitivity. Another effect is thermal bending of the silicon-glass sandwich (like a bimetal, since the silicon and glass have a comparable thickness) that gives a temperature-dependent change in air gap thickness that also gives a change in sensitivity. Therefore, microphone chips made by bonding silicon to glass suffer from temperature drift of the sensitivity, which is undesirable in critical applications of microphones.
Microphone chip designs based on an insulating diaphragm material are often to be preferred from a fabrication point-of-view. There are several electrically conducting diaphragm materials that can be made on silicon wafers. In the table below, a list of conducting diaphragm materials is shown, together with the disadvantages.
Evaporated or sputtered
Lack of stress control
metal
Need for complicated layer protection during
silicon etching

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