Micro Mechanical Transducers (eBook)

Pressure Sensors, Accelerometers and Gyroscopes

Min-hang Bao (Autor)

S. Middelhoek (Herausgeber)

eBook Download: EPUB | PDF

2000 | 1. Auflage
392 Seiten
Elsevier Science (Verlag)
978-0-08-052403-0 (ISBN)

Infos

Some years ago, silicon-based mechanical sensors, like pressure sensors, accelerometers and gyroscopes, started their successful advance. Every year, hundreds of millions of these devices are sold, mainly for medical and automotive applications. The airbag sensor on which research already started several decades ago at Stanford University can be found in every new car and has saved already numerous lives. Pressure sensors are also used in modern electronic blood pressure equipment. Many other mechanical sensors, mostly invisible to the public, perform useful functions in countless industrial and consumer products.

The underlying physics and technology of silicon-based mechanical sensors is rather complex and is treated in numerous publications scattered throughout the literature. Therefore, a clear need existed for a handbook that thoroughly and systematically reviews the present basic knowledge on these devices.

After a short introduction, Professor Bao discusses the main issues relevant to silicon-based mechanical sensors. First a thorough treatment of stress and strain in diaphragms and beams is presented. Next, vibration of mechanical structures is illuminated, followed by a chapter on air damping. These basic chapters are then succeeded by chapters in which capacitive and piezoresistive sensing techniques are amply discussed. The book concludes with chapters on commercially available pressure sensors, accelerometers and resonant sensors in which the above principles are applied.

Everybody, involved in designing silicon-based mechanical sensors, will find a wealth of useful information in the book, assisting the designer in obtaining highly optimized devices.

Textauszug

Chapter 1

Introduction to micro mechanical transducers

Min-Hang Bao    Department of Electronic Engineering, Fudan University, Shanghai, China

§1.1 Piezoresistive pressure sensors

§1.1.1 Brief history

The effect of piezoresistance in germanium and silicon was discovered by C.S. Smith in 1954 [1]. It was found that the resistance of a germanium or silicon filament changed when the material was stressed. The effect of piezoresistance is similar to the strain gauge effect in a metal material, but the differences between them are quite fundamental:

(a) The effect of metal strain gauge is caused by the geometric deformation of the resistor whereas piezoresistance is caused by the change of resistivity of the material,

(b) The effect of metal strain gauge is isotropic whereas the effect of piezoresistance is generally anisotropic, and

(c) The effect of piezoresistance can be two orders of magnitude larger than that of the metal strain gauge effect.

It was believed that the large piezoresistance effect would have some application in sensors, especially in mechanical sensors dominated at that time by metal strain gauges. Soon a semiconductor piezoresistive sensing element (a piezoresistor) was developed and found an application in mechanical sensors.

With the rapid development of silicon technology in the 1960s, the excellent mechanical properties of the material silicon were understood in addition to its versatile electrical and thermal properties. Therefore, efforts to use silicon as a mechanical material were made. First, piezoresistors were made by selective diffusion into a silicon wafer by planar processes so that the silicon wafer could be used as a mechanical diaphragm with integrated piezoresistors on it. When the diaphragm was bonded to a glass or metal constraint by epoxy as schematically shown in Fig. 1.1, a pressure transducer was formed [2]. For the first time, silicon was used as both the mechanical as well as the sensing material in a sensor.

Fig. 1.1 A pressure transducer using a silicon diaphragm

Significant progress was made around 1970 when the silicon substrate with sensing elements on it was shaped by mechanical drilling to form an integrated diaphragm-constraint complex [3]. A pressure transducer formed by this technique is schematically shown in Fig. 1.2. As the whole structure is made out of bulk silicon material, the mechanical performance of the device is greatly improved.

Fig. 1.2 A silicon piezoresistive pressure transducer based on a diaphragm formed by mechanical drilling

The processing technology for the silicon structure shown in Fig. 1.2 was further improved in the mid-1970s when anisotropic etching technology was used for silicon pressure transducers. By using masked anisotropic etching, silicon pressure transducers could be batch-fabricated with the planar process steps, such as oxidation, diffusion, photolithography, etc., originally developed for silicon transistors and integrated circuits [4, 5]. The dimensions of the devices could also be reduced significantly. The silicon “chip” of a pressure transducer made by this technology is schematically shown in Fig. 1.3.

Fig. 1.3 A silicon piezoresistive pressure transducer based on a diaphragm formed by micromachining

The dimensions of the mechanical structures processed can be controlled to an accuracy of microns, the technologies are often referred to as micromachining technologies.

Some basic concepts implied by the pressure transducer shown in Fig. 1.3 are:

(a) The silicon material can be used for the mechanical structure as well as electronic components and sensing elements, and

(b) The mechanical structure of silicon can be batch-fabricated by micromachining technologies.

Numerous innovations and improvements have been made for silicon pressure transducers in the following years and the production volume of silicon pressure transducers has been growing steadily since then, but the basic principles remain unchanged even today.

§1.1.2 Working principles

The structure of the silicon pressure transducer shown in Fig. 1.3 is basically a typical structure of a present-day silicon pressure transducer despite the many structural and technological modifications. To understand the working principles of the device, a more detailed description follows with reference to Fig. 1.4.

Fig. 1.4 Detailed illustrations for a typical silicon pressure transducer (a) bottom view, (b) top view, (c) Wheatstone bridge and (d) cross-sectional view

Fig. 1.4(a) gives a back view of the sensor chip. Using masks (SiO2 or Si3N4) on the frame region the cavity is etched using an anisotropic etchant (typically, aqueous KOH). As the wafer is 100-oriented and the edges of the etching windows are along the <110> directions of the silicon crystal, the sidewalls of the cavity are {111} planes. As the angle between the {111} sidewalls and the (100) bottom is 54.74°, the bottom of the cavity (the diaphragm) is smaller than the etching window by d, where d is the depth of the cavity. Therefore, the size of the diaphragm can be well-controlled by the size of the etching window and the etching depth.

Fig. 1.4(b) shows the front side of the chip (for clearness, the area of the diaphragm is delineated by dotted lines). Schematically shown on the right-hand side of the diaphragm and frame are four piezoresistors formed by boron diffusion or ion implantation on an n-type substrate, the metallization to interconnect the resistors into a Wheatstone bridge and the four bonding pads for power supply and signal output. The cross section along the AA′ line is shown in Fig. 1.4(d). The four piezoresistors are connected to form a Wheatstone bridge as schematically shown in Fig. 1.4(c).

Before the sensor chip can be functional, the chip must be encapsulated. The structure of an encapsulated pressure transducer is shown in Fig. 1.5. The silicon sensor chip is first electrostatically bonded to a glass plate with a hole in the center. The chip-glass combination is then mounted onto the base of a package (also with a hole at the center). After bonding, pads are electrically connected to the leads of the package by wire-bonding. A cap with an input port is then hermetically sealed to the base of the package.

Fig. 1.5 An encapsulated silicon pressure transducer

The pressure to be measured is applied on the diaphragm through the input port of the cap. Suppose that the pressure is positive with reference to the environmental pressure (atmospheric pressure at the rear of the diaphragm).

As the pressure on top of the sensor chip is larger than that on the back, the silicon diaphragm bends downwards. This causes stress in the diaphragm. The stress, in turn, causes a change in resistance of the resistors. For a typical design as shown in Fig. 1.4, the resistance of R2 and R3 goes up and that of R1 and R4 goes down. This will cause an output of the Wheatstone bridge directly proportional to the pressure difference on the diaphragm. Generally speaking, the output of the bridge can be higher than 100 mV with good linearity for a 5 V power supply (higher outputs are possible with larger nonlinearity). This usually determines the nominal maximum operation range of the device. The operation range of a pressure transducer can be from 1 kPa to 100 MPa basically decided by the size and the thickness of the diaphragm.

To meet different application needs, pressure transducers can be packaged to form three types of devices. They are gauge pressure transducers (GP), absolute pressure transducers (AP) and differential pressure transducers (DP).

The pressure transducer shown in Fig. 1.5 is a gauge pressure transducer. This kind of pressure transducer measures a pressure measurand with reference to the environmental pressure around the device.

An absolute pressure transducer measures a pressure measurand with reference to an absolute reference pressure. The reference pressure is usually a vacuum so that it is not temperature dependent.

A differential pressure transducer measures the difference between two pressure measurands. Therefore, a differential pressure transducer has two input ports for the two pressures to be measured. Generally speaking, the sensor chips for the three types of pressure transducers are similar, but the packaging techniques can be quite different. Among them, the package for a gauge pressure transducer is the simplest and the package for a differential pressure transducer is the most difficult.

According to the brief description given above, the working principles of a piezoresistive pressure transducer are based on much theory, including the stress distribution in a diaphragm caused by pressure and the piezoresistive effect of silicon. The stress distribution in a diaphragm will be discussed in Chapter 2, the piezoresistive effect of silicon will be discussed in Chapter 5, and finally the detailed principles and design methods of pressure transducers will be given in Chapter 6.

§1.2...

Produkt-Details

• Erscheint lt. Verlag: 16.10.2000
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Physikalische Chemie
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Technik ► Maschinenbau
• ISBN-10: 0-08-052403-6 / 0080524036
• ISBN-13: 978-0-08-052403-0 / 9780080524030