Semiconductor Industry and MEMS Materials





 
1.Introduction to modern  Semiconductor industry

Since its invention about 40 years ago, the Intigreted Circuits(ICs) has literally changed our world. What we heared, such as CUP, DRAM, SDRAM and so on, are the products(ICs) of modern semiconductor industry. In the past 30 years, the ability of semiconductor products had doubled and the price had decreased by half. Here we have a group of data to describe the rapidly development of IC industy all over the world:

Future projections for silicon technology from the
SIA NTRS[1.3]
Year
1997
1999
2003
2006
2009
2012
minimum feature size
250nm
180nm
130nm
100nm
70nm
50nm
DRAM Bits
256M
1G
4G
16G
64G
256G
DRAM Size(mm2)
280
400
560
790
1120
1580
Transistors
11M
21M
76M
200M
520M
1.4B

Semiconductor industry is playing more and more important role in modern society. We can say, no semiconductor industry, no modern civilization!
 

Leader in Semiconductor Manufacturing and Technology


 

In 2004 IDF(USA), Intel was showing its new silicon based micro chips sets in an un-cut wafer

 
Intel is a leader in semiconductor manufacturing and technology and has established a competitive advantage through its scale of operations, agility of its factory network, and consistent execution worldwide. Intel has 11 fabrication facilities and six assembly and test facilities worldwide. Intel produces the silicon for its high-performance microprocessors, chipset and flash memory components in its fabrication facilities. After the silicon-based products are created, they are sent to Intel's assembly and test facilities where each wafer is cut into individual microprocessors, placed within external packages, and tested for functionality.

In 2003, Intel spent $3.7 billion on capital investments to help build manufacturing capacity and increase manufacturing efficiency. Intel has completed the manufacturing conversion from 200 mm silicon wafers to 300 mm silicon wafers. Chip fabrication costs on 300 mm wafers are at least 30 percent lower than on 200 mm wafers. By the end of 2004, four fabrication facilities will use 300 mm silicon wafers.

 

2.Commonly used MEMS materials
 
 

Category
Material
Property or appliction
Metal
Au,Al,Cu,Ni,Cr
Conductor
Semiconductor
Silicon
Structure,semiconductor
Poly-crystalline silicon
Structure,semiconductor
Single-crystalline silicon
Structure,Anisotropic etch properties
GaAs
Structure,optical properties
Quartz
Crystalline SiO2
Substrate,structures
Polymer
Parylene-C
Nafion
Polymeric materials
Non-metals
Diamond(C)
structure
Others
 
 
 
 

3.
Crystal structure of Silicon

 
 
 
 

source: http://www.webelements.com

 
 
 
 

Si crystals have diamond structure

 
 
 
 
The <111> planes have fewer dangling bonds => <111> planes etches slowly in some etchants The etch-rate ratio can exceed 1,000.

Common anisotropic etchants: 

Potassium hydroxide (KOH), 
Ethylene Diamine (EDP),
Tetramethyl Ammonium Hydroxide(TMAH)


 
  4. Crystal planes  
 
 
  Crystals are characterized by a unit cell which repeats in the x, y, z directions.

  • Planes and directions are defined using x, y, z coordinates.

  • [111] direction is defined by a vector of 1 unit in x, y and z.

  • Planes defined by ^Miller indices ̄

C Their normal direction (reciprocals of intercepts of plane with the x, y and z axes).

 
 

5.Types of silicon wafer

  Geometric representation of crystal Si Definition:

      ^(  ) ̄  specific plane     ^{   } ̄ equivalent plane

      ^[  ] ̄  specific direction   ^<  > ̄ equivalent direction

 

  Wafers are classified as how they are doped(n-type, p-type), and how the Si crystals are oriented as seen from the top of the shape of the wafers are cut differently as below.

 
 
 
 
Characteristics (compare to other MEMS Materials)

 
 
6.Why do we choose silicon as a MEMS structural material?

 
Semiconductors are a class of materials which have the unique property that their electrical conductivity can be controlled over a very wide range by the introduction of dopants. While this property can easily be observed in crystalline, polycrystalline, or amorphous semiconductor materials, crystalline materilas provide the most reproducible properties and the highest performance devices and are almost always used in integrated circuits.
      Dopants are atoms that generally contain either one more or one fewer electrons in their outermost shell than the host semiconductor. They provide one extra electron or one missing electron (a "hole") compared to the host atoms. These excess electrons and holes are the carriers, which carry current in semiconductor devices. The key to building semiconductor devices and integrated circuits lies in the ability to control the local doping and hence the local electronic properties of a semiconductor crystal.


 Naturally, we concern the chemical and physical properties of chemical element-Silicon. Why does the IC industry choose the Silicon as the fundamental material? Why does the Silicon play so important role in the area?
        We will introduce the details in the coming chapter to illustrate the structure of Si crystal.           

       In ICs technology, we will deal with the fabrication of silicon wafer. But in MEMS technology, we just use the commercial products of wafers. The correct choice of wafers are imporatnat issue in MEMS device fabrication.
 

7.Semiconductor Devices

 
 

Understanding of how the basic devices used ICs operate is useful, because it provides some understanding of the objectives we have for IC technology.
       The most commonly used IC devices are:

       1) PN Diodes
       2) Bipolar Junction Transistors
       3)  MOS Transisters

 
 

8 . Wafer cleaning and gettering

 
 
 
 The purpose of wafer cleaning and wafer gettering are to reduce contamination. Due to the very small size of MEMS components, even if those particles we look as very small are too big for these components.For these reseasons, reducing contamination becomes a very important and expensive work in semiconductor industry and MEMS lab. We almost use more than half money to keep the lab clean and reduce waste products.
       Contamination on wafers consists of particles, films such as photoresist which must be removed after they serve their purpose in lithography, and trace levels and element which has not been purposely introduced.
       The detail methods to clean Si wafers will be given in classes.