Introduction

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Introduction[edit]

Welcome to the course!

Lecture date: Friday, 2014.08.29 (this class was not recorded)

This course is primarily concerned with manufacturing—"microfabrication"—of devices made from thin solid films.

The prototype device is the solid state transistor–based integrated circuit—the 'chip' that is used in computers and many other devices for logic and information storage.

Manufacturing Steps[edit]

The processes used to manufacture these devices are based on a relatively few basic steps:

bulk crystal growth (silicon, mostly)

  • make wafers on which devices are manufactured

chemical reactions with surfaces

  • cleaning
  • oxidation
  • etching

thin-film formation

  • evaporation
  • sputtering
  • epitaxy
  • chemical vapor deposition
  • spin coating

photolithography

semiconductor doping

  • solid-state diffusion
  • ion implantation

We will analyze some of these processes using fundamental chemical engineering concepts. For homework, you will watch a video that provides a broad overview of these processes and how they are combined to manufacture a solid state device.

Semiconductor Fundamentals[edit]

Electronic Switches[edit]

Key idea: The transistor-based operations in a modern electronic device are all related to the idea that an electrical signal can be turned off or on, or be amplified, by another (generally smaller) signal. Prior to the invention and commercialization of the transistor, the vacuum tube filled this role. The transistor offers improvement in cost, reliability, and energy efficiency, and began supplanting the vacuum tube in the 1950's and 60's.

Wikipedia has an excellent history of the transistor that puts this technical innovation in perspective.

Semiconductor Materials[edit]

Many semiconductors are composed of silicon, a so-called "semiconductor" material. As the term suggests, silicon is not quite a conductor and not quite an insulator; when a voltage is applied across a piece of silicon, a very small current will flow.

Charge Density and Mobility[edit]

For now, suffice it to say that when an electric field is imposed on a solid material, the current that flows in the material is proportional to the free charge density; in a future lecture, we will explore this relationship in more detail.

The free charge density of a material is related to the energy difference between the valence band and the conduction band in that material:

  • In an insulator, the difference may be several electron-volts (eV); thermal energy is insufficient to promote electrons to the conduction band, and no current flows when an electric field is imposed.
  • In a conductor, the bands overlap; a 'sea' of mobile electrons move easily, and current flows readily when an electric field is imposed.
  • In a semiconductor, the bands are separated by a moderate amount of energy e.g. in silicon by 1eV; thermal energy is sufficient to promote a very small number of electrons to the conduction band, and a very small current flows when an electric field is imposed.

Thermal Energy[edit]

How much kinetic energy is available due to thermal motion to promote electrons? Thermal kinetic energy is equivalent to [math]\frac{3}{2}k_{B}T[/math], where [math]k_{B}[/math] is the Boltzmann constant of [math]1.3806488 × 10^{-23}[/math]; therefore, there is only 0.026 eV available at 300K. Clearly, only a very tiny fraction of the electrons in pure silicon are available for conduction.

Doping[edit]

The free charge density can be dramatically altered by the addition of dopants or impurities into the crystal lattice.

"N-type" dopants are electron (negative charge) donors, e.g. group V elements such as P, As, or Sb in Si. "P-type" dopants are electron acceptors (creating free positive charges, 'holes'), e.g. group III elements.

Why?

  • group V elements have 5 valence electrons
  • group III elements have 3 valence electrons

N-Type Doping[edit]

Consider P, a group V element:

File:N-doped Si.svg
figure 1.X - Phosphorus dopant bonding structure, an "N-type" dopant

extra electron given up easily, leaving behind an immobile positive ion, [math]\text{P}^+[/math].

P-Type Doping[edit]

Similarly, Boron (B) has only 3 valence electrons, so it "accepts" an electron from an adjacent bond, leaving behind a mobile positive hole and creating an immobile negative ion [math]\text{B}^-[/math]:

File:P-doped Si.svg
figure 1.X - Boron dopant bonding structure, a "P-type" dopant

How do holes move? Electrons jump from one bond to the open hole:

[math] \text{Si}\ ^{\bullet}_{\bullet}\ \color{blue}{\text{Si}}\ ^{+}_{\bullet}\ \color{red}{\text{Si}} \rightarrow \text{Si}\ ^{+}_{\bullet}\ \color{blue}{\text{Si}}\ ^{\bullet}_{\bullet}\ \color{red}{\text{Si}} [/math] (an electron moving to the right is equivalent to a hole moving to the left)

[math]N_D[/math] is used to represent density of donor atoms and [math]N_A[/math] is density of acceptor atoms.

The Doping Unit Operation[edit]

By varying the dopant concentration, the availability and nature—electrons or holes—of the free charges can be manipulated. This forms the basis for manipulating how charge flows through the different regions of a semiconductor device. Later in the class we will explore the primary unit operation involved in doping— ion implantation—in detail.

Homework[edit]

For Homework 01, you will be watching a video on the semiconductor manufacturing process. The video is (somewhat humorously) dated, but the processes described still form the basis of modern solid state device construction. After watching the video, you will answer some questions and provide us with some information on your background, interests, and expectations for this class.

Additional Resources[edit]

Drs. Vivek Subramanian and Michel Maharbiz created this video for EE40: EE40 - Semiconductor fabrication overview