Lecture 3: Basic Concepts: Semiconductors - UVicmech466/MECH466-Lecture-3.pdf · Lecture 3: Basic Concepts: Semiconductors ... Note that in a cubic lattice, ... We can view the actual

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MECH 466Microelectromechanical Systems

University of VictoriaDept. of Mechanical Engineering

Lecture 3:Basic Concepts:Semiconductors

© N. Dechev, University of Victoria

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Silicon Structural Properties

Crystal Planes

Bulk Micromachining

Semiconductor Properties

Doped Semiconductors

Overview


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Silicon solid exists in three different forms:

AmorphousRandomly oriented atoms,e.g. Glass

PolycrystallineCrystal grains/domains oriented in random directions, which meet at grain boundaries

CrystalEntire solid is made of an ordered array of atoms


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Silicon crystal has a cubic lattice.

Silicon atoms form covalent bonds with four adjacent atoms, in the form of a diamond lattice structure.

Si Atoms

CovalentBonds

Note: Bonds outside of Cubic Lattice are not shown


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Silicon exhibits different properties along different crystal planes.

This includes properties such as E (young’s modulus), electron mobility, piezoresistivity, and chemical etch rates (for fabrication purposes). z

x

ya a

a


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Miller Indices, Planes

The Miller Indices are a common notation used to identify the planes and directions in a crystal lattice.

To determine the Miller Index of a plane, we use the following procedure:

z

x y

a

Step 0: Identify the plane of interest. For example, the plane shown in pink is one face of the cubic structure.


Crystal Plane (100)

Step 3: Reduce these numbers to the smallest set of integers h,k and l, by multiplying all by a, which yields (1 0 0).Parentheses are used to denote a crystal plane.

Step 2: Take the reciprocals of the three numbers found in step 1.In this example: 1/a , 1/∞ (=0), and 1/∞ (=0).

Step 1: Identify the intercepts of the plane with the x, y and z axes. In this example, we have x = a, y = infinite, z = infinite.

x = a

y = ∞

z = ∞

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Miller Indices, Planes

Since atoms are symmetrical, similar planes have identical material properties.

z

x y

Families of similar planes are denoted with braces {}. Therefore, these three planes belong to the family of planes {100}.

(100)

(001)

(010)

Lattice View of {100} family of planes


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Miller Indices, Planesz

xy

Crystal Plane (110)

(110)

Lattice Top View of {111} family of planes

z

xy

(111)

Crystal Plane (111)

Lattice Top View of {110} family of planes


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Miller Indices, Directions

[100]

Crystal Direction [100]

Step 1: Find the parallel vector that begins at the origin.


Step 2: Reduce the three coordinates to the smallest set of integers [h,k and l]. For example, consider the vector originates at (0,0,0) and ends at (0,1,0). Therefore, the Miller Index direction is [0,1,0].

All collectively in direction family <100>.

[001]


[010]


Note that in a cubic lattice, such as Si, a vector with Miller Index [hkl] is always perpendicular to plane (hkl).

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Miller Indices, Directions

z

x y

[110]

z

xy

Direction family <111>.Direction family <110>.

[111]


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Image of Atomic Structure of Silicon Crystal along the {111} plane[image from Dept. of Synchrotron Radiation Research, in Lund]


Image of Atomic Structure of Silicon Crystal along the {100} plane[image from Dept. of Synchrotron Radiation Research, in Lund]

We can view the actual atomic structure of a silicon crystal using SPM (Scanning Probe Microscopy) technology. Interestingly, the ‘probes’ used in SPM are made using ‘bulk micromachining’ of silicon crystal.


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Main components of the SPM system. Fig. 14.1 from textbook [Chang Liu]

Scanning Probe Microscopy

The basic operation of the SPM technique is shown below.

Chapter 14 in the textbook provides a good overview of SPM technology.


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Interaction Forces used with SPM techniqes. Fig. 14.2 from textbook [Chang Liu]

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Image of SPM probes, Fig 14.12 in textbook[Chang Liu]

Scanning Probe Microscopy

SEM images of passive and active SPM probes.


Image of SPM probe with thermal actuators, Fig 14.17 in textbook [Chang Liu]

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MEMS Microfabrication

There are two main microfabrication methods for MEMS:

•Bulk Micromachining, which is based on the etching and bonding of thick sheets of material such as silicon oxides and crystalline silicon.

•Wet Etching (Anisotropic, or Isotropic)

•Dry Etching (Plasma Etching, Reactive Ion Etching)

•Surface Micromachining, which was discussed in the preceding lecture.


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Bulk MicromachiningAnisotropic Wet Etching

Silicon Substrate

Wafer inserted into High Temp Furnace with oxygen gas, to grow oxide layer


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Silicon Substrate

HF Acid Etch

Oxide is patterned with photolithography (Not Shown)

Oxide


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Silicon Substrate

Wet Silicon Etchant


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Silicon Substrate

54.7˚

<100>

<111>


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Wet Etching of Rectangle Mask,with edges oriented along <110> vectors

Computer simulations of Anisotropic Wet Etching, using ACES freeware.

Wet Etching of Rectangle Mask,with edges oriented along <100> vectors

Wet Etching of Oval Mask,with major axis oriented along [100] vector© N. Dechev, University of Victoria

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Bulk MicromachiningDry Etching (Reactive Ion Etching)

Silicon Substrate

HF Acid Etch

Oxide is patterned with photolithography (Not Shown)


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Silicon Substrate

Reactive Ion Etch


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Silicon Substrate

<100>


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Bulk Micromachining

SEM images of samples:

Anisotropic Wet Etching using KOH withwafer edge aligned along <110> vectors

[Asia Pacific Microsystems, Inc.]

Deep Reactive Ion Etching (RIE) using Bosch Process[Tyndall National Institute]

Gas Phase Etching (Anisotropic Dry Etching) using XeF2© N. Dechev, University of Victoria

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A semiconductor is a material whose conductivity lies between that of a perfect insulator and that of a perfect conductor.

Additionally, the conductivity of a semiconductor can be ‘controlled’ by various means such as:

- intentionally introduced impurities (doping)- externally applied electric fields- temperature variations- mechanical stress- radiation (light)

These methods of control have allowed for semiconductors to be used as thousands of electronic devices, such as:

- diodes- bipolar transistors, field effect transistors (FET)- temperature sensors- force and pressure sensors- solar cells and photo-transistors


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The conductivity of a semiconductor is determined by the number of ‘free charged particles’ in the bulk, and their ability to move through the bulk (mobility).

There are two types of charge carriers:

- electrons

- holes

An ‘intrinsic semiconductor’ is a perfect semiconductor crystal with no impurities or lattice defects.

For equilibrium conditions, for an intrinsic semiconductor:

n = p ≡ ni


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An ‘extrinsic semiconductor’ is a semiconductor with an impurity. Usually a semiconductor that has been intentionally doped to create either:

- a surplus of electrons in the bulk (n-type material)usually done with phosphorus doping

- a surplus of holes in the bulk (p-type material)usually done with boron doping

Periodic Table (Right Side Only)Doping Atoms

[image from HyperPhysics, C.R. Nave, Georgia State]© N. Dechev, University of Victoria

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N-Type Silicon Semiconductor[image from HyperPhysics, C.R. Nave, Georgia State]

P-Type Silicon Semiconductor[image from HyperPhysics, C.R. Nave, Georgia State]


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Conductivity of a semiconductor is the ability to conduct electric current:

where:

Note: Do not confuse σ (conductivity) with σ (stress)

do not confuse E (electric field) with E (Young’s modulus)


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Conductivity associated with p-type silicon is:

(eq. 3.16) where:

Conductivity associated with n-type silicon is:

(eq. 3.15) where:


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Conductivity calculations are based on theory and experimentally determined coefficients.

We will not cover the details of ‘semiconductor conductivity theory’ in this course. However, some useful information and references are provided in the course textbook, pages 49-56.

Determining the conductivity parameters when fabricating semiconductors, or doping materials to create piezoresistive devices is critical.

For the remainder of the course, the ‘conductivity’ or ‘resistivity’ valves for semiconductors will be provided.


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Techniques for Doping of Silicon

Doped silicon can be created in one of two methods:

Using the ‘crystal growth process’, the doping element is introduced and will be distributed uniformly throughout the bulk of the solid.

Using a photolithographic method, selected areas can be doped using a diffusion process or ion implantation.


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Diffusion Doping

Diffusion doping is done using a deposition and baking process.

(a) A temporary mask is patterned onto the silicon.(b) A layer containing a high concentration of the desired dopant element is deposited onto the material (for example, PSG).(c) The chip is then baked at an elevated temperature, which promotes the diffusion of the dopant atoms into the exposed silicon surfaces.(d) The dopant layer is removed by chemical etching.(e) Finally, the mask layer is removed by chemical etching.


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Ion Implantation

Ion implantation involves the ‘insertion’ of ions of one material, into another.

(a) The implant material is ‘energized’ to create charged ions (single atoms).(b) The ions are accelerated using an electric field toward the target.(c) (Optional) a ‘separation’ magnetic field can be applied to remove impurities.(d) (Optional) a deceleration electric field may be used to control the implant energy.(e) This mechanical implantation creates damage to the crystal structure, therefore, a thermal annealing process follows implantation to ‘heal’ some of these defects.


Ion Implantation Process [image from Wikipedia]

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Sheet Resistivity

A concept used to determine the resistance of the doped paths in the silicon. The resistance of the tracks can be determined based on the track geometry and the resistivity.

Consider the following diagram:

Sheet Resistivity units:

Using Ohm’s Law:

Voltage is defined as:

Current is defined as:


t

wL

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Sheet Resistivity

Therefore:

Recall, resisitivity is defined as:

Hence:

Therefore:

However, we can redefine this formula in terms of a new quantity known as ‘sheet resistivity’ by rewriting the above equation as:

Therefore, the ‘sheet resistivity’ is defined as:


t

wL

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Sheet Resistivity

Example of Sheet Resistivity:


SEE CLASS NOTES FOR SOLUTION

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Lecture 3: Basic Concepts: Semiconductors - UVicmech466/MECH466-Lecture-3.pdf · Lecture 3: Basic Concepts: Semiconductors ... Note that in a cubic lattice, ... We can view the actual