Jackson M.J. Micro and Nanomanufacturing

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382 Micro- and Nanomanufacturing

7.13 Problems

Describe the chemical vapor deposition process for produc-

ing micro-cutting tools.

What are the properties of diamond?

Describe the synthesis of diamond.

Characterize the growth of CVD diamond technology.

Describe plasma-enhanced CVDD deposition.

6. Describe RF plasma-enhanced CVDD deposition.

Describe DC enhanced CVD.

8. Describe microwave plasma enhanced CVD.

9. What is hot filament CVD?

10.

How can hot filament CVD be used to coat micro-cutting

tools?

11.

How are substrates selected and pre-treated?

12.

Describe filaments are modified to produce micro-cutting

tools.

13.

Explain how diamond is nucleated and grown.

14.

How is diamond deposited to molybdenum?

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8. Laser Micro- and Nanofabrication

8.1 Introduction

Laser micro-and nanofabrication is based on the interaction of

light with solid matter. As a result of the complex interaction be-

tween light and matter, small amounts of material can be removed

from the surface. Two different phenomena are operating, namely:

pyrolithic processing and photolithic processing. Pyrolithic process-

ing is composed of heating, melting, and ablating the material at the

surface, and photolithic processing is based on the direct breaking of

chemical bonds in a wide range of materials.

A laser is a very intense beam of light that removes material by

breaking atomic bonds. Light can be described as packets of energy,

or photons, that has a wavelength and a frequency. Monochromatic

light is special because it is composed of only one wavelength that

has the same phase, and is coherent compared to incandescent light

that is composed of different wavelengths. Optical energy is trans-

ferred to the electrons by absorption, which essentially increases the

energy of electrons by increasing the vibration of the electron that is

sensed as heat. This chapter describes the basic principle of using

lasers to fabricate features at the micro-and nanoscale.

8.2 Laser Fundamentals

8.2.1 Creation of Monochromatic Light

The laser cavity contains an active medium that is responsible for

the amphfication of light energy by the process of stimulated emis-

388 Micro-and Nanomanufacturing

sion due to the active medium. In fact the word LASER originates

from this process, Light AmpHfication by the Stimulated Emission

of Radiation, and the laser type is often categorized by the medium,

such as CO2 or He:Ne. The cavity operates as an optical oscillator.

Energy is supplied to the medium, which must be activated to begin

the process of creating a laser beam. Triggering or pumping can be

produced by an AC/DC or RF power supply for CO2 or other gas la-

sers,

or by pulses of light created by Nd-YAG laser, or by a chemi-

cal reaction, e.g., iodine laser. This triggers the release of photons

that are trapped in the cavity by a mirror at each end. One of the mir-

rors is only partially reflecting and allows light to escape to form the

beam; the totally reflecting mirror is usually curved to reduce

dif-

fraction losses (Fig. 8.1).

Pump

(DC/AC or

power supply)

r + - - - -

laser beam

totally

reflecting mirror

partially

reflecting mirror

Fig. 8.1. Schematic diagram of the cavity of the laser [1]

For an example case, consider the stimulated emission in carbon

dioxide lasers. The carbon dioxide molecule is naturally stable,

however, it can exist at other discrete energy levels (the molecule

can exist only at specific energy levels; no transition states are al-

lowed). The allowed energy levels are quantized. Increasing the vi-

bration of the molecule due to the high power electric field increases

the energy levels; these vibrations can be either asymmetric oscilla-

Laser Micro- and Nanofabrication 389

tions or symmetric oscillations. The energy of CO2 molecules is

raised from the ground state to (001) by N2 molecules that have a

similar high state energy level to CO2. The molecule has excess en-

ergy compared to its ground state; therefore it drops an energy level

from (001) to (100). This energy is lost in the form of a photon.

There are further energy states the molecules must pass through

(010) to get back to its preferred ground state. A high voltage is ap-

plied to the CO2 gas. The voltage is high enough to cause some of

the N2 molecules to be raised to higher energy levels, which in turn

raises the energy levels of the CO2 molecules. High-energy mole-

cules often lose energy through collisions with the sidewalls. How-

ever, energy can also be lost by spontaneous emission when the en-

ergy level drops from (001) to (100) and a photon is emitted. In this

case the direction of the photon is random. If enough photons are

produced one will have a direction that coincides with the optical

axis of the cavity and will oscillate due to the reflections at each mir-

ror. If a photon collides with a CO2 molecule in the upper (001) en-

ergy level it causes it to lose energy and drop to the (100) level, in

doing so it must emit a photon. The emitted photon will be of the

same wavelength and direction as the incident photon and it will also

be in phase. So each time a photon interacts with an upper energy

level molecule it produces an identical photon. The mirrors at each

end of the cavity have the effect of increasing the path length. This

amplifies the effect of gaining more photons as the photon travels

along many oscillations before exiting the cavity. Exiting is

achieved by passing through one of the mirrors, which are partially

reflecting. The reason for the addition of nitrogen in CO2 lasers is

that nitrogen has only one excited energy level and this upper energy

level is very similar to that of

CO2.

Nitrogen is used to increase the

CO2 to its highest energy level. This leads to high efficiency but CO2

must be kept cool for this effect to happen. To help reduce diffrac-

tion losses and aid in alignment, the mirrors are usually curved; in

fact the length of the cavity and the geometry of its mirrors are im-

portant factors and define many of the laser's characteristics such as

its transverse electromagnetic mode (TEM) mode, or shape of the

beam. Cavities are described as stable or unstable depending on

whether or not the beam converges or spreads out from the cavity

(Fig. 8.2).

390 Micro- and Nanomanufacturing

8.2.2 Stimulated Emission

Atoms can exist only at defined energy levels; there are no transi-

tion states. The pumping mechanism raises the energy level of an

atom, and after a short time the atom tries to return to its original

state of energy. To do this it ejects the energy in the form of a pho-

ton. It is the combined effect of producing many photons that pro-

duces laser light. This rise and decay of energy states to produce a

photon can take place in a number of mediums, which emit different

wavelength photons resulting in different laser types for different

applications.

Pump

(DC/AC or RF power supply)

totaUy

reflecting mirror

laser beam

partially

reflecting mirror

^jvindow

totaUy

reflecting mirror

active medium

totally

reflecting mirror

totaUy

reflecting mirror

gas flow

aerodynamic window

totally

reflecting mirror

Fig. 8.2. Effect of cavity design on beam shape [1]

Laser Micro- and Nanofabrication 391

If the temperature of the cavity is too high the lower energy level

cannot be transferred to its ground state fast enough, thus ceasing the

production of laser light. Hence the maximum temperature that can

be sustained by the system defines the maximum power of the laser.

Carbon dioxide lasers can employ different cooling strategies. Slow

flow lasers achieve cooling through the cavity walls. A uniform gain

is achieved across and along the cavity giving it a good mode and

making it particularly well suited for laser cutting. Fast axial flow

lasers achieve cooling by convection of the gas through the dis-

charge zone. The gas enters cold and leaves hot, usually at a speed

of around 300 - 500 m/s. The configuration of the system produces a

symmetric power distribution of the beam. The cavity length is such

that it produces a low Fresnel number and therefore the beam has a

low order and can easily be focused to a small point. The gain can be

up to 500 W/m. The power these lasers produce is proportional to

the cross-sectional area; therefore producing short lasers would seem

like a good way to keep the design compact. However such designs

produce high mode numbers making the beam difficult to focus eas-

ily. There are other types of laser that work on the same principle of

gaining photons from the energy decay of various particles.

A solid crystal made up of yttrium, aluminum, and garnet with

the addition of neodymium ions makes up the Nd:YAG laser. It is

therefore known as a solid-state laser. It works on a similar principle

to the CO2 laser; high energy levels are achieved by the neodymium

ions and their subsequent loss of energy when returning to the

ground state releases a photon. Energy is supplied by a flashlamp,

and the photons are emitted when the energy level drops from a high

state to a low state. The decay time for each energy level is very

short. After the 1.06 |Lim laser radiation has been emitted a terminal

state is reached. In order to reach the ground state further cooling is

required. Because the pumping efficiency is low, a great deal of en-

ergy has to be put into the system that requires cooling to prevent

thermal distortions of the beam. A krypton lamp powers the Nd^^

ions in the YAG rod. The Q-switch is a way of controlling the beam,

it can be a mechanical chopper, a bleachable dye, an optoelectrical

shutter, or an acousto-optic switch (a piezoelectric material changes

its density when an electric current is applied, which in turn changes