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754 Chapter 5.6: Common Analytical Methods for Surface and Thin Film

Fig.

17.

Typical

RBS

system using a high-energy accelerator and an end station equipped

with the standard RBS pardcle detector,

nuclear reacdon photon detector,

and a forward recoil detector (used for hydrogen detection).

Ion Source

Focus/Steering

N2 Stripper

U"^*

""LITI

He^Beam

20° Move for

Grazing

RBS

FRe6

NRA

Gamma Detector

The end station is as supplied by Charles A. Evans & Associates.

ground. The total energy of the alpha particles is either 2 or 3 Me V for an applied

1 MeV accelerating potential.

The alpha particles are steered using a bending magnet. This acts as a selector

to ensure a pure alpha particle beam. After beam alignment on the sample in the

end station, most instruments are equipped with a collimator to restrict the beam

size to

-2 mm diameter at the sample.

The end station consists of a moderate high-vacuum chamber (10 "^ torr).

Better end stations work in UHV. The end station is equipped with two particle

detectors and a y-ray detector. The particle detectors are used to obtain the RBS

and FReS or grazing-angle RBS data, while the y-ray detector is only used for

y-ray detection. For these analyses, the sample is positioned using a computer-

controlled goniometer stage. Some end stations are equipped with surface analyt-

ical apparatus such as AES to further increase the utility and flexibility of the

instrument.

5.6.4 UHV Generation

System Considerations for Surface Analysis 755

5,6.4

UHV GENERATION AND SYSTEM CONSIDERATIONS

FOR SURFACE ANALYSIS

In the previous sections, it is clear that surface analysis must be performed in

an ultra-high-vacuum ambient. Among the four techniques discussed, there is

considerable parallelism in the analytical chamber construction, vacuum require-

ments, etc.

Over the past 30 years, vacuum technology has undergone great changes. This

book is devoted to making us aware of some of these changes. Vacuum systems

used for commercial analysis of materials depend heavily on the production of

re-

liable ultra-high vacuum at a reasonable cost. As noted in Chapters 2.4 and 4.2 of

this book, most ultra-high-vacuum systems are produced using stainless steel con-

struction and copper metal seals in the form of, e.g., Conflat^^ seals. The metal-

to-metal seals and stainless steel construction can maintain vacuums in the 10~^^

ton* region. Many of the systems are either ion pumped or cryopumped, and in

some cases where gas loads are large titanium sublimation pumps and cold pan-

els are employed.

5.6.4.1

Sample Introduction System and Manipulation

To maintain a good ultra-high-vacuum ambient for surface analysis in an analysis

vacuum chamber, manufacturers equip their apparatus with rapid-turnaround air

lock systems. The detailed design varies, depending on the nature of the tech-

nique and the sample manipulation system in the analysis chamber. Most systems

generally consist of some type of sample manipulation and transfer device, a fast-

pumping system that will take an ancillary chamber from air pressure to near UHV

conditions and a small volume ancillary chamber to reduce the pumpdown time.

Stainless steel construction is used throughout both the air lock and the analy-

sis chamber. The ancillary chamber is typically metal sealed or may have a single

O-ring-sealed port for quick sample introduction. Modern systems are universally

equipped with a small-size turbomolecular pumping system, although some sys-

tems used in the academic areas still employ ion pumps and sorption pumps to

evacuate the air lock chamber. Ion pumps were found to be slow and in many

cases not able to handle the gas load from the samples. Turbomolecular pumps in

the 50 to 200 L/s range are typically employed, depending on the size of plumb-

ing used to rough-pump the system and where the pump is mechanically located.

The mechanical pumps used to back the turbomolecular pumps use an in-line oil

filter that should be changed on a periodic basis to keep the pump oil out of the

756 Chapter 5.6: Common Analytical Methods for Surface and Thin Film

turbomolecular pump chamber (see Chapter 2.3.3). The chambers are equipped

with a dry nitrogen purge system while at atmospheric pressure. This reduces the

amount of water vapor that may adsorb on the stainless steel chamber walls.

Pumpdown times depend on the time the chamber is exposed to air/dry nitro-

gen purge and the sample(s) being introduced into the analysis chamber. Fast

pumpdown times are encountered with materials that do not degas; for example,

metals and inorganic semiconductor and inorganic insulating materials. An intro-

duction system could be made to cycle from air to a good vacuum of

10 ~^

torr in

5 to 10 minutes. However, many polymeric materials tend to degas and hence re-

quire a long pumpdown before introduction into the analysis chamber. Most

degassing must be done in the introduction chamber to keep the main analysis

chamber clean. This is done with reproducible results in all the modern analytical

manufacturers where the base pressure is 10"^ torr or less.

Sample introduction is typically done using a horizontal introduction system.

The details of the systems vary depending on the type of sample manipulation be-

ing employed in the main analysis chamber. Some manufacturers introduce one

sample per pumpdown. The sample is docked on a stage in the high-vacuum sys-

tem. If more than one sample is to be introduced into the chamber, samples must

be handled sequentially. Other systems handle many samples for a single pump-

down. These systems are, again, based on a horizontal introduction system that is

computer controlled or hand operated.

In the analysis chamber, the sample manipulation system varies, again, with

manufacturer but in all cases consists of a minimum of x-y-z-0 manipulation.

Sample position can be computer controlled or manually set. For multisample

analysis, it is necessary to automate the sample manipulation. This is done using

computer-controlled motor drives on the manipulator where the positions of the

samples can be manually read into the system. In some more sophisticated sys-

tems,

sample position can be read into the computer through video selection. This

type of sample manipulation is usable where positioning need not be too accurate.

Repositioning a sample within a few /xm is well within the capability of

the

equip-

ment available.

When performing submicrometer analysis, such as in the case of small-spot

AES or small-spot SIMS, manual manipulation of the sample is preferred. It is

useful to be able to see the point of analysis through video imaging of the sample

(through secondary electron imaging). This is obtained in both AES and SIMS in-

strumentation (both TOF SIMS and dynamic SIMS), x-y-z-0 manipulators are

used to perform the necessary spatial maneuvers required to set up the sample for

analysis. Many goniometers are capable of performing an eucentric alignment.

With this alignment, the sample can be rotated without losing the analysis position.

Fixturing is available for either heating or cooling the sample during the analy-

sis process. In some AES and XPS instruments, cooling and heating are an inte-

gral part of the sample manipulator. In addition to routine measurements, such

References

757

stages can be useful in performing elegant analysis in situ. Other instruments are

equipped with ancillary chambers that provide sample preparation operations such

as cleaning. This equipment is all UHV compatible.

5.6.4.2

In-Line

Use of

Analytical Systems

The semiconductor industry has led the use of highly technical instrumentation

aimed at surface analysis. A large portion of the work that goes into making in-

tegrated circuits and semiconductor devices requires the preparation of clean,

highly oriented, submicrometer thin-film structures on very clean surfaces that

may themselves be thin-film structures. Cleanliness and chemical purity are criti-

cal.

In well-equipped fabrication sites, instrumentation is included in the clean

room facility to perform secondary electron microscopy and in some cases AES

and other structure-related methods. Incorporating analytical instrumentation into

the production of high-technology devices, materials, and structures is essential.

REFERENCES

K. Siegbahn et al.

ESCA,

Atomic, Molecular

and

Solid

State Structure

Studied by

Means

of Elec-

tron Spectroscopy (Almqvist and Wiksells, Uppsala, 1967).

Handbook of

X-ray

and Ultraviolet Photoelectron Spectroscopy, edited by D. Briggs (Heyden,

London, 1977).

Practical Surface Analysis, Vol. 1 edited by D. Briggs and M. P. Seah (Wiley, Chichester, En-

gland, 1990).

J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of

X-ray

Photoelectron

Spectroscopy (Perkin-Elmer, Eden Prairie, MN, 1992).

G. Beamson and D. Briggs, High Resolution XPS of Organic Polymers (Wiley, Chichester, En-

gland, 1992).

6. L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach, and R. E. Weber, Handbook of

Auger Electron Spectroscopy, 2nd ed. (Physical Electronics, Eden Prairie, MN 1976).

Handbook

of Auger

Electron

Spectroscopy (JEOL Ltd., Tokyo, 1982).

8. Auger Electron Spectroscopy, Michael Thompson, et al. (Wiley-Interscience, New York, 1985).

9. Auger Electron Spectroscopy, edited by C. L. Briant and R. P. Messmer (Academic Press,

Boston, 1988).

10.

For example, see G. Hertzberg, Atomic Spectra and Atomic Structure (Prentice Hall, Englewood

Cliffs,

NJ, 1937).

11.

F. J. Himpsel, F. R. McFeely, A. Taleb-Ibrahimi, and J. A.

Yarmoff,

in The Physics and Chem-

istry ofSiOi ond the Si-Si02 Interface, edited by C. R. Helms and B. E. Deal (Plenum Press,

New York, 1988).

12.

J. H. Scofield,

Electron Spectrosc. Rel.

Phenom.,

8 (1976) 129.

13.

G. A. Somorjai and H. H. Farrell, Advon. Chem. Phys., 20 (1970) 215.

14.

J. J. Lander, Progress in Solid State Chemistry, Vol. 2 (Macmillan, New York, 1965).

15.

G. A. Somorjai, Principles of Surface Chemistry (Prentice-Hall, Englewood Cliffs, NJ, 1972).

758 Chapter 5.6: Common Analytical Methods for Surface and Thin Film

16.

Introduction

Surface Chemistry

and Catalysis, Gabor A. Somorjai (Wiley, New York, 1994).

17.

Fundamentals

Surface

and

Thin

Film Analysis, Leonard C. Feldman and James W. Mayer

(North-Holland, New York, 1986).

18.

C. W. Magee, W. L. Harrington, and R. E. Honig,

Rev.

Sci.

Instrum.,

49 (1978) 477.

19.

J. A. McHugh, Secondary Ion Mass Spectrometry, in Methods of

Surface

Analysis, edited by

A. W. Czanderna (Elsevier, New York, 1975).

20.

A. Benninghoven, F. G. Rudenauer, and H. W. Werner, Secondary Ion Mass Spectrometry

(Wiley, New York, 1987).

21.

A. E. Morgan, Secondary Ion Mass Spectrometry, in Characterization of

Semiconductor

Mate-

rials, Vol. 1, edited by G. E. McGuire (Noyes, Park Ridge, NJ, 1989).

22.

Handbook of Modem Ion Beam Materials Analysis, edited by J. R. Tesmer, M. Nastasi, J. C.

Barbour, C. J. Maggiore, and

W. Mayer (MRS, Pittsburgh, 1995).

23.

C. W. Magee and E. M. Botnick,

Vac.

Sci.

Technol.,

19 (1981) 47.

24.

J. H. Thomas, III, and B. L. Bentz, Surface analysis in Electronic Materials

Handbook,

Vol.

Packaging (ASM International, Metals Park, OH, 1989).

25.

Backscattering Spectrometry, W-K. Chu, J. W. Mayer, and N.-A. Nicolet (Academic Press, San

Diego, 1978).

26.

W. D. Mackintosh, Rutherford scattering in Characterization of Solid

Surfaces,

edited by P. F.

Kane and G. B. Larrabee (Plenum Press, New York, 1974).

CHAPTER

6.1

RoII-to-RoII

Vacuum Coating

William

Robbins

Corporate Research Laboratories

Thin Film

Technology Resources

6.1.1

OVERVIEW OF ROLL-TO-ROLL VACUUM COATING

6.1.1.1

History

In 1917, sputtered thin-film resistors deposited on long glass rods were produced

on a commercial basis. In the early 1930s, capacitor materials were being pro-

duced, made of vacuum-metallized paper

tissues,

using such advanced techniques

as a cellulose nitrate smoothing lacquer and silver nucleation of a zinc coating.

This material was produced using a roll-to-roll semicontinuous vacuum coating

process; such vacuum coating is the primary focus of this chapter. The vacuum

process and equipment for this capacitor film were developed in Germany. Fig-

ure

is a photo of

this

early roll-to-roll coater.

In the late 1930s Penning observed that magnetically confined plasmas en-

hanced sputtering rates from cathodes. This is the beginning of magnetically as-

sisted sputtering. Also, large telescope mirrors, eventually 200 inches in diame-

ter, were aluminized by John Strong, and aluminized automotive headlamps were

in production.

From the mid-1930s

mid-1950s,

many vacuum deposition processes evolved

ISBN

0-12-325065-7

$25.00 All rights of reproduction in any form reserved.

761

762

Rg.l.

Chapter

6.1:

RolI-to-RoII Vacuum Coating

1930s roll-to-roll vacuum coater. Source: Courtesy of Collin H. Alexander, Private Commu-

nication.

into commercially viable technology in roll-to-roll systems. In the early 1950s,

Q-meters were first used to measure electrical conductance of deposited films in-

side a vacuum system, on-line, with no contact. Monitoring having noncontact

capability is important to vacuum coating technology, and sophisticated equip-

ment has been developed to monitor properties of deposited materials and process

parameters. By 1953, 3M Retroreflective Sign Material, a material used for traffic

control products, was in production using vacuum deposition as a key step.

6.1.1.2

Overview

With the advent of both plastic film and paper technology, large area, roll-to-roll

vacuum deposition processes became feasible. These strong, thin, flexible plastic

and paper substrates are typically referred to as "webs." Roll-to-roll vacuum coat-

ing systems range from very small laboratory coaters, having webs 4 inches wide,

to large units having a web width of 120 inches or 3 m. Typical commercial units

coat webs that are 40 to 60 inches (1 to 1.5 m) wide; wider webs may provide

6.1.1 Overview of Roll-to-Roll Vacuum Coating 763

low-cost coatings if equipment

optimally operated. The process is versatile, rela-

tively low cost, and nonpolluting.

In large modern roll-to-roll vacuum systems,

roll of web is loaded and threaded

automatically; the chamber is evacuated or pumped down, and deposition is initi-

ated. One roll may weigh 450-3500 kg and have an area of

thousand to 90 thou-

sand square meters. The material is coated during one evacuation cycle, in reality

a semicontinuous process but often termed continuous coating. Evacuation or

pumpdown time may be as short as 5 to 10 minutes, typically 15-30 minutes, be-

fore initiating deposition, even though pressure required for much vapor coating

is on the order of

"^ to 3 X 10 ^ torr. Even for sputtering processes, where

pressures are on the order of 3-6 mtorr (mt), background gases must, in general,

be less than a few percent of the pressure of sputtering gas, and even less for de-

position of materials particularly sensitive to reaction with background gas. Oc-

casionally, a truly continuous air-to-vacuum-to-air process is used, but this ap-

proach is rarely economical for roll-to-roll coating using flexible substrates, and

will not be considered in this chapter. Such an air-vacuum-air process is generally

used, and appropriately so, for coating glass.

Typical web speeds for the most commonly deposited material, evaporated alu-

minum, may be as high as 15 m/s (2950 ft/min). For many other materials and pro-

cesses, a speed of approximately 2 m/s (400 ft/min) is common. Films deposited

in various vacuum processes range in thickness from about

nm to about 600 nm;

thicknesses of 30 to 180 nm are most common. To promote adhesion, modify nu-

cleation and growth morphology of films, a layer is often deposited prior to the

primary thin-film deposit, a practice similar to that used in batch coating. Typi-

cally, such a layer has a thickness ranging from 0.1 to 10 nm. Coatings are gener-

ally limited to a few layers of films, both for economic reasons and because of

limitations imposed by properties of flexible substrates.

Although webs in roll form make these materials useful and convenient as sub-

strates for vacuum coating, they bring with them numerous problems intrinsic to

their properties. These substrate problems include outgassing, low modulus, tem-

perature limitations, and moisture permeability. Substrate properties also con-

tribute to poor adhesion and high corrosion rates of deposited films. These prob-

lems are significantly reduced when coating on inorganic materials, such as glass.

Two very common applications of this technology include production of pack-

aging materials and capacitor films. Vacuum coating systems using wide webs

have been optimized for producing packaging products, typically polyester coated

with aluminum. Wide coating systems also provide materials serving architec-

tural markets where wide web widths are often essential. Roll-to-roll vacuum sys-

tems for capacitor metallization have been optimized at narrower widths, 0.6 to

1 m width, as very thin substrate materials are used in this application. These

capacitor coating systems require a technology to create longitudinal stripes or

bands of uncoated substrate material, typically using oil masking or, in the past,

continuous metal bands running on the substrate as contact shadow masks.

764 Chapter 6.1: Roll-to-RolI Vacuum Coating

6.1.2

TYPICAL PRODUCTS

These are produced by a roll-to-roll vacuum coating in a "semicontinuous" pro-

cess.

Packaging products dwarf all other product applications combined.

Packaging products and vapor barriers

Snack (potato chip) bags

Permeation reduction films

Packaging for medical products

Transfer films

Decorative products

"Polyester" balloons

Wrapping papers and films

Electrical products

Capacitor films

Flex circuitry materials

Microinterconnect materials

Microwave-absorbing materials

Food heating

Stealth products

Transparent conductors

Specialty antennas

Optical products

Retroreflective signs

Mirrored films

Solar control products

Reflective window treatments

Neutral density window films

Spectrally selective window films

Reflective thermal insulation

Evacuated thermal insulation

Protective fabrics

Reflective fire suits

Glare reduction coatings

Color filters

Optical recording materials

Infrared absorbers

6.1.3 Materials and Deposition Processes for RolI-to-Roll Coating 765

Printing products

Offset printing plates

Security inks

Electro-optical products

Photoreceptors

Electrochromics

X-ray detectors

Hard coats, friction reduction materials

6.13

MATERIALS AND DEPOSITION PROCESSES COMMONLY

USED IN ROLL-TO-ROLL COATING

6.1.3.1

Process Concepts

Adhesion to substrate, corrosion resistance, and also reproducible optical and

electrical properties are typically demanded of thin-film coatings. These proper-

ties contribute to the concept of "functional" films. The following six basic pro-

cess steps help achieve functional films during vacuum coating:

Outgas web to avoid contamination downstream and, on occasion, to modify

polymer properties comprising the web.

Prime the web surface, changing chemistry and mechanical properties to im-

prove adhesion, for example, by exposing webs to plasmas of appropriate

composition.

Deposit a thin film to nucleate the subsequent primary deposit; this first thin

film, a nucleating film, helps control growth and morphology of primary film

and may also act as a barrier and adhesion promoter.

Deposit primary film, minimizing voids and impurities.

Deposit protective top coatings.

6. Monitor relevant properties on-line, in a noncontact fashion.

All these steps are generally performed sequentially within a vacuum coater. This

means that the web passes only once from unwind roll to windup roll, all within a

vacuum coater, making for

"one-pass"

process.

The steps are certainly optimized

to fit each product, and each step is not always necessary nor appropriate. For ma-

terials needing minimal functionality, some steps may often be rudimentary, or

even optional.