Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

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776 Chapter

6.1:

Roll-to-RoIl Vacuum Coating

with the additional consideration that little outgassing occurs from the roll of web

until it is unwound during coating. In addition, small shards of polymeric mate-

rial generated by previous operations, especially slitting operations and cinching

of roll on windup, may contaminate system and pumps, unless substrate cleaning

procedures have been used to remove dust and particles prior to coating.

The pumps commonly used in roll-to-roll systems are listed here, and are se-

lected to meet the specific needs of these systems:

Backing, roughing

Mechanical Compressor Pumps

Rotary piston

Rotary vane

Axial screw

Medium vacuum

Roots blowers (Chapter 2.3)

Vapor booster pumps

High vacuum

Diffusion pumps (Chapter 2.4)

Turbomolccular pumps (often with molecular drag sections) (Chapter 2.6)

Capture "pumps" or traps

Cryogenic (helium refrigerator) (Chapter 2.5)

Meissner traps (liquid nitrogen chilled)

Refrigerated traps (CFC refrigerator)

In addition, reaction between residual gas and reactive metal vapors such as alu-

minum or titanium effectively pumps residual gas. This pumping or "gettering"

happens as a by-product of the deposition process, often affecting both product

and process quite dramatically. Commercially manufactured getter and sputter

pumps are not used in roll-to-roll applications.

Diffusion pumps have been the high-vacuum pumps of choice for roll-to-roll

coating. Originally, this choice was due to availability; later, low cost and relia-

bility became primary reasons. Reliability of diffusion pumps is somewhat over-

rated, as they need more maintenance, cleaning, and care than they generally re-

ceive. They also need water-cooled baffles to minimize backstreaming; further,

they do not provide stable pumping performance if operated at gas loads near

their rated throughput.

Cryotraps in various forms are very useful pumps for roll-to-roll processes.

Cryopumps using helium refrigeration are best used alone or with turbo pumps;

cryopumps are generally not used in combination with diffusion pumps. Meissner

6.1.4 Vacuum Systems for Roll-to-Roll Coating Applications 777

traps and CFC refrigerated traps are used especially for water trapping, particu-

larly in conjunction with diffusion pumps.

Reliable pump performance is essential, even when pumps are assaulted by

high gas loads and condensable gas, especially water vapor. Oil-recirculating sys-

tems,

gas purges, and ballast valves help keep oil cool and dry in these systems,

extending both mechanical pump and oil life. Large pumps, with relatively slow

rotational speed, typically operate at lower temperature, wear

less,

and have longer

seal life than smaller units with similar capacity achieved by using high rotational

speed. Tolerance to water vapor and ability to pump water vapor are key specifi-

cations for mechanical pumps used in web coating operations, especially those

used during regeneration of traps or those which pump chambers in which rolls of

web are unwound.

Throughput or mass flow of gas is related to the power input and efficiency of a

pump. Throughput is often more important than pump speed for much roll-to-roll

coating, as pumps are often pushed to mass flow limits, especially by sputtering

processes. All these pumps are pressure and power limited. If driven beyond rated

capacity, mechanical pumps lose oil, may overheat, and seize; jets of diffusion

pumps stall, and chamber pressure rises sharply; capture traps may warm, ex-

pelling previously captured gas.

The supply roll of web should, in general, be isolated from coating chambers

by conductance-limiting slots or rolls. These design features help decrease the

gas from substrate outgassing that flows to coating processes. Isolation between

various sections of a vacuum chamber with various functions are provided by

conductance limits between sections, with separate pumps for each section [6].

This design approach reduces cross-contamination between sequential processes,

often operating at various gas compositions and pressures. Commonly, supply

roll chambers are pumped by large Roots blowers backed by small Roots blowers,

which in turn are backed by rotary piston pumps. This is

reliable package if

prop-

erly operated to minimize water contamination, corrosion, and oil degradation.

Pressures less than 20 torr are tolerated by vapor booster pumps. Roots blow-

ers are best selected with bypass valves permitting high input pressures. Roots

blowers without bypass valves may be started at pressures between 10 and 20 torr,

whereas Roots blowers with bypass valves may be operated at atmospheric pres-

sure and contribute to pumpdown performance at pressures significantly above

20 torr. Under certain conditions of pressure and throughput, Roots blowers oper-

ate with small compression ratios, limiting performance.

Systems are roughed to between 0.1 and 1 torr, depending on types of pumps

used, system volume, and valve-opening speeds. At lower roughing pressures, the

potential for backstreaming of mechanical pump oil must be reduced by using fil-

ters,

traps, and Roots blowers. To minimize backstreaming, diffusion pumps must

not be exposed to high pressures during crossover.

778 Chapter

6.1:

Roll-to-RoII

Vacuum

Coating

6.1.4.2 Pressure Gauging (Chapter 3.1)

Pressure gauges useful for roll-to-roll coating processes are tightly constrained by

the following considerations:

• Stability of zero on longer runs, especially as chamber temperature increases

• Ability to set zero accurately, even if residual background pressure is high, a

common situation

• Response to contaminating materials

• Reproducibility on gauge tube replacement

• Reliability of gauge tube and electronics

• Response to gas mixtures, and whether or not gas factor can be ascertained

THERMAL CONDUCTION [CONVECTION) GAUGES

The wide range and reliability of these gauges make them useful for roughing

control, backing pressure limits and operating interlocks, including safety inter-

locks on power supplies and interlocks controlling ion gauge tubes. Stability of

zero is quite adequate, and can be set using sealed evacuated tubes, useful when

gauge is used in locations where pressures do not get very low. Contamination of

gauge tubes with pump oil may be a difficulty, but coarse sintered metal filters in-

terposed between system and gauge tube alleviate this problem.

ION GAUGES

At the higher pressures,

10 ""^

torr, typical in roll-to-roll coating and especially

in sputtering systems, triode tubes (type 504) or Granville-Phillips Stabil-Ion^^

gauges are particularly useful. A standard Bayard-Alpert (BA) type of ionization

gauge is ineffective above pressures of 2 X

10 ""^

torr, though wide range BA gauge

tubes may be of some value. Some gauge controllers achieve linear performance

even with BA tubes at higher pressures, by using short excitation times. Elec-

trometer zero is automatically set in ion gauge systems even at high pressures,

and zero stability is excellent. Reliability of electronics for driving ionization

tubes is improving.

CAPACITANCE GAUGES

Zero stability is inadequate on older designs, but is becoming better on certain

new equipment. If the system does not achieve pressures that are low enough,

gauge zero can not be accurately set. Certainly, a valuable feature is the measure-

ment of pressures, which are independent of gas composition and gas properties.

6.1.5 Substrates [Webs]

779

6.1.5

SUBSTRATES (WEBS)

6.1.5.1

Web Materials

Polymers (see expanded discussion at end of list)

Polyester (Polyethylencterepthalate) (PET) is the workhorse of this industry

and is readily available in thicknesses from 1.5 to 175 /im, 0.00006-

0.007

inch.

Polyimide (Kapton^M) is used for flexible circuits. This material tolerates

very high temperature, so products made using it are solderable.

Poly amide (Nylon

T^)

Polymethylmethacrylate (Plexiglas^M, Lucite^M)

Polycarbonate (Lexan^^)

Polyethylenenapthalate

Polyethylene

Polypropylene

Polytetrafluoroethylene (Teflon ^^)

Polystyrene

Cellulose acetate

Polyvinylidene Chloride (Saran^^^

Metal foils

Paper products

Kraft and standard papers

Textiles

Woven and felts or nonwovens of natural fibers and synthetic fibers, including

glass fibers

In addition to the wide variety of polymers just listed, there are copolymers, poly-

mer blends, block copolymers, coextrusions, layered, and laminated materials.

Many processing variables determine internal structure and therefore properties

and performance of each polymer system.

As an example, polyester (PET) can have a range of thermal expansion, hygro-

scopic expansion, shrinkage on heating, modulus, and level of trimers. The value

of each of these properties depends on the amount of stretch during orientation,

on temperature and time of heat setting, all of which result in different amounts of

crystallinity. Stretch is a factor that is typically different in the longitudinal direc-

tion of the web (machine direction) from the cross-web direction (transverse di-

rection), creating properties that are anisotropic.

780 Chapter 6.1: Roll-to-Roll Vacuum Coating

Various limitations of polymer substrates are notable. Low elastic (Young's)

modulus is typical, especially in comparison with the modulus of glass and met-

als.

A modulus of 0.5 to 0.8 X 10^ psi is typical for PET, one tenth the value of

glass and metal. In addition, polymers degrade with heat, they hydrolyze and are

permeable to water vapor, often supplying acids or bases to interfaces between

polymer and thin films. Plasticizers in some polymers migrate; vinyl (polyvinyl

chloride) is a good example. Some of these plasticizers easily diffuse to the sur-

face of the web, causing two problems: (1) increased pressure in the vacuum

chamber during deposition as plasticizers evaporate and decompose, and (2) plas-

ticizers form greasy layers on the polymer surface, resulting in bonding difficul-

ties for vacuum deposited films.

All these limitations of polymers tend to speed corrosion and decrease adhe-

sion, resulting in possible product failure. Many polymers also exhibit birefrin-

gence, scattering and surface roughness, degrading optical performance, and com-

promising polarization.

6.1.5.2 Web Handling

The primary goal is to unroll the web to be coated, move it past deposition

sources, including, if necessary, such process steps as outgassing, plasma expo-

sure,

nucleation, and monitoring of properties; then roll it up again. Difficulties

may occur, with wrinkles and creases forming, rolls slipping sideways (telescop-

ing),

or wound too tightly, edges building up faster than the center of the roll,

webs bagging and sagging in the center of the web, scratches generated, and point

defects created.

Before addressing cures for these problems, consider a typical web drive sys-

tem. Such a system has at least three motors, each motor having four quadrant

control. Two of these motors provide web tension, one each for the wind and un-

wind rolls, the third motor drives a roll at user-selected speeds. Motors deliver

torque, which is adjusted by feedback to control web tension. Web tension is de-

termined using either direct measurement, or a system estimating roll diameters

and friction to calculate web tension from motor current. Mechanically, many

web drive problems can be addressed by using appropriate motors and drive sys-

tems that are independently adjustable along web path. Various methods are used

for isolating sections to permit separate wind and unwind tensions. These include

either a pull roll or "pacer" roll, with a "nip" roll sometimes used to hold web

against pacer roll to prevent web slippage. If web that hasn't been outgassed is

used against a high-temperature pacer roll, nip rolls are essential to avoid web

slipping caused by gas lubrication. To avoid edge buildup, windup rolls are often

moved axially back and forth a few millimeters. This motion is slow, hydrauli-

cally actuated, and referred to as "side lay."

6.1.5

Substrates [Webs]

781

Drive motors and tension motors are usually mounted outside the vacuum

chamber. Rotary feed-throughs, sealed with either 0-rings or ferrofluids, permit

transmitting mechanical power from motors into vacuum chambers. 0-ring seals

are often quite acceptable in roll-to-roll coating applications, as pressures are not

extremely low, and constant rotary motion quickly generates a differential pres-

sure drop across a multiplicity of 0-rings, resulting primarily in startup "tran-

sients,"

as differential pressures are established. Drive motors are coupled to pacer

rolls through high gear ratios and tension motors are coupled through low gear ra-

tios,

matching the mechanical impedance of tension motor to web roll. Flexible

couplings, for example, Lovejoy Uniflex^^ brand couplings, are essential on each

side of ferrofluidic feedthroughs to decrease radial loading on such feedthoughs.

For polymer webs, tension typically used is such that web stress is in the range

of 80-600 lb/in^, depending on web modulus. Thin polymer webs, 4-20

jmrn

and

thinner, require controlled acceleration and tendency drives to reduce wrinkling.

Tendency drives provide torque to idle rolls, through bearings, by rotating the

shafts on which idle rolls are mounted, requiring four bearings for a shaft and

roller set. For very thin or low modulus web materials, limited or controlled ac-

celeration rates and "tendency drives" are both useful to reduce the effects of

bearing friction and roll inertia. In comparison to polymeric materials, paper

webs need higher tensile stress, and metal foils may require much higher stress,

especially if contact needs to be maintained between web and a chill roll.

Alignment of rolls, or parallelism between the axis of successive rollers, often

described as "tramming," is critical to minimize wrinkling and creasing of webs.

In addition, web wrinkles are a function of web caliper (thickness), web width,

web tension, web modulus, wrap angles, velocity, span lengths, roller alignment

or tram as already mentioned, roller diameters, roller runout, and friction between

web and rollers.

Arrangement or layout of rollers within the vacuum system plays a significant

role in the sensitivity of

web

wrinkling to roller alignment. This arrangement gen-

erates the degree of wrap of web around each roller, and the free-standing span

between each pair of rollers, both of which determine the precision of required

roller alignment. Defining a span or length (L) of web between any pair of rollers,

and normalizing for a given web width (W), generates an L/Wiaiio. If

the

L/Vi^ra-

tio is increased, roller alignment need not be so precise, but increasing this ratio is

difficult within the confined space available inside typical vacuum

coaters.

Increas-

ing web caliper increases web stiffness, and has the same effect as increasing L/W

ratio,

requiring less precise alignment. The web material or coating may also

cling to rollers, demanding more precise alignment. A system may be aligned or

tranuned for web moving in one direction, and not be tranuned for web moving in

the reverse direction. This is due to the need for web to approach each roller per-

pendicular to the rotational axis of each roller. Wrinkles arise not only from roller

misalignment and rollers that are not precisely cylindrical, but also from poor

782 Chapter

6.1:

RolI-to-Roll Vacuum Coating

web quality, poor stock roll formation, and from nonuniform shrinkage and ex-

pansion of webs, both caused by web heating during film deposition.

A slight collapsing of any vacuum chamber occurs on pumpdown, even in the

stiffest of chambers; this is an elastic flexing of chamber walls. This flexure or

elastic distortion of vacuum chamber walls can easily destroy roller alignment if

not prevented by proper design. Web-handling systems inside a vacuum system are

often mounted on plates quite well isolated from walls of the vacuum system, and

flexible couplings are used on all mechanical feedthroughs. These roll-mounting

plates are best made of steel or hard aluminum, as stainless steel is not dimen-

sionally stable enough. A web drive system mechanically coupled too rigidly to a

chamber that is too flexible cannot be aligned or trammed simultaneously at both

atmospheric pressure and at vacuum, a very bothersome problem, and totally in-

excusable.

Adhesion of freshly deposited films to rollers handling the substrate, especially

if film is oxide free, can cause wrinkles, particularly with web-handling equip-

ment that is not well aligned or trammed. Therefore, the front surface of a freshly

coated film should not be contacted by rollers, if

possible.

This is particularly im-

portant with evaporated metals, as there may be inadequate film-to-substrate ad-

hesion. Lifting of small specks of vacuum-deposited metal from the coated web,

and transferring this metal to rollers is a problem occasionally observed. This

problem, termed "picking," is solved by (1) improving adhesion between metal

film and web through use of plasma priming and barrier layers, (2) rethreading

web in coater to avoid front surface contact, or (3) oxidizing the surface of the

metal film before it contacts a roller.

6.1.5.3 Web Temperature Control and Web Defects

WEB TEMPERATURE CONTROL

Control of web temperature during deposition is an important aspect of control-

ling product quality, and is typically achieved using chilled or heated rolls to

stabilize web temperature and provide web dimensional stability. Films may be

deposited onto webs that are either in free span or held against a temperature-

controlled roll. A free span is particularly useful if web cannot be held uniformly

against such a roll, as is often the case with metal foil webs. Infrared lamps and

ultraviolet lamps aid outgassing from polymer webs.

Heat transfer coefficient in vacuum is typically 140-280 W/m^K (25-50 Btu/

hr-°F-ft^) between web and chill roll for PET and polyimide. This is very sensi-

tive to the amount of outgassing from web, the pressure of trapped gas from this

outgassing, the surface finish of roll and polymer surface finish and polymer com-

pliance

[7-12].

6.1.6 Process Control

783

A convenient formula relating web length to roll diameters and web thickness,

using the same dimensional units throughout, is

WEB DEFECTS

Various defects are often observed in polymer webs and rolls of web. A major

problem of rolls is air that is entrapped into stock rolls prior to vacuum coating,

caused by inadequate web tension, high web speed and method of winding. This

entrapped air may cause telescoping because it expands in vacuum and lubricates

the roll, permitting the roll to slip sideways or telescope in a characteristic

stepped fashion. This air also affects deposition if too much of it flows into the

region of deposition. Web defects include hard bands, which represent thicker

material; baggy web, which is longer in some regions than others; wavy edge-

slitting; point defects, which exist in webs, and are also generated or enhanced

during coating by heat, dirt on rolls, slip pimples, and dirt on webs.

6.1.6

PROCESS CONTROL

6.1.6.1 Monitoring and Control During Deposition

To prevent scratching either the web or the deposited film, when measuring prop-

erties of thin films on moving webs, noncontact measurements are essential, es-

pecially in vacuum. On-line, noncontact monitoring of electrical conductance,

optical transmission and reflection, magnetic properties, and substrate tempera-

ture are readily accomplished, even in vacuum. Significant effort may be required,

however, to achieve these capabilities. Delcom Instruments, Inc. produces con-

ductance monitors that achieve requisite performance. Monitoring equipment

must have excellent gain and zero stability to be useful for roll-to-roll coating, as

zero cannot be reset in the middle of runs. By combining data from the preced-

ing techniques, along with off-line calibrations using XPS, ICP, ellipsometry and

other analytical methods, additional material properties may be measured on-

line,

in a noncontact fashion.

A generally useful concept that is not often applied is the elimination of some

unknowns shared by different measurements to determine additional parameters.

As an example, measurements of both electrical conductance and optical trans-

mission allow determination of volume resistivity and film thickness on-line

784 Chapter

6.1:

RolI-to-RoII Vacuum

Coating

during deposition; for example, in an Al/AlOx cermet used for microwave heat-

ing films. Again using measurement of both electrical conductance and optical

density of a copper film, the oxygen content can also be derived from these mea-

surements, on-line, during deposition. Too much oxygen in copper makes it cor-

rode more easily in some environments. As previously mentioned, both examples

require off-line calibration of properly designed experimental runs followed by

appropriate data reduction. Once appropriate data have been collected, optical

models and effective media approximations permit understanding of deposited

products.

6.1.6.2 Measurement of Thin Films after Deposition

Measurement of thin films on flexible plastic substrates presents a few problems

not encountered with flat, rigid glass substrates. Surface flatness and back surface

reflections can be overcome, permitting optical and ellipsometric measurements

to be made; electrical properties can be measured using a variety of standard and

nonstandard techniques.

Continuous coating processes provide a few advantages: One advantage, for

example, is that the first few feet of material may be discarded, to eliminate mate-

rial produced during "transients" occurring at startup of coating processes. Quite a

different situation exists for deposition onto semiconductors, where such tran-

sient conditions are too often easily incorporated at the beginning of the deposit.

Startup transients are caused by a variety of events, both in batch and continuous

coating processes. Some of these include target cleanup at the beginning of sput-

tering processes, 0-ring leakage through rotary feedthroughs upon starting rota-

tion, adjustment to specifications and source heatup.

Another advantage of continuous coating is that, for some parameters, moni-

toring properties is often easier on moving substrates than on fixed ones; the

moving substrate proceeds out of the deposition zone. At this point measurement

of optical, electrical and magnetic properties of the deposited film are more eas-

ily made, as are measurements of substrate temperatures, all using noncontact

methods. Good process monitoring and control allow successful production of

functional films that withstand all types of environmental attacks and assaults.

6.1.7

SPECIFIC PROBLEMS EXHIBITED BY COATINGS

Functional vacuum-deposited coatings must adhere to the substrate and not cor-

rode nor easily abrade. Consider some environmental causes and mechanisms that

delaminate and corrode films.

6.1.7 Specific Problems Exhibited by Coatings 785

Here is a list of environmental assaults to which functional vacuum-deposited

films may well be exposed during use or subsequent manufacturing steps. Equally,

it is also a list of

tests

to which films might be deliberately exposed. Some of these

environmental assaults include exposure to higher temperatures, acids, bases, wa-

ter, electrical fields and also mechanical wear and flexure. Specifically, tempera-

tures of 325°C are demanded of polyimide-based materials; acid attack must be

resisted by many conductors; bases, either KOH or bleach, must be tolerated; and

boiling water is often used as a quick test for water immersion. Electrical fields

and current flow may cause ion and vacancy migration, cause thin films to heat,

and encourage various electrochemical reactions.

These accelerated tests, or what might be called hypertests, can provide useful

information regarding the nature of mechanisms that may degrade adhesion. These

attacks on a polymer-metal interface often have an electrochemical component.

Although peel and adhesion tests are difficult to interpret and often provide lim-

ited and contradictory information, they become easier to interpret if performed

after exposing products to such hypertests, especially if the products fall apart.

Before considering approaches to improving adhesion, consider corrosion and

abrasion resistance.

6.1.7.1 Corrosion Resistance

The problem with corrosion of thin films is simply stated: A corrosion rate aver-

aging 25 /im/year

(0.001

inch/year) is generally acceptable for most materials ap-

plications as in chemical engineering, automobile

bodies,

and even

bridges.

At this

rate,

however, a film with a thickness of 100 nm has totally corroded in a day or

two.

Often, functional thin films must maintain properties for five or more years.

Here are some techniques to decrease corrosion rates:

• Reduce impurities in the primary deposit, especially oxides, by depositing

films at high rate, and at low background pressure, with partially closed

shutters.

• Reduce voids in deposited material both by nucleating to avoid initial col-

loidal deposits, and by minimizing oxygen content, as noted earlier, with

high deposition rates and low pressures.

• Select those alloys that may form adherent, dense, impermeable oxide films

on their surface. Stainless steel, InconeP^, and MoneF^ are examples.

• Add top coats: Deposit a very thin, dense, adherent and insoluble oxide or a

tightly bound organic film. Such an oxide film functions as do the oxides

formed on alloys noted previously, but is not self-generating, so may be ap-

plied to various materials that otherwise would not produce a self-Umiting

oxide. A typical example of an organic system is the use of 1% benzotriazole

in ViteFM PET to protect copper.