Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology

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Unﬁltered and Filtered Cathodic Arc Deposition 531

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CHAPTER 11

Vacuum Polymer Deposition

Mark E. Gross and Peter M. Martin

11.1 Introduction 532

11.2 Polymer Deposition Process 534

11.2.1 Vacuum Polymer Deposition 534

11.2.2 Monomer Degassing 536

11.2.3 Atomization 537

11.2.4 Cryocondensation Efﬁciency 537

11.2.5 Monomer Cure 538

11.2.6 Adhesion: Plasma Treatment 538

11.2.7 Polymer Thickness Dependence 540

11.3 Plasma Vacuum Polymer Deposition and Liquid Multilayer Processes 540

11.3.1 Plasma Polymer Multilayer Process 540

11.3.2 Extrusion Process Liquid Multilayer Process 543

11.4 Surface Morphology 544

11.5 Multilayer Coatings 546

11.5.1 Ultrabarrier Coating 548

11.5.2 Nanolaminate Coatings 551

11.5.3 Advanced Applications 551

11.1 Introduction

Because of their unique physical, optical, and mechanical properties, thin ﬁlm polymers have

numerous technological and consumer product applications. They offer several advantages

over inorganic thin ﬁlm materials, including low cost, high deposition rates, low optical

absorption, high smoothness, wide variety of compositions, formation of composites, high

breakdown voltage, and high ductility. Disadvantages, however, are low mechanical strength

and wear resistance, low chemical resistance in some cases, high gas permeability, low melting

temperature, small range of refractive indices, and numerous optical absorption bands between

near-infrared and infrared wavelengths. Thus, applications are restricted by many of the above

properties, but are also expanded in many areas. Recent applications include

ultralow-permeability gas and water barrier coatings for organic electronics, thin ﬁlm solar

cells, thin ﬁlm batteries, multilayer optical coatings, thin ﬁlm capacitors, nanocomposites and

nanolaminates on plastic. Among the numerous methods used to deposit organic materials and

532

Vacuum Polymer Deposition 533

polymers, vacuum polymer deposition (VPD), also known as the polymer multilayer (PML)

process, produces ultrasmooth, nonconformal, and pinhole-free ﬁlms at very high deposition

rates over large areas.

Unlike any other vacuum deposition process, VPD ﬁlms actually smooth the surface of the

substrate [1]. All other vacuum deposition techniques essentially bombard the substrate with

species from a source that can bond immediately to the surface (low energy), bond after

moving some distance on the surface, or reﬂect off the surface. These traditional processes

‘grow’ the coating atom by atom or molecule by molecule from the substrate surface outward,

and produce ﬁlms that tend to reproduce the substrate surface on the length scale of the

bombarding species. As a result, substrate surface roughness, from atomic scale upward, is

replicated by the growing ﬁlm. Furthermore, this adatom growth can increase surface

roughness through mechanisms such as shadowing, dislocations, and grain boundary growth

(see Chapters 2, 4, 5, and 12). These growth mechanisms usually increase the roughness as the

ﬁlm grows thicker. A VPD layer, in contrast, does not grow atom by atom upward from the

substrate; a gas of monomer vapor condenses on the substrate as a full-thickness liquid ﬁlm

that covers the entire substrate surface and its features. The liquid ﬁlm is then cross-linked into

a solid layer by ultraviolet (UV) or electron beam (e-beam) radiation. The resulting surface is

glassy with virtually no defects or pinholes. The VPD layer can be combined with

conventional physical or chemical vapor deposition layers (PVD, CVD, PECVD, etc.) to form

low-defect, ultrasmooth thin ﬁlm structures.

VPD technology permits ultrafast deposition of polymer ﬁlms in the same vacuum

environment as conventional physical vapor deposition (sputtered or evaporated) thin ﬁlms.

With this technology, polymer ﬁlms can be deposited on moving substrates at speeds up to

1000 feet/min (∼ 300 m/min) and thicknesses ranging from a few angstroms to 1.3 mm with

excellent adhesion to substrates and thickness uniformity of ± 2%. The VPD process has two

forms, evaporative and non-evaporative. Each begins by degassing the working monomer,

which is a reactive organic liquid. In the evaporative process, the monomer is metered through

an ultrasonic atomizer into a hot tube where it ﬂash evaporates and exits through a nozzle as a

monomer gas. The monomer gas then condenses on the substrate as a liquid ﬁlm that is

subsequently cross-linked to form a solid polymer by exposure to UV radiation or an electron

beam. In the non-evaporative process the degassed liquid monomer is extruded through a

slotted die oriﬁce onto the substrate. It is then cross-linked in the same fashion as in the

evaporative process. Salts, graphite, or oxide powders, and other non-volatile materials can be

deposited in a homogeneous mixture with the monomer. Such mixtures cannot be ﬂash

evaporated, but are required for electrolyte, anode, cathode, and capacitor ﬁlm layers. The

evaporative process can produce thicknesses up to approximately 10 ␮m at speeds as great as

1000 feet/min (∼ 300 m/min). The non-evaporative process can deposit thicknesses from

10 ␮m to about 1.3 mm at substrate speeds approaching several hundred feet per minute.

534 Chapter 11

The VPD process has several distinct advantages over conventional organic and

organic/inorganic deposition processes:



Polymer thin ﬁlms are more cost-effective since polymer layers can be deposited

10–100 times more quickly.



VPD technology permits polymer, polymer electrolyte, metal, and even oxide and

nitride ﬁlms to be deposited in a single pass through a vacuum coater, as a monolithic

structure, in an integrated manufacturing process.



The VPD process eliminates all of the unnecessary handling and laminations involved

in conventional processes, and it also results in better adhesion between layers and

defect-free interfaces.



It is amenable to virtually all types of deposition processes, including roll-to-roll,

in-line, and batch planetary substrate rotation



Capital equipment and raw material costs are comparable to or less than those of

conventional processes.

Another advantage of conventional VPD is that it has been found to be quite efﬁcient at

smoothing rough substrates. Surfaces with a roughness less than 10

A RMS (Roughness

Measurement System) are routinely obtained with UV-cured PML/oxide/PML barrier ﬁlms

deposited on both 150

A RMS polyethylene terephthalate (PET) substrates and 5 ␮m RMS

metal plates. With e-beam cure the roughness on PET is 180

A RMS, higher than the initial

substrate surface roughness. The increased surface roughness with e-beam curing is due to the

electrostatic repulsion of the curing electrons trapped in the liquid setting up a standing wave

pattern prior to full solidiﬁcation.

11.2 Polymer Deposition Process

11.2.1 Vacuum Polymer Deposition

The VPD process involves vacuum ﬂash evaporation of a monomer liquid which is

subsequently condensed as a liquid ﬁlm onto a substrate and then radiation cross-linked to

form a solid ﬁlm. The VPD process can be implemented in either batch or roll-to-roll web

mode. Additionally, VPD can be combined with PVD processes to form novel hybrid

deposition processes. This process works best for monomers with evaporation points less than

∼ 500

◦

C. The process used to ﬂash evaporate the monomer is as follows.



Liquid monomer is ﬁrst degassed and stirred while atmospheric gasses are pumped

away to a suitable base pressure.

Vacuum Polymer Deposition 535

Figure 11.1: Schematic diagram of monomer evaporator [2].



The heart of this process is the monomer ﬂash evaporator shown schematically in

Figure 11.1 [2]. The degassed monomer is then sprayed into the interior of a very hot

enclosure that has a heated pressure bafﬂing expansion nozzle on one end (Figure

11.1). By ‘very hot’, it is meant that the enclosure walls are hot enough to instantly

vaporize the monomer spray, without a pool of liquid forming, yet not hot enough to

pyrolize and/or polymerize the monomer on contact (between 100

◦

C and 500

◦

depending on the monomer used). Under these conditions the monomer evaporates as

quickly as it is introduced into the enclosure. Thus, the mass ﬂow of monomer gas

exiting the nozzle is directly calculable from the monomer ﬂuid feed rate. Bafﬂes in

the enclosure/nozzle system serve to build up pressure (of the molecular gas of

monomer molecules) in the nozzle expansion chamber and to homogenize the

monomer gas. Building the pressure serves to make the molecular gas stream exiting

the slit in the nozzle more uniform across the length of the nozzle slit.



The ﬂow of monomer into the hot enclosure is controlled with a precision metering

pump. Liquid monomer is fed through a capillary tube into an atomizer that distributes

a mist of microdroplets into a heated enclosure. The monomer spray can be created by

536 Chapter 11

a variety of techniques, including an ultrasonic atomizer, fuel injector, or by dripping

onto a high-speed rotating disk. Dispersion with an ultrasonic atomizer gives the best

thickness uniformity



In a web coating process the monomer gas exits the evaporator through a narrow slit as

a uniform line source, condenses as a liquid ﬁlm on a cooled substrate that is moving

past the slit, and the liquid ﬁlm is subsequently polymerized when the substrate passes

a UV or e-beam radiation source. The substrate must be at a lower temperature than

the monomer gas for condensation to occur. Since the evaporator/slit can be

constructed to emit a uniformly dense line of gas then, as long as the total ﬂuid input is

constant, the thickness of the polymer coating on web will be uniform in thickness

along both its length and width. The monomer ﬂuid is atomized to a ﬁne mist and

sprayed over as large an area as possible on the interior of a very hot enclosure to

reduce cooling effects of the liquid on the hot interior surface of the evaporator. If the

rate of monomer mass impingement per unit area is too high, the surface cools too

rapidly and monomer liquid may build up on the interior surfaces of the evaporator. If

such liquid build-up occurs, and the monomer resides on the surface for too long, the

monomer will polymerize inside the evaporator. When the monomer polymerizes

inside the evaporator the resultant polymer ﬁlm interferes with heat transfer between

the surface and the subsequently atomized liquid. Gradually more and more

polymerization occurs, causing heat transfer to continuously degrade, and the rate of

monomer retention within the evaporator continuously increases over the duration of

the run. The VPD process has produced 1 ␮m thick ﬁlms at line speeds of 600 m/min.

The ﬁlm thickness range known to be obtainable with the VPD process extends from a

few tens of angstroms up to tens of micrometers, with the upper thickness limit

largely imposed by the penetration depth of the cross-linking radiation

employed.

11.2.2 Monomer Degassing

The degassing step is necessary to properly introduce the liquid monomer into the vacuum

environment, and is necessary to maintain the vacuum integrity of the chamber, although it is

always good practice to minimize the amount of the atmospheric gasses in order to minimize

contamination of concurrent processes. If the liquid monomer is not degassed prior to injection

into the vacuum, violent expansion occurs as the gas containing liquid enters the vacuum. The

violent expansion causes pulsing and spitting at the injection port, which leads to large, erratic

pulses in the ﬂow of liquid into the evaporator. This subsequently leads to variations of the ﬁlm

thickness along the length of the linearly translating substrate. Several experiments have been

performed to verify that no monomer components are removed during the vacuum degassing

process.

Vacuum Polymer Deposition 537

11.2.3 Atomization

A number of atomizers can be used with the VPD process. For some low ﬂow-rate VPD

methods the liquid monomer will be passed through an ultrasonic atomizing nozzle. The

amount of liquid monomer atomized by the nozzle is controlled by the liquid delivery system

used with a nozzle. There are several liquid delivery systems suitable for this application,

including gear pump, syringe pump, pressurized reservoir, peristaltic pump, and gravity feed.

The ﬂow rate range can be as low as a few microliters per second to up to several gallons per

hour. Ultrasonic atomization produces a low-velocity controlled spray pattern attaining a

narrow distribution of droplet size, normally in the micrometer range.

Factors to consider when selecting an ultrasonic atomizing system for each application are

listed below:



spray pattern desired – frequency of nozzle (25–120 kHz)



characteristics of the liquid monomer, viscosity



ﬂow rate desired – bore of nozzle



frequency generation – most companies that provide ultrasonic atomization equipment

use a broadband frequency generator enabling the use of several different nozzles with

one supply.

Ensuring an uninterrupted ﬂow of liquid monomer and tuning the correct power setting for the

ultrasonic nozzle to produce a uniform spray are critical to achieving a uniform polymer

deposition. A monomer ﬂow that is erratic and does not keep the nozzle bore full will result in

pulsed deposition and resulting non-uniform polymer ﬁlms, and the deposition will pulse.

Tuning the power for the ultrasonic nozzle has an effect on the spray pattern and monomer

droplet size. For process development purposes, it is important to be able to see the

atomization process while adjusting the nozzle power, typically from 2 to 10 W. A power

setting too low will result in monomer dripping off the nozzle, a power setting too high may

produce the incorrect spray pattern and damage heat-sensitive monomers.

11.2.4 Cr yocondensation Efﬁciency

The extremely high polymer deposition rate is not due solely to the rate at which the monomer

is metered into the VPD ﬂash evaporator, but also depends on condensation efﬁciency,

substrate temperature, evaporation temperature, and other process conditions, as described

below. High molecular weight monomers have dew points well above ambient temperatures so

that monomer gas readily cryocondenses at ambient and higher temperatures. Thus, when the

stream of monomer gas exiting the ﬂash evaporator strikes the cooled substrate, a very high

percentage of the gas immediately condenses as a liquid ﬁlm. Condensation efﬁciencies as

538 Chapter 11

high as 95% have been demonstrated for hexane diol diacrylate monomer (or HDODA),

MW = 214, evaporated at 230

◦

C and condensing on a substrate held at 5

◦

C. However, when

the substrate temperature is raised to 25

◦

C, condensation efﬁciency for HDODA drops below

60%. When the substrate temperature is raised to 66

◦

C virtually no HDODA condenses.

However, tripropyleneglycol (400) diacrylate monomer, MW = 508, condenses on a 66

◦

substrate at 36% efﬁciency when evaporated at 290

◦

C. Therefore, the VPD rate is a very

strong function of both the evaporation and condensation temperatures, monomer species, line

speed, and monomer feed rate.

11.2.5 Monomer Cure

Once condensed onto the substrate, the liquid monomer must be cross-linked to form a

polymer. Liquid monomer is cured, or cross-linked, using either high-intensity UV light

radiation or an electron beam. Because temperature-sensitive substrate materials are generally

used, and because it is too slow, thermal curing is not used in this process. However, as ﬁlm

thicknesses exceed 2–3 ␮m, only UV cure must be employed owing to the low energy

distribution of the VPD e-beam source. The e-beam source is typically a diode sputter cathode

enclosed in a grounded enclosure that has a slit in the wall facing the sputter glow discharge,

and operated at about 15 kV. Low-energy electrons emitted from the plasma impinge upon the

open slit, exit the box and initiate polymerization of the liquid monomer ﬁlm. These electrons

have energies ∼ 15 keV and below, and cannot penetrate more than 2–3 ␮m into the liquid

monomer.

UV wavelengths most suitable for photoinitiators used to cure acrylate monomers range

between 200 and 400 nm. Several types of lamp are suitable for generating these wavelengths,

including high-pressure mercury arc, and electrodeless and medium-pressure mercury arc

lamps. A typical UV source is a fusion variable power supply lamp system. This system can

vary 600 W/in (240 W/cm) power from 25% to 100%, and is designed to generate intense UV

radiation in the 200–400 nm range. The wavelength can be tuned to optimize absorption by the

photoinitiator used by selecting the correct bulb; type V 400–500 nm, type D 350–400 nm,

type H 240–320 nm, and type H+ 210–320 nm. Output power can also be tuned for the cure

desired. High-intensity UV light initiates a chemical reaction that cures the ink or the coating

almost instantaneously.

11.2.6 Adhesion: Plasma Treatment

Polymer/metal or polymer/oxide multilayer stack structures have a wide range of optical,

packaging, semiconductor, and display applications. A primary concern in the deposition of

such stacks is adhesion at polymer–substrate, polymer–metal and metal–polymer interfaces.

This is particularly critical for noble metals such as silver (Ag) because of the low chemical

Vacuum Polymer Deposition 539

reactivity and absence of physical interlocking effects due to smooth substrate surfaces created

by VPD coatings. The substrate surface must be sufﬁciently activated to promote an

interaction with the depositing metal atoms. By removing surface contaminants, incorporating

new and more reactive functional groups, and cross-linking weak subsurface layers,

low-temperature plasma has proven to be very efﬁcient for such applications. In addition, the

plasma process is fully compatible with all other vacuum technologies, allowing in-line and

high-speed processing of polymer/metal multilayer structures. The interaction of a plasma

with the substrate surface results from the synergetic actions of energetic electrons, ions,

atoms, molecules, and UV photons. Depending on the gas used, the interaction of plasma

species with a polymer surface can lead to etching, surface activation, and deposition of thin

ﬁlms. Surface activation is generally accomplished with O

, or Ar gases, and is strongly

dependent on the chemical structure of the substrate surface. Table 11.1 summarizes adhesion

studies using these gases and e-beam bombardment. In situ X-ray photoelectron spectroscopy

(XPS) measurements indicate that plasma exposure causes destruction of initial C

O and

C O groups in the ﬁlm and substrate. Other functional groups such as amide (NH C O)

structures are also created, but at a much slower rate [3]. As a result, activation treatment

conditions differ and must be optimized for each substrate surface. Plasma treatment can also

inﬂuence the surface roughness of the substrate material, also shown in Table 11.1 [3]. This

topic is addressed in Section 11.4.

Table 11.1: Summary of plasma adhesion enhancement [3]

Sample Gas/EB Treatment O/C

RMS surface roughness

(J/cm

) (nm)

PET 0.67 13.2

PET Ar 8 0.85 7.9

PET Ar 34 2.04 5.2

PET EB 10 0.67 6.8

PET EB 40 2.05 4.7

PET N

80.40 2.5

PET N

34 1.81 9.6

PET O

80.93 10.6

PET O

34 1 20.2

VPD 0.43 0.1

VPD Ar 34 0.41 0.1

VPD EB 34 0.39 0.1

VPD O

34 0.69 0.1

VPD O

34 0.42 0.2

Ratio determined by X-ray photoelectron spectroscopy. High O/C indicates increased plasma damage compared to native PET

(C/O = 1.5).

540 Chapter 11

Adhesion of metal layers to VPD thin ﬁlms can be enhanced by plasma pretreatment of the

cross-linked polymer, and depends on the chemistry of the plasma [3]. For example, adhesion

of silver to a VPD acrylate ﬁlm was excellent when it was pretreated with a nitrogen (N

)

plasma, while poor when exposed to an argon (Ar) plasma. Similar ﬁndings were reported for

several other polymeric systems [4]. Thus, both the incorporated nitrogen and plasma

enhanced surface energy due to plasma bombardment contribute to the improved Ag/polymer

adhesion.

11.2.7 Polymer Thickness Dependence

Polymer thickness depends on two factors: substrate temperature and monomer pressure at the

substrate surface. Increasing the substrate temperature decreases the deposited thickness,

while increasing monomer pressure at the substrate surface increases the deposited thickness.

Thickness is directly proportional to cryocondensation efﬁciency, as presented earlier, and

decreases rapidly with increased substrate temperature. An increased number of monomer

molecules directly results in increased monomer pressure and higher deposition rate.

During the VPD process, substrate temperature can be monitored by several methods, the most

widely used of which is an infrared sensor located in the vacuum system. Film thickness is a

strong function of substrate temperature, particularly at lower temperatures. The components

of the monomer blend used can also shift this thickness versus temperature plot.

Monomer pressure is normally measured in the evaporation chamber with a heated

capacitance manometer. Pressure in the evaporation source is directly proportional to the ﬂow

of monomer into the source.

11.3 Plasma Vacuum Polymer Deposition

and Liquid Multilayer Processes

11.3.1 Plasma Polymer Multilayer Process

The plasma polymer multilayer process (PPML) is a hybrid deposition process that combines

key features of three existing vacuum deposition methods: the VPD process, plasma

polymerization and plasma-enhanced chemical vapor deposition (PECVD). PPML is a high

deposition rate process for solid ﬁlms used for high molecular weight/low vapor pressure

liquid, or even liquid/solid suspension, monomer precursors. The gas resulting from ﬂash

evaporation of a liquid monomer mixture, or from a suspension of insoluble solid particles in

liquid monomer, is used as the support medium for a glow discharge in a PECVD-like process.

Owing to the high molecular weight/low vapor pressure nature of the precursors, the plasma of

the ﬂash-evaporated gas cryocondenses at an extremely high rate on substrates at ambient and

lower substrate temperatures. Upon condensation, the liqueﬁed plasma immediately begins to