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

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CHAPTER 18

Jet Vapor Deposition

Paul Komarenko, Michael Drago, Michael Gorski, Takashi Tamagawa

and Bret Halpern

Jet Process Corporation, 57B Dodge Avenue, North Haven, CT 06473, USA

18.1 Introduction 881

18.2 Principles of JVD Sources and Systems 882

18.3 Relative Motion between Jet Sources and Substrates 885

18.4 Three JVD Jet Sources 890

18.4.1 Wirefeed to Hot Filament Source 890

18.4.2 Hollow Cathode Sputter Jet Source 891

18.4.3 Wick Jet Capillary Feed Source 892

18.5 Generation and Deposition of Atoms and Clusters 893

18.6 Application of JVD to Solder and Related Layers 894

18.7 Summary 897

18.1 Introduction

Jet Vapor Deposition

(JVD

) uses high-speed inert gas jets to carry ﬁlm components to

substrates. This transport takes place in a fast ﬂow system, mechanically pumped to pressures

in the range of 1 torr. Developed by Jet Process Corporation (JPC) and now used

commercially, JVD has been applied to a wide range of thin and thick ﬁlm applications,

demonstrating an adaptability to new problems, and a versatility for making complex materials

quickly and economically.

Our work in the past shows that JVD is particularly well suited to solder deposition for

microelectronic packaging, where there is a growing need to solder small electronic devices on

ever smaller circuit features at higher density. Deposition of the required solder bumps, often

of expensive alloys such as eutectic AuSn, taxes the capability of standard techniques to

produce correct properties in an economical way. JVD is thus fast becoming a method of

choice, and JPC has become an important supplier of solder layers and related adhesion,

diffusion barrier, and protective cap layers.

JVD’s signature innovation is a novel vapor source: a sonic jet in low vacuum which transports

condensable atoms, molecules, or clusters, to a substrate and deposits them efﬁciently in a

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small area. Larger areas are coated by arranging for relative two-dimensional (2D) motion

between substrate and one or more jet sources. The combination of jet sources and

mechanisms for jet–substrate relative motion characterizes JVD.

The use of the jet in low vacuum for deposition, ﬁrst investigated by Halpern [1] and later by

Schmitt and Halpern [2, 3], has opened new pathways to both microscopic property control and

manufacturing efﬁciency. JPC has developed several jet sources based on different principles,

which are now basic components of a multiple jet, moving substrate process [4] for depositing

metals, semiconductors, dielectrics, oxides, nitrides, and organics. These materials can be

grown by JVD in many forms: multicomponent, alloy, multilayer, host–guest, and nanocluster

ﬁlms. The growth rates are high, approaching in some cases micrometers per minute over areas

of several hundred cm

. Deposition is carried out without toxic precursors or efﬂuent, on

substrates that remain cool, near room temperature. By straightforward control of jet source

conditions, JVD can generate and deposit either atoms or nanoclusters, both at high rate.

Atoms and clusters behave differently in deposition, particularly in penetration of photoresist

mask windows needed for solder bump deposition. All these features of JVD are valuable for

deposition of solder and related layers. JVD is deceptively simple, but it has a fortunate

combination of useful characteristics, which will be discussed in the following sections:

Section 18.2: Principles of JVD sources and systems

Section 18.3: Relative motion between jet source and substrates

Section 18.4: Three JVD jet sources

Section 18.5: Generation and deposition of atoms and clusters

Section 18.6: Application of JVD to solder and related layers.

Apart from solder deposition, the previous edition of this Handbook described a variety of past

applications showing the range of JVD. These applications and related references are brieﬂy

summarized in the Appendix.

18.2 Principles of JVD Sources and Systems

Jet sources can have different designs, but all are mounted on ﬂanges aimed into a

low-pressure deposition chamber. A fast ﬂow of inert gas through each jet source is sustained

by a mechanical pump and Roots blower [5, 6] with pumping speeds typically in the range of

5000–10,000 liters/minute, but sometimes higher or lower. These pumps comprise a reliable,

inexpensive vacuum technology.

A typical jet source is a 3 inch (7.62 cm) diameter cylindrical nozzle, with an endplate having

an exit oriﬁce ∼ 1 cm in diameter. Helium or argon, and occasionally nitrogen, is supplied to

the nozzle and exits from it as a jet. Ordinarily the nozzle pressure P

is several torr, and the

Jet Vapor Deposition 883

Figure 18.1: Generalized jet source, showing nozzle, main jet, and wall jet. The nozzle exit

diameter is typically 1 cm. Metal atoms are vaporized near exit, deposit in localized area.

Trajectories before impact: atom, diffusive; cluster, straight.

downstream pressure P

is 1 torr or less, but the range may be wide: in different applications

both pressures can be higher or lower. When the ratio P

> 2, the ﬂow is critical, and the jet

emerges at the speed of sound; for He at 298 K, the exit velocity is ∼ 10

cm/s. There is

considerable latitude in the source shape, oriﬁce size, and operating parameters of a jet source.

Figure 18.1 shows a generalized jet source.

The jet can entrain any atomic, molecular, or cluster vapor, and convey it to a substrate for

condensation. A vapor source is placed upstream of the nozzle exit, a region in which the

carrier gas is accelerating toward the speed of sound. Depending on the material, vaporization

can be effected by several techniques, such as thermal evaporation, glow discharge sputtering,

and microwave plasma or direct current (DC) discharge reaction. Atoms of gold, for example,

injected upstream into the He ﬂow will be swiftly captured and transported downstream by the

jet. On a ﬂat substrate placed perpendicular to the jet, and several centimeters downstream, a

bright gold deposit will quickly appear; virtually all the gold atoms deposit in a circular zone

comparable to the nozzle exit area. The deposit is symmetric but non-uniform, being thicker

toward the center. The capture efﬁciency for metal atoms is generally high, and for clusters

even higher; Figure 18.1 indicates the trajectories of an atom and a cluster before impact on

the substrate. We will discuss later the designs of three sources based on different vapor

generation approaches.

The use of single or multiple jets in JVD synthesis follows from the structure of a single inert

gas jet. The jet contour can be made visible by atomic emission from a plasma ignited in the

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jet, or by chemiluminescence, say from active N

. The structure of free jets issuing from

small-diameter nozzles at high pressure has been amply described [7], but particularly useful

features arise under JVD conditions, where the nozzle exit diameters are relatively large,

∼ 1 cm, and the nozzle pressures are usually between 1 and 10 torr.

On exiting the nozzle, the jet expands into a zone of reduced density in which nearly all atoms

move in the same direction at the speed of sound. In JVD this zone is nearly collision free; it

terminates at the Mach disk, located a distance x ∼ 0.67 d

)

1/2

downstream of the nozzle

of diameter d

[7]. The pressure ratio usually lies in the range 2 < P

< 10, and the Mach

disk is located several nozzle diameters, or several centimeters, downstream. Beyond the Mach

disk the density rises sharply to the downstream background value, the jet speed drops to

∼ 3 × 10

cm/s, determined by the system pumping speed, and collisions again occur in the jet.

Further downstream the jet diverges little, under typical JVD conditions, and remains fairly

collimated over distances of tens of centimeters.

Jet collimation has important consequences. Because axial transport is much faster than radial

diffusion, the jet remains a spatially distinct source independent of other nearby jets, so that

several jets can be operated close to each other.

A heavy atom injected into the light carrier jet will travel with the jet, but will also execute a

‘random walk’. For example, an atom of Au injected on the jet axis will diffuse a distance x

away from the axis during the transit time to the substrate τ = L/v according to

∼ 2 Dτ ∼ 2DL/v (18.1)

where D is the diffusion coefﬁcient, L is the distance to the substrate, and v is the jet speed

downstream of the Mach disk. For L ∼ 10 cm, v ∼ 3×10

cm/s, and D ∼ 600 cm

/s (diffusion

in He at 1 torr), the diffusion distance is ∼ 4 mm. Therefore most Au atoms will have diffused

only a few millimeters from the axis by the time they arrive at the substrate; they are

effectively trapped within the jet, and cannot easily escape. This already suggests a high

capture efﬁciency on the substrate, which is particularly important for expensive metals. In a

real jet source, vaporization does not occur exactly on axis, but from an extended area such as

a hot W coil, so that a fraction of Au atoms are nearer the jet boundary and can escape more

readily. But the fraction lost turns out to be small.

On impact with the substrate the free jet ﬂares radially into a wall jet. Visualization

experiments show that the transition from free jet to wall jet is sharp; the thickness of the wall

jet is only a few millimeters. When the Au atoms arrive near the substrate, the wall jet can be

expected to carry some of them away, as well as broaden the deposit. But a thin wall jet favors

deposition; metal atoms in the wall jet can still diffuse fast to the substrate. In the wall jet too,

only a small fraction of gold atoms are transported far enough to be lost.

Jet Vapor Deposition 885

We conﬁrmed the high capture efﬁciency of Au atoms by mass balance experiments; a known

length of Au wire is vaporized in the nozzle, and the weight deposited downstream is

measured with a microbalance. We found that 95% of the Au wire vaporized in the nozzle

deposited on the substrate, with the remainder carried away in the wall jet. Jet collimation and

localized deposition can be regarded as the JVD equivalent of line of sight deposition in high

vacuum. The deposition efﬁciency is far higher in JVD, however: all vaporized material goes

in one direction, is conﬁned to the jet, and most of it deposits in the impact zone.

The large inertia of Au atoms entrained in the jet may also favor high capture efﬁciency. When

the axial He jet transforms sharply into a radial wall jet, heavy Au atoms turn the corner only

with difﬁculty, and tend to move straight toward the substrate [8]. We have not yet determined

the relative importance of diffusion and inertia in the wall jet region under our operating

conditions, but studies of deposition under conditions of much higher pressure ratio P

and

much smaller nozzle diameter than JVD do show inertial effects under restricted conditions

[9]. In JVD cluster deposition, inertia plays an important role, to be discussed later.

Simultaneously ﬂowing jets are not only spatially distinct, but the operation of one does not

alter the upstream nozzle conditions of any other. As long as P

exceeds ∼ 2, the pressure

in the nozzle remains constant despite changes in downstream P

resulting either from

changes in pumping speed or from injection of gas downstream. If the pump can sustain

critical ﬂow, the jets all emerge at the speed of sound, and neither information nor mass can

propagate upstream. Therefore the calibration of one jet source is unaffected by simultaneous

operation of other jets. This makes it possible to run several jet sources together to make

multicomponent ﬁlms of accurate composition.

Another example of the beneﬁts: we vaporize many metals in a hollow cathode sputter jet, or a

wirefeed jet, and then oxidize the growing metal ﬁlm with O

or O atoms injected

downstream. Inside the nozzle, the metal sputtering target or any other metal vapor source is

then shielded from the oxidants downstream by sonic ﬂow out of the nozzle; a comparable

protection in high-vacuum PVD is nearly impossible.

In sum, JVD’s jets in low vacuum have a relatively simple behavior, despite variations in

density and speed near the Mach disk. The main jet is fast, collimated, and well deﬁned, and

the wall jet is thin. Consequently, the jet delivers metal or other vapor efﬁciently; deposition is

localized, ﬁlm growth is fast, and radial wall jet ﬂow does not greatly broaden the deposit.

Multiple jets are basically independent.

18.3 Relative Motion between Jet Sources and Substrates

In order to deposit uniform ﬁlms over larger areas, the jet source and substrate are moved

relative to each other in two dimensions [4]. Solder must be deposited on a range of substrates

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Figure 18.2: Schemes for 2D relative motion between substrate and jet sources. Different

combinations of rotation and translation are possible.

including ﬂat patterned silicon wafers of 2, 4, 6, and 8 inch diameter, rectangular substrates of

similar area, and small components such as CuW heat sinks of irregular shape and millimeter

dimensions. JVD systems must be versatile enough to coat single wafers or batches, by

providing the 2D relative motion between jet and substrate. As shown in Figure 18.2, one can

move the source or the substrate (or even the gas jet itself) with motion control electronics

giving appropriate combinations of rotation and translation. Motion is transmitted

mechanically through chamber walls via bellows assemblies and ferroﬂuidic seals, or by

stepper motors within the chamber. Figure 18.2 also suggests the various combinations that

have been implemented at JPC. For example, in Figure 18.3 substrates are mounted on a

carousel which can both spin rapidly and translate slowly along its axis; the carousel motion is

computer controlled, and various motions can be programmed. The jet is aimed radially at

the carousel. If the carousel is only spinning, a band of deposition appears around it; if the

carousel is also vertically scanned at constant speed, the band broadens to cover the entire

carousel surface, and that of any substrate on it. The deposit thickness is uniform because all

areas are exposed for equal times to a constant jet ﬂux. This approach is ideal for batches of

small or moderate sized substrates, where the ﬂatness of the substrate does not depart much

from the curvature of the carousel, so ﬁlm thickness remains uniform. To keep the size of

carousels and chambers manageable, this batch processing is limited to 4 inch diameter wafers

or 4 × 4 inch squares.

Jet Vapor Deposition 887

Figure 18.3: A spinning, oscillating carousel for batch deposition of uniform ﬁlms on small wafers

or small substrates of other shape.

Several jet sources can be aimed at the carousel. To make a layered compound, the jet sources

are run in sequence, as in Figure 18.4, for different times determined by the required layer

thickness. To deposit a multicomponent compound, several jets are run at the same time, while

the substrate is sweeping through all the jets at high speed. Relative motion between substrate

Figure 18.4: Sequential deposition of layers such as Ti, Pt, and Au, using multiple jet, moving

substrate approach for substrates on a carousel.

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and several jet sources can often be made so fast that micromixing occurs at submonolayer

level. For example, lead zirconate titanate (PZT) can be made using three metal jets of Pb, Zr,

and Ti, plus a plasma jet of oxygen, by choosing the carousel spin frequency so that less than

one monolayer of any metal is deposited on any substrate during its time of passage through

any of the jets. The three components are then well mixed and quickly converted to oxide by

the oxygen plasma jet.

Larger substrates, such as 4, 6, or 8 inch wafers, or a limited number of small substrates, are

better processed with a single wafer, spin–scan approach, as in the right side of Figure 18.2.

The wafer is mounted on a fast spinning platter; the spin provides one degree of motion, and

slow translation of the jet source or the platter provides the second degree. For example,

the jet source can be mounted on a sliding vacuum ﬂange, while the wafer spins around a

ﬁxed axis; the sliding jet source can then scan the wafer periodically across a diameter. The

scan speed is slow at the wafer’s edge and rapid through the wafer’s center, to equalize

residence times. The correct relation between scan speed and position must be found by

experiment, but this approach gives uniform ﬁlms and economical use of material for single

wafer coating.

To coat a single wafer with a sequence of metal layers, we move the entire spinning platter

past a sequence of jet sources. The simplest arrangement of sources is either a straight line or a

circle. The spinning wafer visits each jet source in sequence, performing a slow scan back and

forth in front of each source to give the desired multilayer sequence. For example, Figure 18.5

shows a compact index arm conﬁguration in which several sources are arranged in a circle.

Figure 18.5: Index arm deposition system for depositing a sequence of layers on a single wafer.

The wafer spins rapidly through an angle θ, while the index arm moves through angle φ, visiting

each jet source in turn and executing a programmed periodic scan through an arc of angle φ in

front of each source.

Jet Vapor Deposition 889

The index arm ﬁrst positions one edge of the spinning wafer in front of the ﬁrst source; the arm

then executes a scan by sweeping the wafer back and forth through a short arc from one edge

of the wafer through the opposite edge. The index arm then indexes to the next jet source,

where the spin and scan motion is repeated. The same process can be done for a straight line of

sources, but the index arm conﬁguration allows a smaller, more compact chamber. At JPC, we

use this index arm conﬁguration for solder coating of a single wafer (4, 6, or 8 inch); a typical

solder deposition might require a standard Ti + Pt + Au bond barrier, followed by a thick layer

of AuSn or other solder, requiring four sources in the chamber. The ability to provide all

solder-related layers for small numbers of substrates, without vacuum break, is important for

customer research and development.

Spin–scan deposition on a single wafer results in a radially symmetric ﬁlm, and this makes

possible a direct measurement of ﬁlm density. Symmetry results because the spin is very fast,

around 800 rpm, and the scan is slow, several minutes for a 6 inch wafer. The ﬁlm volume is

found by taking proﬁlometer height readings along a radius, and then doing a numerical

integration. The mass of the ﬁlm is measured with a microbalance; a 10 ␮m AuSn ﬁlm on a

6 inch wafer weighs > 2 g. We found that the density of AuSn deposited via JVD was ∼ 95% of

bulk, suggesting low porosity and low internal surface for adsorption of contaminants that

impede wetting when the solder melts.

The high density result is also interesting in view of the extremely high local deposition rate

and low substrate temperature in JVD, factors which should favor lower density. The average

deposition rate is 10 ␮m over the 6 inch diameter area in ∼ 40 minute, already substantial. But

the local, instantaneous deposition rate is about 100 times as great, of the order of micrometers

per second, since the entire ﬂux of AuSn is delivered through a small jet area of ∼ 1cm

.Itis

also worth emphasizing that the substrate temperature always remains low in spin–scan

operation, because no point on a substrate remains opposite a jet long enough to become hot,

no matter how metal vaporization is accomplished inside the nozzle. In single wafer coating,

the ﬁnal wafer temperature never reaches 80

◦

C, so the process is relatively cool. Moreover,

even when the wirefeed rate is increased by a factor of 4, with a corresponding increase in

local deposition rate, the measured density remains high. We believe that this behavior is

peculiar to low melting metals, such as the solder alloys, which have low Tammann

temperatures [10], implying a surface atom mobility that favors dense ﬁlms, even at the low

substrate temperatures and high local rates of JVD. This is fortunate, since solder layers are

generally thick, and must be deposited at a high rate if the process is to be economical, and on

substrates that must remain cool if photoresist masks are to be undamaged.

In sum, the contributions of several jets can be integrated even at high overall growth rate. Jets

operated singly in a prescribed time sequence yield multilayer structures; jets operated

together give alloys or doped ﬁlms, or, in a reactive mode, compounds such as oxides and

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nitrides. The multiple jet, moving substrate strategy has proven very successful at JPC,

particularly for solder layer deposition, in both single wafer and batch modes.

18.4 Three JVD Jet Sources

Virtually all work in JVD is done with three jet source designs: (1) wirefeed to hot ﬁlament jet;

(2) hollow cathode sputter jet; and (3) capillary feed wick jet.

18.4.1 Wirefeed to Hot Filament Source

Shown in Figure 18.6, this is the mainstay used at JPC for commercial production of solder

coatings [6, 11]; AuSn can serve as an example. A ﬁne wire of AuSn alloy is fed continuously

Figure 18.6: This deposition source combines the main features of the hot ﬁlament wirefeed and

the e-jet plasma JVD sources. High argon ion density permits substrate cleaning, as well as ion

bombardment of the growing ﬁlm at low energy and high current.