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

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Jet Vapor Deposition 891

against an electrically heated W coil structure located just upstream of the nozzle exit. On

contact, the AuSn wire melts, wets, and vaporizes; individual Au and Sn atoms are swept

downstream with the jet and deposit on the substrate, concentrated within a circle ∼ 1cmin

diameter; radial diffusion of Au and Sn in the jet gives the AuSn deposit a Gaussian proﬁle. At

constant AuSn wirefeed rate the ﬂux of Au and Sn atoms is constant, and the AuSn deposition

rate is proportional to wirefeed rate, considerably simplifying thickness calibration. The Au to

Sn ratio of the wire is preserved in the growing ﬁlm.

Inclusion of a second hot ﬁlament as a source of thermionic electron emission enables electron

avalanching and ignition of a low-voltage, high-current Argon plasma, which is then carried

by the jet to the substrate. In addition to high-rate deposition, this e-jet source can supply Ar

ions at high density, ∼ 10

ions/cm

, for substrate precleaning or ion bombardment of the

growing ﬁlm [12–15]. The wirefeed to hot ﬁlament can be used for metals that do not alloy

with and degrade the hot tungsten coil structure. This includes all metals commonly used for

solder deposition, such as AuSn, InSn, and other alloys, etc., if they can be purchased as wire.

Very high rates are possible with this wirefeed jet source; even a relatively slow wirefeed can

give deposition rates approaching micrometers per minute over a 4 inch wafer. Most of JPC’s

commercial deposition of solders is carried out with the hot ﬁlament wirefeed and e-jet

combination, in both atomistic and cluster modes.

18.4.2 Hollow Cathode Sputter Jet Source

This jet source is based on sputtering at high pressures of ∼ 1 torr, and has been in use for

many years at JPC. A hollow cathode provides a target for argon glow discharge sputtering of

metal atoms, and also plays the role of a cylindrical nozzle. Argon is let in at one end, ﬂows

through the hollow cathode, and forms the sonic jet which sweeps sputtered atoms to the

substrate. JVD sputter jet sources give much lower deposition rates than the wirefeed; they are

limited by power dissipation, and the hollow cathode must be encased in a water-cooled

copper block. However, they can generate a vapor of virtually any metal, and are appropriate

for thin ﬁlms in the several thousand

Angstrom range, as for example layers of Ti, Pt, and Au

required as adhesion, diffusion barrier, and protective layers prior to solder bumping. The

deposition rate can be calibrated against Ar

sputter current; when the gas ﬂow conditions are

ﬁxed, this calibration is stable and accurate.

Clusters can also be generated in the sputter jet. Although the atom generation rates are

much lower than the wirefeed, and therefore local gas phase metal atom concentrations are

lower, nevertheless we have found conditions suitable for cluster initiation, growth, and

deposition.

892 Chapter 18

Figure 18.7: Wick jet source showing basic components: molten solder crucible, wire str and wick,

and hot ﬁlament for vaporization.

18.4.3 Wick Jet Capillary Feed Source

The wick jet, shown in Figure 18.7, has three components connected in series inside the nozzle:



a reservoir of molten solder, at a temperature somewhat above the melting point;



a wick fabricated from tungsten wires, at temperature greater than that of the reservoir;



a hot zone vaporizer consisting of a tungsten ﬁlament at very high temperature.

One end of the wick dips in the molten solder; the other is connected to the hot zone. The

junction of the wick and hot zone is positioned upstream of the nozzle oriﬁce. The wick and

hot zone are Joule heated to their respective temperatures by a simple combination of

alternating and direct electric currents (AC and DC).

The wick jet works by capillarity. As metal atoms vaporize from the hot zone, surface tension

draws more liquid metal from the reservoir, through the wick, to the hot zone. In steady state

the rates of vaporization and replenishment are equal.

The wick jet is universal for all solder metals, because all solder metals are compatible with

hot tungsten. Wick transmission is sufﬁcient to maintain an extremely high vaporization rate at

the hot zone, and therefore a high deposition rate downstream. A major economic advantage of

the wick jet is its use of bulk material as the supply for the reservoir; the material cost is thus

as low as possible, with impurity level the only variable. Surface oxides ﬂoat to the top of the

reservoir, but are not drawn up the wick to be vaporized. Alloy compositions that do not lend

themselves to wire drawing are easily handled as a melt, so there is no constraint on solder

composition. Other advantages include the absence of moving parts, simple construction, and

ﬂexible form. The wick is particularly adaptable for nanocluster generation, and we ﬁrst

Jet Vapor Deposition 893

Figure 18.8: Indium solder bumps deposited by JVD, 3 ␮mon5␮m pitch. (a) Photoresist mask;

(b) lift-off; (c) as-deposited bumps. (Photograph courtesy of Alcatel-Lucent Bell Labs.)

observed cluster deposition using a wick jet. The solder bumps in Figure 18.8 were produced

with an experimental wick jet design.

18.5 Generation and Deposition of Atoms and Clusters

With modest changes in pressure and ﬂow, JVD jet sources give a choice of generating either

metal atoms or metal nanoclusters. Cluster deposition is advantageous for solder layers,

because atoms and heavier clusters will ﬁll holes in photoresist masks in very different ways;

cluster deposition is highly directional.

In the normal JVD range, at pressures ∼ 1 torr and high jet speed, only single metal atoms

deposit, but not clusters. Cluster formation must be initiated by ﬁrst stabilizing a dimer. This

requires a three-body collision of two metal atoms and one carrier gas atom, or else three metal

atoms. While contributions of heterogeneous processes in the nozzle cannot be ruled out [16],

three-body collisions are highly improbable in the jet: the metal atom and Ar or He

concentrations are normally too low, and the transit times from nozzle to substrate are too

short. The time for a metal atom to undergo a three-body collision, using a typical three-body

recombination rate constant [17],is

∼ 10

/(M)(Ar) ∼ 0.1/P

(18.2)

894 Chapter 18

where (M) and (Ar) are gas-phase concentrations (no./cm

) of metal atoms and Ar atoms, and

, P

are the pressures in torr of metal atoms and Ar atoms. For P

= 0.01 torr and

= 1 torr, τ

∼ 10 s. Since this far exceeds the normal transit time from nozzle to substrate,

dimers cannot form.

Nevertheless, nanoclusters can be generated at high rate by moderate changes in several JVD

conditions. From eq. (18.2), changes in carrier gas pressure, jet speed, source to substrate

distance, identity of carrier gas (third body), and metal vaporization rate will all affect the

dimer three-body nucleation rate. The time for subsequent cluster growth will be inﬂuenced by

the distance from vaporization point to nozzle exit, and also exit to substrate distance.

Moreover, all of these parameters can be conveniently altered to switch a jet source from atom

generation to nanocluster generation. The transition is fairly sharp, but readily controllable.

For example, at constant carrier gas ﬂow rate, partially closing the pump valve will lower the

jet speed, and increase the pressures of carrier gas and entrained metal atoms, which provides

a simple way to initiate clustering.

Once dimers are formed, subsequent growth to trimers, tetramers, and nanoclusters is

relatively fast. An energized trimer will be formed from a stabilized dimer by addition of a

metal atom. This trimer has an excess energy of ∼ 1 atom heat of vaporization, and a ﬁnite

lifetime with respect to loss of that atom via unimolecular decomposition; if there are enough

collisions with cold carrier gas atoms during that lifetime, enough excess energy will be

removed to stabilize the trimer. Similar reasoning applies to a tetramer. As the number of

atoms in the cluster increases, energy is more readily delocalized, and the unimolecular

lifetime becomes longer. In JVD sources the carrier gas remains relatively cool, so that at

accessible JVD pressures small clusters are quickly stabilized and growth is fast. The ﬁnal

cluster size then depends on the available concentration of metal atoms in the jet, which can be

controlled by wirefeed rate or a sputter jet current. Cluster size can also be inﬂuenced by

varying the distance to the substrate and the jet speed. In JVD wirefeed and wick jet sources

the overall rate of cluster deposition is very high, at least equal to the rates of atom deposition,

so that cluster deposition is fast and economical.

In sum, JVD sources can be adjusted in several ways to control cluster nucleation and growth

rates. The freedom to choose between atom and cluster deposition has proven useful for solder

layer applications.

18.6 Application of JVD to Solder and Related Layers

Electronic components must often be bonded to metal pads on an integrated circuit by solder

[18]. One approach is to use a preform cut from solder foil to the correct dimensions and

placed manually between the component and the pad. But as circuit dimensions diminish,

placement of small preforms becomes difﬁcult, and there is a limit to how thin a preform can

Jet Vapor Deposition 895

be cut and handled. Under these conditions, it is better to deposit a photoresist mask that

exposes only the solder pads, and then vapor deposit a solder layer over the entire patterned

wafer. After photoresist lift-off, the contact pads remain coated with solder, ready for

component placement.

Since the total contact pad area is only a small fraction of the wafer area, the lift-off approach

may seem wasteful of expensive solders such AuSn. Some solders can be deposited

electrochemically on just the contact pads, but it can be difﬁcult to electrodeposit alloys with

precise composition, where even small errors will affect the solder melting point, a critical

property. JPC customers report incomplete satisfaction with electroplated AuSn, a frequent

reason for turning to JVD.

In the past decade JPC has reﬁned the JVD solder deposition technology [19], and is now able

to supply all elemental and alloy solders, such as In, Sn, AuSn, InBi, InSn, SnAg, SnInAg, and

SnCuAg. Solder bumping places severe requirements on process economics and material

quality, and JVD addresses those requirements with several advantages:



high deposition rates for atoms and clusters;



accurate composition and thickness control;



in situ ion etching to assure adhesion;



alloy rather than component layer deposition;



minimal material waste;



adaptability to large and small substrates;



compatibility with conventional photoresist and lift-off technology;



single wafer or batch processing.

Prior to solder coating, wafers must be precleaned, particularly if they have patterned resist

layers, where organic residue can compromise solder adhesion on pads. In JVD, the substrate

can be cleaned with a high ﬂux of low-energy argon ions generated by an e-jet source. Even

without bias, the plasma sheath at the wafer surface provides a ∼ 8–10 V potential for

bombarding ions, which is effective for removing a few monolayers of resist. An alternative is

to use a JVD microwave discharge jet to generate high levels of atomic oxygen that removes

organics by fast conversion to CO and CO

even at room temperature.

Almost all solders are currently deposited at JPC using the hot ﬁlament, wirefeed jet source,

though indium must be done with the wick jet. All solder metals and alloys are compatible

with the wirefeed source; none will alloy with or damage hot tungsten, so that the jet source

lifetime is very long. Alloy wire of correct solder composition, for example eutectic AuSn

896 Chapter 18

80/20, is vaporized into the jet as Au and Sn atoms; the right composition is reproduced in the

depositing AuSn ﬁlm. Precise composition, and therefore melting point, is a key requirement

for a solder ﬁlm, and customers take pains to verify it, by energy-dispersive X-ray spectroscopy

measurements, melting/freezing point measurements, and ﬁnally by the performance of the

solder in bonding. Solder layers can be from several to 20 ␮m thick, and both the wirefeed and

wick jet sources can make such layers at high rate. For example, with 15 mm AuSn wire, fed at

2 inches per minute, a 6 inch diameter wafer will be uniformly coated in ∼ 45 minutes. The

wirefeed rate can be greatly increased, and the coating time reduced proportionately.

Solder bumps of micrometer dimension and area densities of millions/cm

can be made using

conventional photoresist lithography [20, 21], but reliable manufacturing of such arrays is still

a challenge. Moreover, as the circuit features become smaller, ﬁlling small holes in a

photoresist mask adds to the challenge. Most patterned wafers received at JPC present large

area pads several tens of micrometers wide. If the solder layer is to be several micrometers

thick, then the resist walls must be about twice as high. For large area bond pads the aspect

ratio (photoresist thickness/pad width) will be substantially less than 1. For these low aspect

ratio cases, the wirefeed atomistic deposition of individual Au and Sn atoms is a reliable

approach; on the pad the deposited solder has the correct thickness and a ﬂat proﬁle. However,

small-area pads, with lateral dimensions ∼ 10 ␮m or less, will still require several micrometers

of solder. The photoresist will still be thick, and the windows in the photoresist will have a

higher aspect ratio. We found that as the aspect ratio approached 1, small holes were difﬁcult

to ﬁll, if we tried to ﬁll them by depositing atoms.

The difﬁculty has the following origin. Although the jet is collimated and traveling

line-of-sight normal to the substrate, the metal atoms inside it are not; they collide frequently

with He or Ar atoms, their velocities are randomized, and they impact the ﬁlm at angles that

can be far from perpendicular. Some of these impacting metal atoms will not ‘see’ the bottom

of a deep hole. For small windows with aspect ratio of order 1, resist walls cut off atoms

coming from angles far from the normal, and shadowing is severe. In addition, ‘breadloaﬁng’

from deposition on the sidewalls will further constrict the resist window as deposition

continues. As a result, the solder bump is no longer ﬂat, but dome shaped; moreover, it will be

signiﬁcantly thinner than needed. One can compensate by depositing excess metal, but this

wastes material and time.

JVD provides an effective alternative: generate and deposit heavier nanoclusters. The

nanoclusters grow inside the nozzle, are accelerated by the jet, and travel toward the substrate.

A massive nanocluster will be hit many times by randomly directed carrier gas atoms, but its

transverse velocity components remain extremely small. The trajectory of the nanocluster then

remains parallel to the jet, and it impacts the growing ﬁlm at a right angle. In consequence,

there is little shadowing by resist walls, and since sidewall deposition is also reduced, there is

reduced breadloaﬁng [22]. The resist window is therefore ﬁlled by solder clusters from the

bottom up, giving a ﬂat-topped solder bump of undiminished thickness. The sequence of

Jet Vapor Deposition 897

scanning electron micrographs in Figure 18.8 shows the photoresist mask, the lift-off process

under way, and an array of indium (In) bumps made from In nanoclusters deposited by JVD

[20]. The bump diameter is 3 ␮m, while the pitch is 5 ␮m. The ﬂat top on the In bumps is

noticeable. These are quite small-diameter bumps, and there has been some breadloaﬁng,

evident in the slightly conical shape. In other work, in which In was deposited in 4 × 4 and

6 × 6 ␮m windows, breadloaﬁng had much less of an effect, and the bump sides were less

sloped. It is worth emphasizing that this cluster ﬁlling process gave uniform deposition over 6

and 8 inch diameter wafers, and was carried out at economically high deposition rates, e.g.

5 ␮m of In over a 6 inch wafer in < 15 minute.

18.7 Summary

JVD’s jet in low vacuum strategy is applicable to many kinds of thin ﬁlm. In addition to simple

metal ﬁlms, these include multilayers, alloys, multicomponent oxides and nitrides, organic and

host–guest ﬁlms, cluster and cluster embedded ﬁlms.

The jet in a low vacuum strategy is economical to implement. Even relatively small,

inexpensive mechanical pumps will maintain critical ﬂow conditions, providing high-speed

collimated jets in a small-footprint, ﬂexible apparatus. The carrier gas ﬂows represent only a

small material cost. The batch process turnaround time in a JVD chamber is a matter of

minutes.

Mechanisms for relative motion between jet source and substrates, combined with the use of

several jets simultaneously or in sequence, allow design of processes for substrates of many

sizes and shapes, for production of many complex ﬁlm materials. JVD metal sources are based

on techniques such as glow discharge sputtering or direct vaporization, which can deposit

nearly every metal in the Periodic Table, with no toxic metal precursors or harmful exhausts.

In JVD, rates of metal deposition are generally high because the jet entrains a large partial

pressure of metal atoms, travels at the speed of sound, remains collimated, and deposits atoms

with a high capture efﬁciency in a localized area. Even if metal atoms comprise less than 1%

of the jet, the deposition rate will approach micrometers per minute over a 4 inch wafer. Rates

of cluster deposition are equally high.

Solder layer deposition in particular is naturally suited to JVD, as a consequence of high

deposition rates, short pump down times, efﬁcient material use, and plasma precleaning.

Much of JVD’s versatility arises from the use of independent jets and relative motion schemes

to give multilayer structures. These capabilities have now been focused on the vapor

deposition of solder metals and alloys, addressing a growing need in microelectronic

packaging technology. JVD can provide not only the solder layers, but also the required

adhesion, diffusion barrier, and protective cap layers. JVD employs several jet source designs

that can run at high deposition rate, while ensuring the required solder properties: precise

898 Chapter 18

composition, melting point, purity, and adhesion, independent of substrate size and shape.

Moreover, the ease of converting from atom to cluster deposition has important consequences

for jet deposition of solder layers, signiﬁcantly extending downward the size range of solder

bumps that can be cleanly deposited. All of the common solder elements and alloys are

compatible with JVD jet sources, and can be vaporized and deposited with accurate

preservation of composition, in atomic or cluster form. Localized deposition and high capture

efﬁciency make JVD intrinsically economical for costly materials such as AuSn.

Appendix Survey of Miscellaneous JVD Films and Applications

Here we brieﬂy review some past work that illustrates the range of capabilities of JVD.

Cu + Au Multilayer Electrodes

We used Cu and Au jet sources in sequence, to make thin ﬁlm electrodes in production runs for

AT&T Bell Laboratories and a US Navy application. The substrates were 50 mm diameter

piezoceramic wafers less than 1 mm thick; these required a 1.5 ␮m Cu electrode, ﬂashed with

50 nm Au, on both sides of the wafer, with a 0.25 mm border free of metal and cleanly deﬁned

at the perimeter. Wafers were mounted on the carousel in accurately machined receptacles

which supported the wafer, oriented it toward the jets, and deﬁned the border.

Al, Al

Microlaminates

Jet sources of Al and O

were used to deposit thick microlaminates consisting of 50 nm layers

of Al alternating with 5 nm of Al oxide [23] at high rate. The microlaminates coating exhibited

a hardness equal to that of microlaminates made by sputtering [24], a far slower ultrahigh

vacuum process.

PZT: Ferroelectric FRAM Non-Volatile Memories

Four jet sources simultaneously supplied Pb, Ti, Zr, and O

to Pt-coated, heated Si wafers

mounted on a spinning carousel. The Pt barrier layer was also deposited by JVD. The resulting

1 ␮m PZT ﬁlm of PZT was deposited in less than 1 hour [25], and exhibited excellent values of

remanent polarization, coercive ﬁeld, switching endurance, and dielectric constant, equaling or

exceeding those obtained with more conventional methods.

PZT: Pyroelectric Detectors

PZT was deposited on 18 substrates of 2 inch diameter mounted on a carousel, using three jet

sources of Zr, Ti, and PbO, in a 0.5 torr downstream O

pressure [26, 27], with substrates at

Jet Vapor Deposition 899

550

◦

C to promote perovskite growth. The remanent polarization, coercive ﬁeld, pyroelectric

coefﬁcient, and dissipation factor, compared favorably to bulk PZT.

Nickel Ferrite Ceramic Films

We deposited ceramic ﬁlms of stoichiometric NiFe

using a single wirefeed plus e-jet

source; NiFe

alloy wire was vaporized thermally inside the nozzle. The source was run in

plasma mode, so that a 1 inch square alumina substrate in front of the jet was heated to

T ∼ 600

◦

C, in part by ion–electron surface recombination. NiFe

ﬁlms of thickness

>25␮m were grown at rates ∼ 100 ␮m/hour. Films were nearly crystalline, with good

magnetic and physical properties as characterized by magnetization, stress-induced anisotropy,

and hysteresis loops [28].

Electronic-Grade Silicon Nitride

Silicon nitride ﬁlms of remarkable electronic quality were grown on a room-temperature

substrate using a microwave discharge jet source [29] that produces Si atoms, Si-bearing

molecular fragments, and N atoms from SiH

and N

. In a metal-nitride semiconductor (MNS)

capacitor the electrical behavior was superior to that of any previously reported [29–33],as

characterized by breakdown strength, radiation hardness, interface trap density, etch rate in

buffered oxide etch, index of refraction, breakdown strength, density of interface states,

reduced tunneling current, and low H content.

Fiber Coating

Despite the directionality of the JVD jet source, we showed that a thin ﬁber oriented

perpendicular to the jet could be coated uniformly. This is possible because the collision mean

free path in the jet at 1 torr is ∼ 100 ␮m; ﬁbers of smaller diameter will be struck uniformly

from all directions by metal atoms, as if in high vacuum. We have coated 12.5 ␮m alumina

ﬁbers with thousands of

Angstroms of Cu and Al, with uniformity veriﬁed by scanning

electron microscopy.

Coating of Thermally Sensitive Membranes

The low substrate temperature capability of JVD is being exploited in the coating of Au and Pt

as ﬁne line electrodes on a 9 ␮m PVDF piezoelectric membrane through a foil mask. PVDF is

thermally sensitive, and can be depoled at T ∼ 350

◦

C, so the low-temperature capability of

JVD is critical.

900 Chapter 18

Ceramic Host–Organic Guest Films

Using multiple jets including a novel organic vapor jet [34], we trapped complex guest organic

molecules such as rhodamine B and methyl red in a range of host ceramic ﬁlms: SiO

, SiN

, and MgO, at high guest concentrations ∼ 1%. Using this approach we demonstrated

deposition, patterning and photobleaching of Methyl Red/ceramic to make thin ﬁlm wave

guides. We made a thin ﬁlm solid host dye laser by trapping rhodamine 6G, and also an

acid–base indicator using trapped methyl red [35]. Ceramic host–organic guest ﬁlms can be

made by mechanical or sol gel methods [36], but these are multistep processes, limited to

soluble species, often including time-consuming thermal treatment. In JVD the host–guest

combination is generated in minutes, at room temperature, and by a vapor deposition

technique compatible with existing semiconductor microelectronic processing.

Polymer Deposition: Parylene

We constructed a parylene–N vapor jet source [37] in which the cracking reactions of the

Gorham process [38] were carried out inside a small nozzle structure, which then accelerates

the active fragments to the substrate. In addition, we mixed into the main ﬂow a stream of

atomic hydrogen. The results were a greatly enhanced deposition rate, enhanced adhesion,

increased hardness, and a reduction in dielectric constant from 1. 65 to ∼ 1.47. X-ray

diffraction showed an amorphous ﬁlm, with no hint of crystallinity. There is a dramatic effect

of H atoms on parylene–N deposition rate and properties.

Deposition on Liquid and Paper Surfaces

JVD’s low vacuum conditions enable deposition on liquids of sufﬁciently low vapor pressure,

and on surfaces not tolerated in a high vacuum system. Thus, we were able to deposit Au and

other metal atoms on liquid water, glycerol, and epoxy surfaces, thereby generating Au

colloids [1, 39]. We deposited 5 ␮m of Cu on a strip of paper 2 inches wide and 3 feet long,

wrapped around a spinning carousel, using a wirefeed source. The adhesion was excellent, and

the Cu coating carried ∼ 10 A without any sign of paper carbonization or Cu delamination.

References

[1] B.L. Halpern, J. Colloid Interface Sci. 86 (1982) 337.

[2] J.J. Schmitt, B.L. Halpern, US Patent 4,788,082 (1988).

[3] J.J. Schmitt, B.L. Halpern, US Patent 5,725,672 (1998).

[4] B.L. Halpern, J.J. Schmitt, J. Vac. Sci. Technol. A 12(4) (1994) 111111623.

[5] B.L. Halpern, J.J. Schmitt, J.W. Golz, D.L. Johnson, D.T. McAvoy, J.Z. Zhang, Y. Di, in: Society of Vacuum

Coaters, Proceedings of 35th Annual Technical Conference, Baltimore, MD (March 22–27, 1992).

[6] B.L. Halpern, J.J. Schmitt, Y. Di, J.W. Golz, D.L. Johnson, D.T. McAvoy et al., Metal Finishing

(December 1992).