Method and apparatus for forming low dielectric constant polymeric films

A method and apparatus for forming a low dielectric constant polymeric film on a substrate, by liquid delivery of a parylene precursor reagent, in the form of an organic solution or a neat liquid, subsequent flash vaporization of the neat liquid or organic solution, pyrolytic "cracking" of the precursor to form the reactive monomer and/or reactive radical species, and condensation and polymerization of the monomer and/or reactive radical species to form a low dielectric constant polymeric film on the substrate. The low dielectric constant polymeric film may comprise a parylene film, formed from a precursor such as [2.2]paracyclophane, an alkyl- and/or halo-substituted derivative thereof, or an analogous compound of a p-xylene derivative.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for forming low 
dielectric constant (low k) polymeric films on a substrate, as an 
interlayer dielectric (ILD) material for fabrication of microelectronic 
device structures. The low k material may for example comprise parylene or 
a substituted derivative thereof. 
2. Description of the Related Art 
Copper currently is of great interest in metallization of very large-scale 
integration (VLSI) devices, due to its low resistivity, low contact 
resistance, and ability to enhance microelectronic device performance by 
reduction of RC time delays. 
The concurrent use with copper metallization of a low dielectric constant 
material likewise provides reduction in the RC time constant, to further 
enhance device performance. 
Among low dielectric constant materials, conventional SiO.sub.2 dielectric 
materials display values of the dielectric constant, k, near 3.2. For 
lower dielectric constant materials, potential candidate materials include 
polymers such as parylene as an interlayer dielectric (ILD) material. Some 
examples of the parylene family are given below: 
##STR1## 
Parylene displays a dielectric constant in the range of from about 2.2 to 
about 2.4. The use of parylene and/or related materials as interlayer 
dielectrics require that the dielectric material be easily deposited 
conformally over typical device topographies. Only then will increased 
device performance in the microelectronic device structure be realized, 
such as a VLSI device operating in the giga-hertz range in which the low k 
material is utilized as an ILD to electrically isolate signal lines, e.g., 
of copper, from each other. 
A major manufacturing issue in the deposition of parylene-based dielectric 
materials, such as parylene-N, relates to the need for accurate and 
controlled delivery of the dimeric para-xylylene ([2.2]paracyclophane) as 
a starting material delivered to the thermal "cracking" chamber. In the 
cracking zone, reactive monomers and/or radical intermediates are formed 
for subsequent formation of the desired polymer. Co-reactants, such as 
cross-linking agents or co-monomers, may be added to form a specific 
polymer or to tailor the polymer properties, including the modulus, the 
dielectric constant, the thermal stability and the device performance 
properties. 
Parylene-N films grow on a cooled substrate (at temperatures on the order 
of -20.degree. C.) in a vacuum environment by condensation of gaseous 
p-xylylene, a reactive monomer. The monomer, however, is not stable at 
room temperature conditions, but exists rather in the form of dimers. A 
vapor stream of the monomer can be readily generated by cracking the dimer 
vapor that is supplied from a stable crystalline solid dimer source. The 
dimer cracks at temperatures between 500 and 750.degree. C., with 
600.degree. C. typically being employed for such dissociation. 
Alternatively, the p-xylylene monomer can be created by cracking analogous 
compounds that contain a p-xylylene unit or derivative thereof. 
Current dimer vaporization approaches employ the sublimation of the solid 
dimer as a feedstock to a cracking unit. Determined by the nature of a 
sublimation approach, the material transport rate of this process, 
however, is difficult to control. As a result, the stream of monomers 
exiting the cracking unit is not constant with time, which leads to an 
irreproducible deposition process. Liquid sources for parylene film 
formation may be vaporized in bubblers, but control of the vapor phase 
concentration of these low volatility compounds is difficult, and leads to 
poor reproducibility of the deposition process. 
For microelectronic applications, control of the vapor concentration is 
critical, since it determines the growth rate, conformality of the 
resulting film, and reproducibility of the film formation process. In such 
applications, thickness must be tightly controlled for operability and 
adequate performance characteristics. Another problem associated with 
systems which couple pyrolysis with sublimation or bubbling is that the 
process equipment aggregately has a large footprint. For example, the 
process equipment may occupy an inordinately large volume of clean room 
space and thereby be undesirable for manufacturing environments. 
It would therefore be a significant advance in the art to provide a method 
and apparatus for conveniently and economically providing low dielectric 
constant polymeric films such as parylene on a substrate with better 
reproducibility, and without the attendant problems of the prior art 
approaches for such film formation. 
It therefore is the object of the present invention to provide such method 
and apparatus for the formation of low dielectric constant polymeric films 
on microelectronic device substrates, to enhance the device performance 
and reduce RC time delays. 
Other objects and advantages of the present invention will be more fully 
apparent from the ensuing disclosure and appended claims. 
SUMMARY OF THE INVENTION 
The present invention in one aspect relates to a method of forming a low 
dielectric constant polymer film, e.g., a parylene film, on a substrate, 
including the steps of: providing a precursor comprising a polymer source 
reagent; heating the precursor to flash vaporize same; heating the flash 
vaporized precursor to pyrolytically crack the polymer source reagent, 
yielding a precursor vapor which includes a polymer source monomer and/or 
reactive radical species; and contacting the precursor vapor with the 
substrate under conditions producing condensation of the polymer source 
monomer and/or reactive radical species, to form a low dielectric constant 
polymeric film on the substrate. 
The precursor in such method may be constituted by a liquid solution 
wherein the polymer source reagent is dissolved in an organic solvent 
medium. 
In a specific aspect, the present invention relates to a method of forming 
a low dielectric constant (low k) parylene film on a substrate, by liquid 
delivery of a precursor therefor. The precursor may be in the form of a 
neat liquid, or alternatively an organic solution containing the precursor 
monomer such as a dimeric [2.2] paracyclophane, or an alkyl- and/or 
halo-substituted derivative thereof. The precursor is subsequently flash 
vaporized, followed by in-situ pyrolytic "cracking" of the flashed vapor 
to form the monomer and/or reactive species, and condensation leading to 
polymerization of the monomer and/or reactive species to form a polymeric 
film of parylene on the substrate. 
In such a method, liquid precursor solution may be supplied at suitable 
elevated temperature for supply to the vaporizer to effect flash 
vaporization thereof, as for example by provision of the precursor 
solution in a temperature controlled oven. Such elevated temperature 
supply in the case of parylene provides increased solution concentration 
of the precursor, e.g., dimeric [2.2] paracyclophane. Solvents for the 
liquid precursor solution include ethers such as tetrahydrofuran (THF), 
glyme solvents, glycols, alcohols, ketones, aldehydes, amines, aryls, 
pyridine and other compatible hydrocarbon and oxyhydrocarbyl solvents. 
Another aspect of the invention relates to a polymer film growth system, 
comprised of a source of organic solution containing precursor, e.g., in 
the case of parylene, dimeric [2.2] paracyclophane, a substituted 
derivative of dimeric [2.2] paracyclophane, or a chemically analogous 
liquid material, joined in liquid flow communication with a vaporizer, 
with the vaporizer in turn joined in vapor flow relationship to a 
"cracking" zone to generate reactive intermediates and/or monomers and 
lastly to a deposition chamber constructed and arranged to contact the 
precursor vapor monomers and/or radicals with the desired substrate to be 
coated. 
In such system, the vaporizer and "cracking" zone may comprise a stacked 
disc unit, or a tubular porous metal membrane unit, wherein the respective 
discs or tubular porous metal membrane constitute a thermally conductive 
high surface area medium serving both as a vaporization matrix and a flow 
restriction matrix, to induce turbulence in the precursor vapor flow and 
increase the residence time to realize optimal cracking to the monomer or 
radical species, as hereinafter more fully described. 
Other aspects and features of the present invention will be more fully 
apparent from the ensuing disclosure and appended claims.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF 
The vapor-phase deposition of polymeric films such as parylene may be 
achieved in the practice of the invention by liquid delivery of an organic 
solution containing precursor, e.g., in the case of parylene, dimeric 
[2.2] paracyclophane, an alkyl- or halogen-substituted derivative thereof, 
or a chemically analogous compound that reacts to provide a monomer and/or 
radical intermediate thereof. The precursor solution is subsequently 
"flash" vaporized, followed by pyrolytic "cracking" of the dimer to form 
the monomer and/or reactive species, and by condensation of the monomer 
and/or reactive species to form the polymeric film, e.g., of parylene or a 
parylene-like material. 
Alternatively, the method of the invention may be carried out wherein the 
precursor is a liquid. For example, the parylene source reagent may 
comprise a derivative of p-xylene whose methyl and/or phenyl ring hydrogen 
substituent sites are at least partially substituted with substituents 
independently selected from the group consisting of halo, alkoxy, 
acetyloxo, acetylthio, sulfone, azide and amide, wherein halo substituents 
may in turn be independently selected from iodo, fluoro, bromo and chloro. 
With respect to the formation of parylene films on a substrate, the 
chemical process is shown below for the vapor-phase decomposition of [2.2] 
paracyclophane leading to the deposition of a parylene-N film. 
##STR2## 
In the practice of the invention as applied to forming a low k parylene 
film on a substrate, generally a precursor solution of the solid parylene 
source reagent in an organic solvent medium is provided. The parylene 
source reagent may be dimeric [2.2] paracyclophane or a substituted 
derivative of dimeric [2.2] paracyclophane, in which one or more of the 
extracyclic carbon atoms in the methylene groups (--CH.sub.2 --) of the 
dimeric [2.2] paracyclophane molecule is mono- or di-substituted with 
substituents such as alkyl, halo (iodo, fluoro, bromo, or chloro), alkoxy, 
or other suitable substituents. 
Preferably, such carbon atoms are unsubstituted, or else such carbon atoms 
are partially or wholly substituted with fluorine to provide a fluorinated 
or perfluorinated derivative of dimeric [2.2] paracyclophane. This 
provides a parylene film designated as parylene-F. 
The parylene source reagent may also be an analogous compound containing a 
p-xylylene unit. The structure of the analogous compound may be a p-xylene 
derivative, in which the protons in the methyl groups or on the phenyl 
ring are partially or wholly substituted with substituents such as halo 
(iodo, fluoro, bromo or chloro), alkoxy, acetyloxo, acetylthio, sulfone, 
azide, amide and others. The analogous compound(s) may be liquid(s) at 
room temperature. In such a case, however, the neat source reagent may be 
used and delivered directly to a vaporizer/cracking unit without use of 
any organic solvent. 
The precursor source reagent is subjected to elevated temperature 
conditions to flash vaporize and subsequently to pyrolytically crack the 
parylene source reagent and yield a precursor vapor including a parylene 
source monomer. Flash vaporization of the dimeric [2.2]paracyclophane from 
the organic solvent precursor solution may be carried out with 
vaporization conducted at temperatures from about 150.degree. C. to about 
300.degree. C., e.g., at 200.degree. C. Cracking is carried out at 
temperatures on the order of about 500.degree. C. to 750.degree. C., e.g., 
about 700.degree. C. 
The vaporization and cracking may be carried out at any suitable process 
conditions. The preferred pressure conditions are subatmospheric or 
near-atmospheric pressure conditions. The temperature, pressure and other 
conditions may be varied in the process of the invention, to achieve a 
desired process and film formation result, as may be readily determined 
within the skill of the art without undue experimentation. 
Once the vaporization and cracking have been carried out, the monomers 
and/or reactive species in the vapor phase are transported to the 
deposition zone. This deposition zone may comprise a chamber in which the 
substrate element to be coated with the polymeric film is mounted on a 
suitable support or susceptor structure, optionally with cooling of the 
support or susceptor structure. 
The substrate element may be maintained at a temperature in the deposition 
zone that facilitates the condensation of the monomer from the vapor phase 
onto the substrate, to polymerize in-situ and form the desired polymeric 
film. The substrate temperature may for example be below ambient (room) 
temperature, e.g., on the order of -20.degree. C., to effectuate the 
deposition, although any suitable temperature conditions may be employed 
in the broad practice of the present invention. Preferred temperature 
levels for the deposition are from about -60.degree. C. to about 
30.degree. C. 
In the deposition step, the monomer vapor is contacted with the substrate 
under conditions producing condensation and polymerization of the source 
monomer on the substrate element. In the case of parylene film formation, 
the result is a low k dielectric polymeric film of parylene or a 
substituted derivative thereof, depending on the identity of the parylene 
source material. 
In some instances of the practice of the present invention, it may be 
advantageous to add co-reactants during the polymerization process, e.g., 
co-reactive monomers that are co-polymerizable with the monomer, to 
produce a wide array of polymeric materials, thereby facilitating the 
deposition of polymers with varying dielectric properties and/or thermal 
stabilities (i.e., cross-link densities). 
In the method of the invention as applied to the formation of parylene 
films, the precursor solution containing the parylene precursor and the 
organic solvent medium may be made up with any suitable solvent species, 
including single component solvents, as well as compatible solvent 
mixtures. 
Solvents that may be usefully employed in the broad practice of the 
invention include hydrocarbyl and oxyhydrocarbyl solvents, including 
ethers such as tetrahydrofuran (THF) and glyme solvents, glycols, 
alcohols, ketones, aldehydes, and other compatible hydrocarbon and 
oxyhydrocarbon solvents. More highly preferred solvents include 
tetrahydrofuran, furan, tetrahydropyran, pyran, pyridine, benzene, toluene 
and glyme solvents. 
Parylene film formation in the practice of the invention may be carried out 
in a polymer film growth system in which a supply of the precursor 
solution comprising a parylene source reagent in an organic solvent medium 
is arranged for selectively dispensing the precursor solution to the flash 
vaporizer. The flash vaporizer/cracking unit is arranged to very rapidly 
volatilize the precursor solution and to pyrolytically crack the parylene 
source reagent and yield a precursor vapor including a corresponding 
parylene source monomer. 
The flash vaporizer/cracking unit is joined in vapor flow communication 
with a contacting chamber constructed to hold a substrate therein for 
contacting with the precursor vapor. The vapor flow communication between 
the flash vaporizer/cracking unit and the deposition chamber consists of 
means such as conduits, tubing, pipes, lines, manifolds, flow passages, 
channels, or any other suitable vapor transport structures by which the 
vapor formed in the vaporizer unit is flowable to the contacting chamber. 
Similar means may be used to interconnect the supply or source vessel of 
the precursor solution with the vaporizer/cracking unit. 
In the deposition chamber, appropriate conditions are maintained for 
contacting of the precursor vapor with the substrate to produce 
condensation of the parylene source monomer to form a polymeric film of 
parylene on the substrate. The monomer-containing vapor for such purpose 
may be introduced in the deposition chamber by a variety of means. These 
may include vapor nozzles, spray heads, showerhead devices, or the like, 
to ensure that the vapor is contacted with the substrate element in a 
manner that results in a desired degree of uniformity, with respect to the 
product parylene polymer film on the substrate element surface. 
The flash vaporizer/cracking unit may be constructed in any suitable 
manner, but in one preferred embodiment described more fully hereinafter, 
the flash vaporizer unit comprises an array of stacked disc elements, each 
formed of a thermally conductive high surface area material. In another 
preferred embodiment of the flash vaporizer unit, such vaporizer unit 
comprises a tubular porous metal element formed of a thermally conductive 
high surface area material. 
It will be appreciated that the vaporization and cracking steps may be 
conducted in separate process units that are interconnected by suitable 
precursor vapor flow passage means. However, for purposes of maximizing 
thermal and process efficiency, it is generally preferred to utilize a 
consolidated unit for carrying out such steps, as disclosed more fully 
hereinafter. 
Considering again the precursor solution containing the parylene source 
reagent and the organic solvent medium, it may be desirable in some 
instances to store the precursor solution at elevated temperature 
conditions, so as to enhance the concentration of the parylene source 
reagent in the solution. Thus, the solubility of the particular parylene 
dimer in an organic solvent may be low in a specific solvent, but such 
solubility increases with increasing solution temperature. For example, 
the solution may be supplied from a vessel maintained in an oven or 
thermally controlled enclosure at temperatures of from about 40.degree. C. 
to about 130.degree. C. 
The use of a temperature controlled oven for solution storage and liquid 
delivery of the solution is therefore one method for providing an 
increased solution concentration and delivery rate in the practice of the 
invention, for high efficiency thin-film formation of the parylene 
polymer. 
The liquid delivery techniques usefully employed in the broad practice of 
the invention include any of the techniques known to those skilled in the 
art for providing controlled volumes of liquid at suitable temperatures to 
a heated vaporization zone. 
The liquid delivery and flash vaporization method of the present invention 
achieves a substantial advance in the art over prior art solid source 
sublimation processes since such prior art processes are limited by the 
delivery rate of the [2.2]paracyclophane reactant to the "cracking" zone. 
The liquid delivery and flash vaporization method of the invention 
therefore enables better control over the delivery rate of the "cracked" 
monomer and film growth rates of the resulting parylene polymer. 
Additionally, modification of the polymeric properties of the product 
parylene film can be realized by altering the structure of the parylene 
dimer starting material. For example, substituted derivatives of 
[2.2]paracyclophane can be synthesized and used to deposit low dielectric 
constant films with good thermal stability. Various substituted 
derivatives of [2.2]paracyclophane are described in X. Zhang, B. Wang, R. 
Tacito, D. Price, J. F. MacDonald, C. Steibruchel, R. J. Gutman, S. P. 
Muraka and T. P. Chow, New York SCOE Multi-level Interconnect Review, 
November 1994, the disclosure of which hereby is incorporated herein by 
reference in its entirety. 
Preferred derivatives of [2.2]paracyclophane include fluorinated 
[2.2]paracyclophane precursors. An illustrative chemical sequence for 
fluorinated [2.2]paracyclophane cracking and vapor-phase deposition of 
tetrafluoroparylene (parylene-F) film is shown below. 
##STR3## 
Using the liquid delivery approach, controlled delivery of precursor and 
fast growth rates of parylene films are readily achieved, as required for 
single wafer microelectronics applications. 
As an example of a specific embodiment of the method of the present 
invention, a tetrahydrofuran solution containing dimeric 
[2.2]paracyclophane is flowed at suitable flow rate to a stacked frit 
vaporizer/cracking unit including a vaporizer zone and a cracking zone. 
The solution is flash vaporized in the vaporizer zone of the 
vaporizer/cracking unit at a temperature of about 200.degree. C., followed 
by cracking the dimeric [2.2]paracyclophane into reactive monomer species 
in the cracking zone of the vaporizer/cracking unit at a temperature of 
about 700.degree. C. The cracked vapor comprising the monomer species then 
is flowed into a contacting chamber for contact of a cooled substrate 
therein, e.g., at a temperature of about -20.degree. C., with the reactive 
monomer species of the dimeric [2.2]paracyclophane. The contacting chamber 
is suitably maintained at a pressure of from about 4 to about 7 Torr, to 
produce condensation of the reactive monomer species and form a polymeric 
film of parylene on the substrate. 
It will be appreciated that the method of the invention may be widely 
varied in practice, as regards the pressures, temperatures, flow rates and 
precursor compositions employed. 
Referring now to the drawings, FIG. 1 is a schematic representation of an 
illustrative process system 10 for the liquid delivery, vaporization and 
transport of a parylene precursor, e.g., a THF solution of 
[2.2]paracyclophane, to form a low k dielectric film on a substrate 12 
mounted on a support structure 13 in the deposition chamber 14. 
In this system, the pump 16 is connected to the check valve 18 by a 
precursor solution flow line 20, which may for example comprise a length 
of Teflon.RTM. tubing. The check valve 18 is connected to the 
vaporizer/cracking unit 19 by a flow line 22, which may for example 
comprise a stainless steel capillary tube. A vapor flow line 26, e.g., a 
flexible stainless steel tube, connects the vaporizer to the deposition 
chamber 14, which may for example comprise a quartz bell-jar CVD reactor. 
The deposition chamber 14 is connected to a pressure gauge 30 by pressure 
tap line 32, to monitor the pressure in the deposition chamber during 
operation. The chamber 14 is also connected by line 34 containing flow 
control valve 36 to liquid nitrogen cold trap 38. The liquid nitrogen cold 
trap 38 is in turn connected by line 40 containing flow control valve 42 
to the vacuum pump 44. 
In operation, the entire system may be evacuated to low pressure, e.g., 0.8 
Torr, following which the vaporizer/cracking unit and the stainless steel 
tubing may be heated to elevated temperature. The vaporizer/cracking unit 
and lines may be heated for such purpose using tubular furnace and heating 
tape respectively, and the temperatures are measured and controlled by 
thermocouples and standard temperature controllers. 
The gas-phase deposition of parylene and/or similar polymeric films can 
therefore be achieved in the practice of the present invention via liquid 
delivery of an organic solution containing the solid dimer di-p-xylene, 
subsequent "flash" vaporization, "in-situ" pyrolytic "cracking" of the 
dimer to form the reactive monomer, condensation, and polymerization of 
the monomer to form polymeric films of parylene. 
The reaction scheme and product film of parylene-N are shown in FIG. 2, 
wherein the starting material [2.2]paracyclophane is shown being converted 
under elevated temperature cracking conditions to the corresponding 
paracyclophane monomer. The reactive monomer in turn, under low 
temperature condensation/polymerization conditions, yields a parylene-N 
film 50 being formed onto the substrate 52. 
In addition to [2.2]paracyclophane, compounds analogous to di-p-xylylene 
that are liquids at room temperature and that crack into the reactive 
monomer species may be deployed to form parylene polymer films in the 
broad practice of the present invention. 
The liquid source must be converted to a stable flow of p-xylylene monomer 
or radical species before introduction to the deposition chamber, and such 
conversion is readily achieved by the use of a vaporizer/cracking unit as 
described more fully hereinafter. 
There are several objectives that the vaporizer/cracking unit must meet. 
These are summarized as follows: 
1) The vaporizer/cracking unit must vaporize the incoming liquid feedstock 
and provide a constant and stable stream of vaporized dimer or other 
source of the monomer species. 
An organic solution containing the di-p-xylylene dimer, or a liquid source 
of an analogous compound that contains a p-xylylene unit, is injected into 
the vaporizer/cracking unit. Liquid flow rates for this purpose may for 
example be from about 0.01 ml/min to about 10 mL/min, in the 
vaporizer/cracking units hereafter described. The vaporizer/cracking unit 
must completely vaporize the liquid feed stream. 
Liquid delivery with flash vaporization provides a major advance over 
sublimation or bubbling because of the improved control over the feed rate 
of the source reagent and the resulting vapor concentration. Typically the 
vaporization process is carried out at 200.degree. C., and is often 
conducted with a carrier gas. 
Vapor pressures characteristic of representative source chemicals for the 
parylene-N film growth process are summarized in Table 1 below. 
TABLE 1 
______________________________________ 
Temperature Di-p-xylylene 
Liquid sources 
(degrees C.) (Torr) (Torr) 
______________________________________ 
50 .03 75 
75 .3 140 
100 3 200 
150 103 
______________________________________ 
2) The vaporizer/cracking unit must crack or pyrolyze the di-p-xylylene 
vapor stream into reactive p-xylylene monomer and/or radical species. 
A typical organic polymer cracking process will occur if the vapor 
temperature is between about 500.degree. C. and about 700.degree. C., with 
temperatures in the vicinity of 600.degree. C. being preferred. It is 
important to efficiently crack all of the dimer and to maintain a constant 
and controlled vapor stream of p-xylylene monomer. In order to ensure 
complete cracking of the vapor stream, the residence time of the vapor in 
the cracking zone must be maximized, and thermal gradients across the 
vapor stream must be minimized. 
The following techniques are usefully employed to increase residence time: 
a) The pressure in the cracking zone should be kept as high as possible 
within overall process constraints. By placement of a flow restriction 
element in the vapor flow path of the cracking zone, the average residence 
time that a molecule spends in the hot cracking zone is increased, 
compared to the case where no flow restriction element is present. This is 
due to the molecular density of the vapor in a fixed volume being a 
function of the pressure. Molecules enter and leave the fixed cracking 
volume at the same rate as determined by the system conductance and 
pumping speed, but at higher pressure there will be more molecules in the 
cracking zone. On average, a molecule will stay in the hold-up volume of 
the cracking zone longer, with increasing pressure of the hold-up volume. 
The pressure in the upstream vaporization zone must be below the vapor 
pressure of the precursor to prevent condensation, however. Vapor pressure 
of the monomer is assumed to be lower than the vapor pressure of the dimer 
and other source chemicals. Another pressure constraint is imposed by the 
organic solvent used in the precursor composition. The pressure in the 
cracking zone should be kept below approximately 100 Torr to prevent the 
formation of soot from decomposition of the solvent. 
Because the vapor pressure is a strong function of temperature, it is 
desirable to have the vaporization and cracking zones isolated from one 
another. For a controllable vaporization process, the temperature of the 
vaporization zone is preferably maintained at approximately 200.degree. C. 
This is much cooler than the cracking zone, which is preferably maintained 
at approximately 600.degree. C. If a higher temperature, such as the 
cracking temperature, is used in the vaporization zone, the vapor phase 
concentration of reactive monomer species would be less uniform in the 
system and vary more with time, because of the difficulty of maintaining 
constant conditions (temperature and pressure) at the vaporization 
surface. Providing proper temperature gradients in the system between the 
vaporization and cracking zones is as important as providing proper 
pressure gradients in the system. 
A further constraint on the cracking zone pressure is that the outlet 
pressure must be compatible with the film growth process pressure. In 
order to grow conformal coatings of parylene-N, the growth must be 
performed at vacuum pressures (typically on the order of 100 millitorr) 
with the substrate most preferably being within the temperature range of 
about -50.degree. C. to 25.degree. C. The vaporization/cracking unit must 
therefore provide an output in this pressure range. The vacuum character 
of the process facilitates the vaporization, but inhibits the cracking 
operation because of the decreased residence time. 
b) The fluid flow path length should be as long as possible in the cracking 
(high temperature) zone. 
The vaporizer/cracking unit should also be designed to provide a vapor 
stream with a uniform radial temperature gradient across the flow stream, 
to yield the most uniform generation of monomer or reactive species. To 
minimize thermal gradients, and hence improve control of the vapor stream 
composition, the following techniques may be implemented: 
vapor passages should be employed that have small effective diameters, 
e.g., of micron dimensions; 
turbulent flow should be induced; and 
mixing of the vapor stream should be induced. 
Two illustrative vaporizer/cracker unit designs based on flash vaporization 
technology are discussed hereinafter. Both of these designs employ a 
thermally conductive, high surface area element as a vaporization 
structure and as a flow restriction structure, to increase the residence 
time of the vapor in the cracking zone and to induce turbulence in the 
vapor stream. 
Although the ensuing discussion of porous media is directed to porous metal 
media formed by powder metallurgy, the porous media could be formed of 
materials such as micro-channeled media, wool, sponge, collapsed screen 
matrix elements formed from a metal, or any other high thermal 
conductivity material of suitable form. 
The first illustrative design of a precursor solution vaporizer/cracker 
unit 60 is shown in FIG. 3 and includes an array of porous metal disks 65 
placed in series along a tube 61. The disks create pressure drops, and 
induce localized turbulence as the vapor flows through the medium of each 
disk. The disks may have identical or graded porosity to permit management 
of the pressure profile. 
On the exit of the fluid from the porous medium, the flow will approximate 
plug flow; it will have a uniform velocity profile, and a more uniform 
thermal profile. The porous metal disks are placed close enough together 
to prevent the development of laminar flow following passage of the vapor 
through the porous metal membrane. The colder temperature and the shorter 
residence time present in the center of a laminar vapor flow stream would 
allow un-cracked dimer to pass. As a result, reactive monomer yield would 
be very low. The provision of a unit maintaining plug flow conditions in 
operation is therefore a very important aspect of the preferred practice 
of the present invention. 
In the FIG. 3 vaporizer/cracking unit, the containment tube 61 serves as a 
housing defining an interior fluid flow passage therethrough. A capillary 
tube 62 injects the liquid precursor source reagent onto a porous metal 
membrane element 64. The liquid may impinge directly or be sprayed onto 
the porous metal membrane element 64. 
A carrier gas inlet 63 communicates with the containment tube 61 to supply 
preheated carrier gas to the system. The porous metal membrane element 64 
has a high surface area from which the injected liquid can vaporize. This 
porous metal membrane element 64 is positioned at an area in the tube 
where the temperature is approximately 200.degree. C. 
A series of metal disks 65 are mounted in longitudinally spaced-apart 
relationship to one another along the length of the interior passage 
bounded by containment tube 61. The metal disks 65 cause localized 
turbulence, thereby enhancing thermal uniformity, reestablishing plug 
flow, and setting up pressure drops which increase residence time in the 
cracking zone. 
A smaller diameter tube 66 is provided at the discharge end of the main 
containment tube 61, as shown, for discharging cracked precursor vapor 
from the vaporizer/cracker unit 60. 
The main containment tube 61 may be disposed in a tube furnace 67 for 
maintaining suitable temperatures for the vaporization and cracking 
operations. 
In the FIG. 3 vaporizer/cracker unit, the temperature upstream of the 
porous metal membrane 64 may therefore be on the order of 200.degree. C. 
and the temperature downstream of the porous metal membrane 64 may be on 
the order of 600.degree. C. 
Another design of a vaporizer/cracker unit 70 is shown in FIG. 4. This 
design is different than the FIG. 3 design in that the vapor is forced to 
flow axially through the pores of a tubular porous metal membrane 
throughout the cracking zone. This arrangement helps to maintain 
turbulence, increase molecular collisions, improve heat transfer, and 
manage the pressure gradient. 
The FIG. 4 vaporizer/cracking unit 70 comprises a containment tube 71, and 
a capillary tube 72 that injects the source chemistry onto a porous metal 
membrane 74. A carrier gas inlet 73 supplies preheated carrier gas to the 
system. 
The porous metal membrane 74 provides a high surface area from which the 
injected liquid can vaporize. This membrane is positioned at an area in 
the tube where the temperature is approximately 200.degree. C. 
A coarse porous metal disk 75 is provided downstream of the porous metal 
membrane 74, and acts as a radiation shield. The system features a 
cylindrical porous metal medium 76 with a solid core 84 that is press fit 
into the containment tube. This medium 76 serves as the cracking zone for 
the vaporizer/cracking unit. 
The medium 76 includes thousands of interconnected capillary pores. As the 
vapor passes through these pores, it is heated to approximately 
600.degree. C. and cracked to the reactive monomer and/or radical species. 
The large surface area to volume ratio of this medium 76 maximizes heat 
transfer from the metal walls to the gas. Since the distance that energy 
has to travel through the porous medium is small, the porous material 
temperature stays uniform. 
The porous medium 76 may be of any suitable material of construction, e.g., 
stainless steel, and may be fabricated by sintered metal or powdered metal 
techniques to provide a porous matrix element having suitable porosity, 
tortuosity and void volume characteristics. In like manner, the disk 
elements and membrane elements of the vaporizer/cracking units may be 
formed of any suitable conductive metal and by any appropriate processing 
techniques. 
Using the porous medium 76 as illustratively shown in FIG. 4 also provides 
a gradual pressure drop from inlet to outlet. At the entrance to the tube, 
the pressure is high and residence times are long. The pressure gradually 
reduces as the vapor passes through the porous medium. This gradual 
pressure reduction is preferred over a more sudden pressure drop such as 
that occurring for example across an orifice. As a gas expands it must 
absorb some amount of heat to maintain its temperature, and even more to 
increase in temperature. By flowing the vapor through a continuous, 
conductive, "heating" medium, energy is efficiently transferred to the 
vapor and the added energy requirements associated with the gas expansion 
can be readily supplied. 
The cracking section of the vaporizer embodiment shown in FIG. 4 may be 
fabricated by pressing a metal core 84 into a commercially available 
porous metal cylinder 76, and in turn pressing this subassembly into a 
correspondingly-sized bore in a containment tube 71. These components of 
the cracking assembly are desirably chosen and fabricated so that the core 
element and exterior tube maintain intimate contact with the porous 
cylinder and seal the pores on the interior and exterior surfaces of the 
porous tube even at high temperatures. Preferred materials of construction 
for the porous cylinder 76 and the other porous elements in the 
vaporizer/cracking unit include 316L stainless steel, nickel, and 
tantalum-plated stainless steel. The porosity of the porous metal cylinder 
76 is chosen so as to give appropriate pressure drops under applicable 
process flow conditions and in turn to optimize precursor residence times 
and maximize the cracking efficiency. 
The cross-section 80 of a region in the cracking zone is shown in FIG. 5, 
as including the solid core 82 surrounded by the porous metal cylinder 76 
and the outer containment tube 71. The cylindrical porous metal medium 
therefore serves as a high efficiency cracking zone. As the vapor passes 
through its pores, the vapor is heated and cracked to the reactive monomer 
or radical species. 
In one specific illustrative embodiment, for the cracking of precursor 
vapor for the deposition of parylene-N polymeric films, the porous 
cylinder 76 may be fabricated from a 100 micron pore size porous 316L 
stainless steel medium, and may have a 0.63 inch outside diameter and a 
0.48 inch inside diameter, with a length of approximately 2 inches. 
FIG. 6 shows a plot of the approximate pressure drop expected across such a 
porous cylindrical medium as a function of the length of the cylindrical 
medium, for a representative parylene-N film growth. The plotted points 
are based on conductance measurements made with a 1/16" thick porous 
stainless steel disk with a porosity of 100 microns, and a gas temperature 
of 200.degree. C. This plot ignores the effects of the expansion of the 
vapor caused by heating and entrance and exit effects, but it gives a 
rough estimate of the pressure profile. 
The vaporizer/cracking unit designs described with reference to FIGS. 3-5 
represent compact, inexpensive, efficient configurations for generating 
vapor streams of parylene monomer or reactive species required to produce 
conformal parylene-N thin films suitable for interlayer dielectrics. These 
designs achieve effective control of vapor phase concentrations, good 
vapor phase uniformity of reactive species, and efficient heating of the 
vapor as required to achieve cracking. They also represent small equipment 
footprints and excellent energy efficiency. 
The features and advantages of the present invention are more fully shown 
by the following non-limiting examples wherein all parts and percentages 
are by weight, unless otherwise expressly stated. 
EXAMPLE 1 
In a specific operation of a system of the type shown and described with 
reference to FIG. 1 hereof, the transport and vaporization of the 
[2.2]paracyclophane were examined using the vaporizer/cracking unit via 
liquid delivery. Silicon wafers were used to collect the condensed 
effluent from the vaporizer for infrared (IR) spectral analysis to 
determine the identity of the condensate. 
The precursor solution included [2.2]paracyclophane dissolved in THF at 
room temperature with a saturated solution concentration of 0.038 M; 20 mL 
of this precursor solution at room temperature was pumped into a standard 
frit-type vaporizer/cracker (heated to 150.degree. C.) at the rate of 0.5 
mL/min. The system pressure increased to .about.2 Torr from 0.8 Torr 
during the run. The liquid nitrogen cold trap 38 positioned upstream of 
the vacuum pump 44 collected the transported materials. After the 
experiment was completed, a white, solid deposit was observed on the walls 
of the quartz reactor and the surface of the silicon wafer. 
To determine the identity of the condensed white deposit, infrared (IR) 
spectroscopy was used to analyze the solid. The white condensate on the 
silicon wafer was analyzed by FTIR versus a blanket silicon wafer used as 
a reference. FIG. 7 shows FTIR spectra obtained for (top spectrum) 
[2.2]paracyclophane starting material, (middle spectrum) condensed 
[2.2]paracyclophane after thermal transport of a THF solution at 
150.degree. C., and (bottom spectrum) THF solvent used in the preparation 
of the solution for liquid delivery. 
From these FTIR data, it was concluded that the white solid collected on 
the silicon wafer during this experiment was [2.2]paracyclophane. It was 
clean and free of significant contamination, indicative of effective 
transport and vaporization of this precursor using the liquid delivery 
approach. 
No detectable incorporation of THF was observed in the white solid 
condensed on the silicon wafer, based on the FTIR comparison with the THF 
solvent. It appeared that the solvent was pumped away completely during 
the transport process of the [2.2]paracyclophane. That is, no solvent 
contamination was present in the condensed solid film (THF was not 
detected). 
EXAMPLE 2 
In a system of the general type shown in FIG. 1 hereof, a THF 
solution-containing [2.2]paracyclophane was flowed to a stacked frit 
vaporizer/cracking unit. The thermal "cracking" zone was held at 
700.degree. C., and the vaporizer temperature was approximately 
200.degree. C. The flow rate was 0.5 mL/min, and the solution 
concentration was 0.038 M. The cracked precursor vapor then was contacted 
with a cooled (-20.degree. C.) substrate surface for film deposition. 
The deposition chamber pressure varied between 4 and 7 Torr during the run. 
30 mL of the solution was pumped to the "vaporizer/cracking" zone, and the 
exiting vapor was condensed onto a silicon wafer at -20.degree. C. The 
substrate was removed from the reactor. Cleavage of the substrate provided 
visual confirmation that a polymeric film was formed on the substrate. 
Further, some of the film could be peeled from the substrate surface. The 
film on the silicon wafer was analyzed by FTIR spectroscopy. 
FIG. 8 shows the FTIR spectrum obtained for the resulting thin film 
deposited from the THF solution of [2.2]paracyclophane after thermal 
transport, in-situ "cracking" at 700.degree. C. and 
condensation/polymerization of the reactants at -20.degree. C. The 
spectrum showed the characteristic IR absorption bands of parylene-N 
polymer. 
These results demonstrate the ability of the parylene film formation 
process of the present invention to form polymeric material on a substrate 
upon condensation/polymerization of the cracked precursor vapor comprising 
the p-xylylene monomer. 
The process of the invention while particularly usefully employed for 
formation of parylene films may also be utilized to form other low k 
dielectric film coatings on substrates, by a corresponding methodology, 
viz., of liquid delivery, vaporization, in-situ "pyrolysis" of the 
reactant to provide reactive species for subsequent polymerization, and 
deposition and formation of the low k dielectric polymer on the substrate 
(e.g., an integrated circuit or other microelectronic device structure). 
Accordingly, while the invention has been shown and described herein with 
reference to various illustrative features, aspects and embodiments, it 
will be appreciated that the invention is susceptible to being embodied in 
other forms and variations, as will readily suggest themselves to those of 
ordinary skill in the art, based on the disclosure herein. The invention 
therefore is to be construed and interpreted as including within its 
spirit and scope all such alternative forms, variations, features, aspects 
and embodiments.