Printed circuit manufacture employing a radiation cross-linkable photo-polymer system

In order to achieve a low dielectric constant and in order to improve the constant temperature resistance of radiation-sensitive synthetic resin lacquers, a photo-polymer system which is composed of a furyl acrylic acid esterified epoxy resin with phenoxy or epoxy end groups is employed as coating (2, 13, 4) on a substrate in the manufacture of printed circuits, particularly in multi-layer format. The cross-linking ensues with light in the wavelength range from about 150 through 400 nm preferably in the presence of a sensitizer without any following hot-hardening. As a consequence of its good solubility, the product can be easily processed and requires no intermediate layers when in a multi-layer format. A further area of employment lies in the field of integrated semiconductor circuits in VLSI technology when producing negative photo-resists.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention lies in the field of methods for the manufacture of printed 
circuits, particularly in a multi-layered format, which employ 
photo-sensitive polymer systems that are cross-linkable by radiation. 
2. Prior Art 
Methods for the manufacture printed circuits using radiation cross-linkable 
photo-polymer systems have been proposed in the German Patent applications 
P 34 24 119.1 corresponding to USSN 749,588 filed June 27, 1985 and P 34 
39 606.9 corresponding to USSN 762,513 filed Aug. 5, 1985 now U.S. Pat. 
No. 4,732,843, respectively. In the method described by German patent 
application P 34 24 119.1, cinnamic acid-epichlorohydrin-bisphenol A 
compounds which contain reactive hydroxy or hydroxymethylene groups at the 
bisphenol building block are employed as initial products for the 
polymerization in order to shorten the exposure time of 
radiation-sensitive enamels. The thermal loadability of products of these 
compounds is in fact extremely high, but such products when 
homopolymerized display only slight solubilities in standard organic 
solvents, such solubilities requiring an involved processing. Also, 
disadvantageous herein is a relatively low storage stability for the 
unexposed materials. 
In the method described in patent application P 34 29606.9, polymer systems 
on an enamel base are employed both in order to achieve a low dielectric 
constant and in order to improve the constant temperature stability. In 
these polymer systems, linear fluoro-polymers having at least two reactive 
end groups per polymer molecule are employed as initial substances for 
conversion with radiation-sensitive substances. The conversion of the 
fluorinated initial substances and the linking thereof to the photo-active 
substances occurs in several method steps and in an involved way. 
Particularly due to the typically partly fluorinated solvent being 
employed, such method step combination also specifically requires involved 
processing technology in the subsequent multi-layer format. 
BRIEF SUMMARY OF THE INVENTION 
More particularly, the present invention relates to a method for the 
manufacture of printed circuits, particularly in multi-layer format, as 
such are employed in micro-electronics, wherein 
(a) a photo-sensitive-polymer system which cross-links in response to 
applied radiation is coated as a layer upon a substrate, particularly a 
substrate comprising a metal carrier foil; and 
(b) the desired wiring structure in such layer is produced by light 
imaging; and thereafter 
(c) the unexposed portions of the imaged photo-polymer system are dissolved 
away. 
The photo-sensitive-polymer system employed is a furyl acrylic 
acid-esterified epoxy resin. The exposed substrate metal (resulting from 
the layer imaging and dissolving) may be reinforced by metallizing, such 
as by electro-plating or by current-less (electroless) metalization. Also, 
further layers over a desired wiring structure may be generated in the 
same fashion by the same steps of coating, structuring, and metalization. 
A principal object of the present invention is to provide a method for the 
manufacture of printed circuits, preferably in multi-layer format, for 
micro-electronic components, wherein the method utilizes simple steps and 
results in a product which, due to the employment of the chosen 
photo-polymer, can forego the addition of a hardening agent. Thus, one can 
forego a process step of hot-hardening, or, alternatively, of tempering 
the exposed substrate. The method should also be employable for use in the 
manufacture of integrated semiconductor circuits in VLSI (i.e., very large 
scale integration) technology. 
This object is achieved by the provision of a method employing as indicated 
a photo-polymer which is a furyl acrylic acid-esterified epoxy resin with 
phenoxy end groups or epoxy end groups. Such resin preferably has a 
molecular weight of less than about 15,000. A cross-linking of such resin 
is implemented with light radiation preferably having a wavelength in the 
range from about 150 through 400 nm in the presence of a photo-sensitizer 
without subsequent hot-hardening. More preferably, the light radiation 
wavelength ranges from about 300 to 400 nm. 
Another object of the present invention is to provide laminate structures 
containing at least two contiguous layers one layer of which is comprised 
of an electrically conductive etchable metal, and the second layer of 
which is comprised of a photo-sensitive polymer that is irradition 
cross-linkable, such laminate structures having been fabricated by the 
method provided herein using the photo-polymer taught herein. 
Other and further objects, aims, purposes, features, advantages, 
embodiments, applications, and the like will be apparent to those skilled 
in the art from the teachings of the present specifications taken with the 
accompanying drawings.

DETAILED DESCRIPTION 
The photo-polymer systems employed in the present invention when employed 
as a coated layer upon a metal substrate achieve a low dielectric constant 
and display a constant temperature resistance as regards radiation 
sensitive synthetic resin lacquers. 
It lies within the scope of the present invention to add sensitizers and 
photo-initiators, as well as co-sensitizers, to the light-sensitive 
polymer in total concentrations ranging from about 0.5 through 5 weight 
percent (based on total photo-polymer composition). 
Derivatives of benzophenone, benzoin or acetophenone preferably come into 
consideration as sensitizers or photoinitiators. Tertiary amines can be 
employed as co-sensitizers. 
The employment of reactive diluants, preferably on an acrylate base and in 
a concentration range of from about 5 through 30 weight percent based on 
total photo-polymer composition also lies within the scope of the present 
invention. 
The conversion and cross-linking of a furyl acrylic acid-esterified epoxy 
resin occurs in the following way: An exemplary starting epoxy resin (for 
example, Araldit.sub.tm Gt 6099) is characterized by the structure: 
##STR1## 
Similarly, an exemplary starting light sensitive group containing 
esterification agent is a furyl acrylic acid chloride, such as 
3-(2-furyl)-acrylic acid chloride. The reaction can be represented as 
follows: 
##STR2## 
where: R is 
##STR3## 
R'is 
##STR4## 
Upon irradiation with UV-light, a cross-linking of the light-sensitive 
polymer occurs in a known way due to reaction of the acrylic 
##STR5## 
double bonding. 
Phenoxy end groups can also be present instead of the epoxy end groups, as 
just shown. 
A photo-polymerizable epoxy resin is in fact known from German OS No. 26 35 
929; however, a hot-hardening format agent is required therein for 
cross-linking. The multi-layer formatting ensues by means of pressing. 
A printed circuit having light-sensitive epoxy resin groups can also be 
derived from German patent No. 23 42 402; however, the cross-linking 
therein also occurs upon employment of a hardener acting under heat. 
Moreover, the multi-layer circuit therein is manufactured by pressing. 
German patent No. 24 08 893 contains a radiation-hardenable mask based upon 
polycarboxylic acid-esterified epoxy compound. Such a hardenable compound 
is not employed for structure generation, and, thus, for the manufacture 
of printed circuits. Rather, such is employed as a paint material and for 
producing printing inks. 
Light-sensitive epoxy cinnamates, or, alternatively, phenoxy acrylates 
suitable for the manufacture of planographic printing plates and copying 
layer carriers are known from German OS No. 25 03 526. The emphasis 
therein, however, is placed on the production of high molecular weights in 
the base polymers (at least about 15,000 to 20,000) and on the omission of 
sensitizing agents. Reactive diluents are not provided. The utilization 
made of these products is of a completely different character than that 
involved in the present invention. 
The photo-polymer used in the invention yields a product which exhibits the 
following properties and advantages over known products: 
(1) The dielectric constant .epsilon..sub..tau. is lower than 3.5; this 
enables low interconnect spacings; 
(2) The glass transition temperature of the cross-linked homopolymer 
already lies about 170.degree. C.; a high temperature stability derives 
therefrom; 
(3) A high UV sensitivity derives, which is about 5 to 6 times better than 
is achieved from utilizing the corresponding cinnamic acid compounds; 
(4) The addition of reactive diluents allows the setting of predeterminable 
optimum values for the respective properties identified about under (1) 
through (3); 
(5) The phenoxy resin product contains no hydrolyzable chloride; 
E-corrosion phenomena (aging and climate test) typical for epoxy resins 
are thus avoided; 
(6) The good solubility in organic solvents enables simple processing; and 
(7) Intermediate layers are no longer required given multi-layer 
structures. 
EMBODIMENTS 
As is apparent from the foregoing specification, the invention is 
susceptible of being embodied with various alterations and modifications 
which may differ particularly from those that have been described in the 
preceding specification and description. For this reason, it is to be 
fully understood that all of the foregoing is intended to be merely 
illustrative and is not to be construed or interpreted as being 
restrictive or otherwise limiting of the present invention, excepting as 
it is set forth and defined in the hereto-appended claims. 
The application of the photo-polymer system used in the present invention 
for the manufacture of a multi-layered wiring structure is described below 
with reference to an exemplary embodiment. 
Referring to FIG. 1, there is seen a copper foil which is employed as a 
carrier, to which a layer 2 of a photo-cross-linkable insulative polymer 
system as taught in the present invention is applied by immersion or by 
spray lacquering, for example, to achieve a (dry) layer thickness of from 
about 5 through 20 .mu.m. Before such application, about 1.5 weight 
percent (on a total composition weight basis), for example, of a 
sensitizer, such as, for example, Michler's ketone, is admixed with such 
polymer system. Such layer 2 is exposed (imaged) and developed with UV 
radiation in such a manner that the particular plate-throughs to be 
inserted thereinto, that is, the terminal points of the product chip, 
arise as plate-through passages 3 positioned in the insulating layer 2. 
The irradiation is accomplished in a contact or projection method upon 
employment of a mask (not shown) which covers the region of the passages 3 
of the layer 2 (negative resist). The covered parts or passages 3 are 
subsequently dissolved out with a selected organic solvent (preferably 
aromatic), such as, for example, toluol, xylol; a ketone, such as 
methylethyl ketone, an ester, such as 2-ethoxy-ethyl-acetate, a 
chlorinated hydrocarbon, such as trichloroethylene, or the like. A 
chemical cross-linking occurs in the exposed parts 13 of the layer 2 which 
prevents the dissolution of these parts 13 by such solvent action so such 
parts 13 remain as an insulating layer designated 13 as a whole. After the 
manufacture of the passages 3 for the platethroughs allocated to this 
first insulator layer 2 (13), the passages 3 are filled up with a 
material, such as, for example, copper, having good electrical 
conductivity, such filling being accomplished in conjunction with the 
copper foil 1 serving as carrier by means of electro-depositing 
metalization (identified as plate-throughs 23 in FIG. 2). 
Referring to FIG. 2, it is seen that a further layer 4 of the 
photo-cross-linkable insulative polymer system of the invention is 
subsequently applied to the imaged and developed insulating layer 13 
containing the plate-throughs 23 in the same fashion as described above in 
reference to FIG. 1. Interconnect recesses 5 and further plate-through 
passages 6 are similarly (as described in FIG. 1) inserted into this 
further layer 4. The exposure and developing of the layer 4 likewise is 
accomplished in the same manner as described above in reference to FIG. 1. 
In addition to the plate-through passages 3 of the first insulator layer 2 
(13), trough-shaped recesses 5 for further desired interconnects are 
generated in the second insulator layer 4. These recesses 5 are arranged 
so that at least one plate-through 23 of the first insulator layer (2, 13) 
projects into a recess 5 or layer 4. For building up of the interlayer 
interconnects, the recesses 5 are provided with a metallization by means 
of electroplating. Further plate-throughs and interconnects can then be 
applied by means of an appropriate repetition of the manufacturing steps 
set forth above. The addition of a hardening agent of the polymer system, 
and, thus, a curing process, can be omitted. The irradiation (imaging) 
with UV-light in the presence of a photo-sensitizer leads to an adequate 
cross-linking, or, alternatively, hardening of the polymer. 
The wiring format does not require any copper intermediate layers. The high 
thermal loadability which is required is established. Processing with 
known resist techniques is not problemmatical. Thus, not only is the 
method for manufacturing such a structure simplified, but also the 
reliability of the electrical characteristics of the incorporated circuit 
is also improved. 
For this reason, and also because of the very low Dk and the good solvent 
dissolution, the polymer system of the invention is excellently well 
suited for use as a high-temperature-resistant negative resist in the 
manufacture of integrated semiconductor circuits in VLSI technology 
wherein the formation of dimensionally true microstructures, or, 
alternatively, patterns is of great significance. 
Although the teachings of our invention have herein been discussed with 
reference to specific embodiments, it is to be understood that these are 
by way of illustration only and that others may wish to utilize our 
invention in different designs or applications. 
1. Preparation of furylacrylic acid 1 
Close to the instructions of J. Johnson.sup.(a) 144 g freshly distilled 
furfural (1.5 mol), 230 g acetic anhydride (2.25 mol) and 147 g potassium 
acetate (1.5 mol) are placed into a 2-1 round-bottomed flask fitted with a 
reflux condenser. 
The flask is heated for 4 h at 150.degree. C. with stirring. Subsequently 
the reaction mixture is cooled to 70.degree. C., diluted with 1,5 l of 
water and after adding 20 g of activated charcoal boiled for another 10 
min. at 90.degree. C. 
The solution is filtered while still being hot with suction to avoid early 
precipitation of furylacrylic acid. 
The filtrate is acidified to Congo red by addition of a 1:1-solution of 
concentrated hydrochloric acid in water (change in colour from red to 
blue). 
After being cooled to 20.degree. C., preferable with stirring , the acid is 
filtered with suction and washed a few times with ice water. One obtains 
greenish (traces of indicator), after boiling with activated charcoal 
white crystals, which are dried at room temperature and high vacuum. 
The yield is 109.4 g (53%). The acid 1 melts at 137.degree. C. 
2. Preparation of 3-(2-furyl)-acrylic acid chloride 2 
By modification of the instructions of T. Sasaki.sup.(b) 88 g (0.64 mol) of 
furylacrylic acid 1 are dissolved in 250 ml of benzene to avoid the 
exothermic reaction when adding thionyl chloride (.apprxeq.150 g; 1.25 
mol) drop by drop in excess. 
The reaction mixture is subsequently heated on a boiling water bath for 2 h 
(reflux condenser). 
Benzene and excessive thionyl chloride are removed by a water jet pump and 
the residue is distilled at 2-10 torr and 80.degree. C.-105.degree. C. 
The brown-yellow distillation product unfortunately becomes rigid in the 
cooler and sometimes offers trouble by clogging the cooler and has to be 
brought into the flask with a heat gun. The pungent smelling crystals are 
soon getting darker at the air and therefore should be stored in the 
absence of light and air. 
The yield is 83.5 g (83.7%). The white, crystalline chloride 2 melts at 
33.degree. C. 
3. Preparation of the furylacrylic acid esterified epoxy resin 3 
10 g unmodified epoxy resin Araldit.sub.TM GT6099, 2.multidot.10.sup.-3 
mol) are placed in a 3-1 three-necked flask with a reflux condenser and 50 
ml of toluene are added. The resin does not dissolve in toluene. Now 11.67 
g (7.multidot.10.sup.-2 mol) liquefied chloride 2, corresponding to a 
double excess, are filled into the flask and stirring is started. During 
heating the temperature in the flask must remain between 60.degree. C. and 
70.degree. C. 
Start of reaction is indicated by evaporation of gaseous hydrochloric acid 
and temperature-rise. 
Moreover the epoxy resin dissolves more and more. The reaction is 
additionally held 3 h at 110.degree. C., and finished when all of the 
epoxy resin is dissolved. 
After the hydrochloric acid has escaped completely the solution reacts 
neutral to lacmus. 
The cooled reaction product is added dropwise to a 3-1 flask filled with 
isopropanol, where it precipitates as a white or grey agglutinating 
thread. 
The pasty product is purified in an ultrasonic bath with boiling water and 
afterwards dissolved in 1 1 methylethylketone while stirred thoroughly. 
Once again it is precipitated in isopropanol, which then is removed by a 
water jet pump. 
After drying at 30.degree. C.-40.degree. C. in high vacuum one obtains a 
white powder. 
The yield is 11.31 g (82.7%). 
(a) J. Johnson, Organic Synthesis, Col. Vol 3, E. C. Horning, Ed., Wiley, 
New York 1962, S. 426 
(b) T. Sasaki Biochem. Z. 25, 275 (1910) 
Working example 
The furylacrylic acid esterified epoxy resin is dissolved in xylene to give 
a 30% (weight) solution, 2% (weight, referring to the resin) of Michler's 
Ketone is added. The solution is spin-coated to an electropolished 
copperfoil. Within one step of coating layer thicknesses between 3 .mu.m 
and 10 .mu.m can be achieved (4000 rpm to 1500 rpm, respectively). The 
thickness may be increased to 50 .mu.m by repetitive spin-coating. 
Prebake: typical 2 h 60.degree. C. 
Exposure: for 4 .mu.m layer 10 sec., 25 mW/cm.sup.2, 365 nm probe, soft 
contact. 
Development: spraying or dipping with toluene, max. 60 sec stopped by 
propanol-2 
Postbake: 30 min., 140.degree. C. 
Resolution: better than 10 .mu.m 
Working Example 
Physical properties of cured furylacrylic acid esterified epoxy resin: 
Glass transition temperature Tg: 112.degree. C. 
Dielectric constant .epsilon.r (25.degree. C., 100 kHz): 3.09 
dielectric loss factor tan .delta. (25.degree. C., 100 kHz)&lt;0.01 
Mixtures: 
(i) with 1.5% (weight) pentaerithritoltriacrylate Tg: 124.degree. C. 
(ii) with 2% (weight) pentaerithritoltetraacrylate Tg: 164.degree. C. 
.epsilon.r and tan .delta. don't change within the limits of experimental 
error.