Method of making adhesive metal layers on substrates of synthetic material and device produced thereby

A method of improving the adhesion between a synthetic substrate and metallized layers deposited thereon. A glass resin layer is spin-coated onto an epoxide substrate. The glass layer is covered by a photoresist layer which is roughened by reactive ion etching. The roughened contour of the photoresist layer is transferred via reactive ion etching to form a perforation pattern in the glass layer. The substrate is then etched vertically and horizontally to produce recesses in the substrate having overhanging walls. A thin copper layer is sputtered onto the substrate and copper conductors are sputtered onto the thin copper layer, the copper layers filling the recesses. The recesses and overhangs form mortices in the substrate, and the copper layers within the recess form tenons which fittingly engage with the mortices to produce adhesion between the substrate and the metallized layers in the order of 1000 n/m.

TECHNICAL FIELD 
The invention relates generally to a method of forming adhesive metal 
layers upon substrates of synthetic material by structuring the substrate 
surfaces to be coated prior to metallization, and more specifically to a 
method of forming metallic conductors on epoxide resin substrates. 
BACKGROUND ART 
Circuit cards with conductors printed thereon have been manufactured by use 
of a subtractive method. In the subtractive method, copper foil is 
laminated onto a suitable substrate made of a synthetic material, and a 
photoresist is applied onto the copper foil. The photoresist is exposed 
and developed to produce the desired pattern of conductors by etching 
excess copper from the surface of the circuit card. As a result of this 
method, copper conductors can be made having widths larger than the 
thickness of the copper material laminated onto the circuit card. However, 
in view of the increasing pattern densities of circuit cards and the 
decreasing widths of the conductors currently used in the art, the 
subtractive method can no longer be utilized (owing to the lateral 
underetching of the photoresist masks). 
With the ever increasing demand for the production of thin conductors, a 
number of additive metal deposition methods have been developed. In such 
an additive method, a thin copper foil is laminated onto the surfaces of 
of synthetic substrates. Instead of the unwanted copper being etched off, 
conductors are grown on the regions of the copper laminate which are not 
covered by a photoresist mask layer. Subsequently, the surplus copper of 
the laminated foil is etched off. The main problem inherent in such 
additive methods is providing a solid and reliable adhesion between the 
conductors applied by electroless deposition and the substrate of the 
circuit card. 
Several adhesion-promoting methods known in the art will now be discussed 
in more detail. 
One of the methods known in the art for increasing the adhesion between the 
conductive metal layers (which are applied by electroless deposition) and 
the substrate involves roughening the surface of the substrate to be 
coated by use of an abrasion process. Specifically, this method involves 
the steps of impressing a relief on a roughened surface; swelling and 
roughening the surface by means of acids, bases or solvents; using 
adhesion-promoting intermediate layers and embedding in the adhesion 
promoter foreign substances removable by means of acids or bases; or vapor 
depositing adhesion-promoting intermediate layers. It is also known (e.g. 
from German Offenlegungsschrift No. 24 45 803) to prepare the entire 
surface of a carrier plate by intensive, even repeated wet sand blasting. 
German Offenlegungsschrift No. 24 25 223 discloses another method for 
improving the adhesion of metallic layers on the surface of a synthetic 
substrate. A solution of zinc oxide, copper (II) oxide, and sodium 
hydroxide is deposited on an aluminum foil to form a micro-nodular 
surface. The aluminum foil is then applied to the surface of the synthetic 
substrate. The aluminum foil and the zinc are removed in an etch bath, and 
copper conductors are applied by electroless plating onto the roughened 
surface of the synthetic substrate. 
German Auslegeschrift No. 27 13 391 teaches a method for making a carrier 
material for printed circuits. In this method, the carrier material is 
coated with a thin copper layer and is transported through one or several 
roller pairs. A slurry is applied to the rollers or to the surface of the 
copper layer. The slurry contains quartz powder, glass powder or similar 
material. The rollers cause the surface of the copper layer to become 
micro-roughened by pressing the quartz or dust particles into the copper 
layer. A covering layer is then applied to the micro-roughened surface of 
the copper layer, and conductors are provided by metal deposition onto 
those areas of the copper layer not coated by the covering layer. 
In more recent methods for making printed circuits, the copper is applied 
by vapor deposition in a vacuum or by sputtering onto laminates of 
synthetic material. These methods have not been used widely because the 
resulting thin copper layer has a relatively poor adhesion to the 
synthetic material laminate. Previously known roughening methods, such as 
those discussed above (as well as the roughening of epoxide resin surfaces 
in an oxygen plasma or the sand blasting process described in IBM 
Technical Disclosure Bulletin Vol. 25, No. 5, October 1982, p. 2339 by H. 
Meuller, J. Schneider, and F. Schwerdt), do not produce sufficient results 
because during the roughening process the thickness of the entire epoxide 
resin layer is reduced to a point where the embedded glass fiber fabric is 
adversely affected. 
Finally, German Offenlegungsschrift No. 29 16 006 describes a method of 
making adhesive metallic layers on non-conductive surfaces, in particular 
on surfaces of synthetic material, wherein the surface regions to be 
coated are roughened by means of etching and are exposed prior to the 
etching process to a source of high energy radiation. 
Thus, a need has arisen in the art for forming metallic layers on a 
non-conductive surface such as a synthetic material in which the metallic 
layers have a high adhesion characteristic (in the order of 1000 n/m) with 
respect to the non-conductive surface. As discussed above, the methods 
currently used in the art for increasing adhesion between a metallic layer 
and a synthetic substrate are unatisfactory where the metallic layer is 
either sputtered or deposited onto the substrate. Further, such adhesions 
have proved to be insufficient where the composite structure is to undergo 
further metallization steps in which thermal strains are applied. 
Moreover, the above-discussed bonding materials and adhesion promoters are 
expensive, and their uniform application involves considerable effort. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved method for forming 
metallic layers with high adhesion characteristics on a non-conductive 
surface. 
Another object of this invention is to provide an improved device having 
metallic layers with high adhesion chracteristics on an non-conductive 
surface. 
It is an object of the invention to provide a method for generating 
discrete recesses on the surfaces of substrates made of synthetic 
material, said recesses forming a foundation for subsequent metallization 
processes. 
The foregoing and other objects are realized by the manufacturing method of 
the present invention, wherein recesses with overhangs are produced in the 
surface of a synthetic substrate prior to metallization. A glass resin 
layer, preferably 0.2 m in thickness, is spin-coated and cured on the 
surface of a synthetic substrate to be metallized. A photo- or electron 
beam resist is applied in a layer thickness between 1.0 to 4.0 .mu.m onto 
the glass resin layer. The assembly is then heated to a temperature of 
80.degree. to 90.degree. C. in order to remove at least a portion of the 
solvent from the resist. The resist layer is then roughened by reactive 
ion etching in a CF.sub.4 plasma. Secondary electron microscope (SEM) 
recordings of the thus roughened photoresist show that the roughness 
structures and between 0.1 and 3 .mu.m in depth. In the next step, and 
also by reactive ion etching in a CF.sub.4 plasma, this roughness contour 
is transferred to the glass resin layer in such a manner that a 
perforation pattern corresponding to the photoresist roughness contour is 
formed therein. The thus obtained glass resin matrix is used as a mask for 
a subsequent reactive oxygen etching process which is executed in the same 
reactor. First, recesses of approximately 2 .mu.m in depth are etched with 
an oxygen plasma through the glass resin matrix into the synthetic 
substrate. After the pressure of the oxygen plasma has been fixed, these 
recesses are recessed still further, creating an overhang structure within 
the substrate. The glass resin layer is then removed and a thin copper 
layer of approximately 0.2 to 0.5 .mu.m in thickness is applied by 
sputtering. Following the application of a photoresist mask layer, a 
conductive pattern is deposited on the thin copper layer through 
electroless deposition by means of an additive process. The thickness of 
the copper layer applied by electroless deposition is approximately 35 
.mu.m. The surplus copper applied by sputtering is etched off after the 
removal of the photoresist mask. In the same manner, a copper layer can be 
applied instead of conductive patterns. 
By use of the manufacturing method of the present invention, discrete 
recesses with overhangs are produced in a synthetic substrate, forming 
discrete mortises therein. When subsequent metallized layers are deposited 
onto the synthetic substrate, the metal fills the mortises to form 
discrete metallic tenons. The fitting engagement between the discrete 
mortises in the synthetic substrate and the discrete metallic tenons 
projecting from the metallized layers greatly enhances the adhesion 
between the substrate and the metallized layers. More specifically, 
adhesions in the order of 1000 n/m can be reached by use of the 
manufacturing method of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIGS. 1A to 1E show the transfer of the photoresist material roughness into 
a subtrate of epoxide resin by means of reactive ion etching, using an 
etching barrier of spun-on glass resin. This structured substrate surface 
ensures a particularly high adhesion of metal on substrate. 
In FIG. 1A, a substrate material is used which is made up of a glass fabric 
which is drawn through an epoxide resin solution. The resin-impregnated 
glass fabric is subsequently cured to a predetermined degree under the 
influence of heat (i.e., the epoxide resin portion of the glass fabric is 
partially polymerized up to a predetermined state of partial curing). The 
partialy cured substrate material (hereinafter referred to as a "prepreg" 
in the B-state) is cut into plates of a predetermined size. The plates are 
fitted with separating foils and are assembled into stacks. The stacked 
plates are then fully cured in a laminating press at temperatures of 
130.degree. to 180.degree. C. and pressures of 500 to 2000 N/cm.sup.2. 
Additional and specific details concerning the substrate material 
processing, and the composition of a typical epoxide resin, are disclosed 
in U.S. Pat. No. 3,523,037. 
A glass resin layer 2 is deposited on the epoxide material 1 by spin-on or 
immersion. In the preferred embodiment, the resin is spin-coated onto a 
spinning substrate 1. The rotation speed of substrate 1 is approximately 
4200 rpm. Layer 2 preferably consists of a 0.2 .mu.m thick layer of 
polydimethylsiloxane resin (which is produced by Owens, Ill. under the 
designation 650), and dissolved in N-butylacetate, with 1 g resin to 10 ml 
solvent. The polydimethylsiloxane resin 2 is cured in nitrogen at a 
temperature of approximately 120.degree. to 140.degree. C. for 10 to 15 
minutes. A layer 3 consisting of a photoresist or an electron beam resist 
is spin-coated on top of resin 2. Layer 2 can be prepared to receive the 
photoresist or electron beam resist material by means of hexamethyl 
disilazane, or the A-1100 silane marketed by Union Carbide Corporation. 
Layer 3 can be composed of any kind of resist materials which are used for 
coating and which adhere well to glass resin layers. Furthermore, the 
resist materials must be thermally stable and removable via reactive ion 
etching. The preferred photoresist material consists of a 
phenol-formaldehyde resin and 3,4-dihydroxybenzophenone-4-naphthoquinone 
(1, 2) diazide(2) sulfonate as a reactive component. This photoresist can 
be obtained from Shipley under the designation AZ 1350J. The photoresist 
is spin coated in a layer thickness of approximately 1.0 to 4.0 .mu.m, and 
exposed for approximately 20 to 30 minutes to a temperature of 
approximately 80.degree. to 90.degree. C. During this period, part of the 
solvent is driven out of the resist. 
Subsequently, as shown in FIG. 1B, photoresist layer 3 is roughened by 
reactive ion etching. The structure of FIG. 1A is placed into a chamber 
for an RF sputter etching, as described, for example, in U.S. Pat. No. 
3,598,710. Photoresist layer 3 is roughened by reactive ion etching, with 
the following parameters: 
______________________________________ 
gas: CF.sub.4 
flow rate: 30 cm.sup.3 /min. 
pressure: 40 .mu.bar 
energy density: 0.5 watt/cm.sup.2 
etch period: 5 to 10 minutes 
______________________________________ 
The relatively high energy density of 0.5 watt/cm.sup.2 causes a roughening 
or deformation of the photoresist. 
Subsequently, as shown in FIG. 1C, the roughness profile produced in 
photoresist layer 3 is transferred as a perforation pattern into glass 
resin layer 2. The roughness transfer is effected by reactive ion etching 
with the following parameters: 
______________________________________ 
gas: CF.sub.4 
flow rate: 30 cm.sup.3 /min. 
pressure: 40 .mu.bar 
energy density: 0.2 watt/cm.sup.2 
etch period: 15 minutes 
______________________________________ 
With the glass resin layer 2 now having a perforation structure as shown in 
FIG. 1C, recesses 4 (see FIG. 1D) are made in the surface of the epoxide 
resin substrate 1. Recesses 4 are formed by means of reactive ion etching 
the substrate through the perforation structure in an oxygen atmosphere. 
The vertical reactive ion etching is performed with the following 
parameters: 
______________________________________ 
gas: O.sub.2 
flow rate: 100 cm.sup.3 /min. 
pressure: 6.0 .mu.bar 
energy density: 0.2 watt/cm.sup.2 
etch period: 10 to 15 minutes 
etch depth: 2 .mu.m 
______________________________________ 
Vertical etching is ensured owing to the low pressure of the oxygen plasma 
between 4 and 6 .mu.bar. 
By further reactive ion etching in an oxygen atmosphere, as shown in FIG. 
1E, the recesses of epoxide resin substrate 1 can be increased in depth, 
and an overhang structure 5 can be produced to form mortises in the 
substrate 1. Reactive ion etching is performed by means of the following 
parameters: 
______________________________________ 
gas: O.sub.2 
flow rate: 100 cm.sup.3 /min. 
pressure: approximately 250 .mu.bar 
energy density: 0.2 watt/cm.sup.2 
etch period: 6 minutes 
etch depth: approximately 4 .mu.m. 
______________________________________ 
The pressure of the oxygen plasma defines the size of the overhang. As 
shown by secondary electron microscope (SEM) recordings, a clearly 
detectable overhang can be achieved with an oxygen pressure of 
approximately 250 .mu.bar. In the case where glass resin layer 2 is 
allowed to remain on substrate 1, it is irremovably bound thereto after 
copper plating of the surface of substrate 1. However, the glass resin 
layer 2 can be removed, e.g. by means of a dry etching process, at this 
point in the process. 
As shown in FIG. 1F, a copper layer 6 is sputtered onto the epoxide resin 
substrate 1 structured in accordance with the invention. The copper is 
sputtered onto substrate 1 by means of magnetic field-supported high rate 
cathode sputtering. This kind of high rate cathode sputtering (i.e., in 
which the plasma is concentrated directly in front of the cathode and the 
substrates are totally exposed to the flow of the sputtered material only 
partially exposed to the bombardment of secondary electrons), is described 
in IBM Technical Disclosure Bulletin Vol. 20, No. 4, September 1977, pp. 
1550 to 1551 by D. R. Hunter et al. Typical high sputtering rates for 
copper are approximately in the 2.5 .mu.m/min. range. Copper layer 6 is 
0.2 to 0.5 .mu.m/min. range. Copper layer 6 is 0.2 to 0.5 .mu.m in 
thickness. Note that a copper layer 6 with a layer thickness of less than 
0.2 .mu.m does not, in and of itself, lead to a high adhesion of the 
copper on the substrate surface. However, the adhesion values for the 
sputtered-on copper layer which are measured at stripes of 25 mm width, 
and extrapolated to one meter, are within a range of approximately 1000 
n/m. 
Subsequently, after the application of a photoresist mask layer (not 
shown), patterned copper conductors are deposited by electroless 
deposition and by means of an additive process onto the sputtered-on 
copper layer 6. While sputtered copper layer 6 is approximately 0.2 to 0.5 
.mu.m thick, the electroless-deposited copper 7 has a layer thickness of 
approximately 35 .mu.m. For depositing the copper, a long term bath 
produced by Photocircuits - Kollmorgen with the following operating data 
can be used: 
______________________________________ 
pH value: 12.6 standardized with NaOH 
CuSO.sub.4.5 H.sub.2 O: 
10.5 g/l high purity copper salt 
HCHO (37%): 3.5 ml/l as a reduction agent 
NaCN: 26 mg/l as a ductility promoter 
AeDTA: 17.5 g/l as a complexing agent 
temperature: 53 .+-. 1.degree. C. 
______________________________________ 
The deposition rate for this process step is approximately 2.5 .mu.m per 
hour. Subsequently, the photoresist mask layer is removed in a known 
manner, and the thin sputtered-on copper layer 6 is etched off in a flash 
etching process without the conductors being substantially affected. 
By means of the above-described surface structure of the substrate, the 
adhesive properties of the metallized layers can be considerably improved. 
The size of the recesses and the form of the overhangs (i.e., the shape of 
the mortises) can be determined by the specially spin-coated layers of 
glass resin and photoresist material, by the selection of the plasma and 
pressure, and by the duration of the etch period. Apart from epoxide 
resins, it is also possible to similarly pre-process and subsequently 
metallize other synthetic materials, e.g. polyimides. Copper can be 
sputtered onto the synthetic material laminates structured in accordance 
with the invention, as described above. However, coating can also be 
effected by vapor deposition in a vacuum, or (after an activation of the 
substrate surface with tin (II) chloride hydrochloric acid and palladium 
chloride, or with a palladium activator containing organic protective 
colloids) by reductive deposition from a bath onto the structured active 
surface. 
The method as disclosed by the invention is advantageously used for the 
copper plating of epoxide resin foils and boards. In this manner, high 
quality printed circuits with hitherto unequalled adhesion properties can 
be made at reasonable costs. 
It is to be understood that modifications can be made to the teachings of 
the present invention without departing from the spirit and scope of the 
appended claims.