Isolated bump circuitry on tape utilizing electroforming

A process for electroforming a bump circuit and a lead circuit on a tape to be used for making contact with integrated circuit chips and the like. By means of the process, an inverse bump pattern is produced. To this end, a first metal plate is coated with a photoresist on one surface and suitable apertures are made in the photoresist in a conventional manner. Now indentations are chemically etched, partly through the uncovered metal surface to form a bump circuit. Next the photoresist is completely removed and the metal plate is again covered with a layer of photoresist. After the photoresist is developed again to form a lead circuit, the metal is chemically etched partly through the uncovered metal to form a lead circuit while at the same time further etching the bump circuit indentations. These indentations are now covered with metal by various plating steps. For example, the indentations may be covered successively with layers of gold, nickel, copper, and an additional layer of gold. The original metal is then removed by chemical etching. Either the remaining photoresist may remain or else the remaining photoresist is completely removed and a new layer of photoresist is applied over the entire structure before the original metal is removed by chemical etching. The final product is an electroformed lead and bump circuit on a tape to provide a flexible carrier.

CROSS REFERENCES TO RELATED APPLICATIONS 
The present application is generally related to the copending application 
to Dugan entitled "Method of Making Circuitry With Bump Contacts" Ser. No. 
873,460, and to the application to Dugan and Phillips entitled "Bump 
Circuits on Tape Utilizing Chemical Milling" Ser. No. 873,453, both 
applications being filed concurrently herewith and both applications being 
assigned to the assignee of the present application. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention. 
The present application generally relates to bump and lead circuits 
arranged on a flexible tape, and particularly relates to a method of 
electroforming both the bump circuit and the lead circuit and to deposit 
it on a flexible tape. 
2. Description of the Prior Art. 
Carriers providing a bump circuit on one surface and a lead circuit on the 
other are well known in the art. They are used for making electrical 
contact with the terminals of semiconductor devices such, for example, as 
integrated circuit chips. The leads provide connections from the 
semiconductor terminals to other elements. The bumps or pads disposed on 
the surface opposite the lead circuit serve the purpose of facilitating 
contact with the semiconductor terminals. This is usually effected by a 
heat ram for thermally bonding the bumps to the terminals. 
It is conventional practice to generate the lead and bump circuits by 
chemical etching. In other words, a plate of a suitable metal, such as 
copper, for example, is subject to chemical etching through a photoresist 
which has been illuminated through a mask and developed to expose the 
metal at places adjacent the bumps which are to be provided. 
In accordance with the present invention, both the bump circuit and the 
lead circuit are formed, not by a chemical etching process, but by 
electroforming. In this connection, reference is made to a patent to Sanz 
et al., U.S. Pat. No. 3,350,250. This patent relates to a method of making 
printed wire circuitry. Specifically, FIGS. 14-18 of the patent disclose a 
particular fabricating technique for a printed wire circuit. Here a 
temporary base member is drilled to provide a recess or indentation. This 
in turn is covered with a layer of conductive material to coat the recess. 
The conductive material forms a funnel-shaped protrusion which may be 
inter-nexted with others to form electrically conductive interconnections. 
However, these protrusions are hollow. 
Reference is also made to a patent to Pritikin, U.S. Pat. No. 2,692,190. It 
discloses various fabricating steps, again for making inlaid circuits. It 
should be noted that these inlaid or printed circuits are not provided 
with bumps. 
The patent to Gigoux, U.S. Pat. No. 3,913,223, discloses a method of 
manufacturing a double-sided circuit, such as a printed circuit having 
raised through-hole eyelets shown particularly in FIGS. 3-5, 10 and 11. 
Basically an aperture is made in a laminated sheet which is then covered 
with metal to provide a through-hole eyelet. It should be noted that none 
of these patents relate to the type of bump and lead circuits on a tape to 
which the present invention is directed. 
Other patents relate generally to various circuit fabrication techniques. 
Thus, the patent to Jackson, Jr., et al., U.S. Pat. No. 3,508,980, relates 
to an integrated circuit structure. It is made with dielectric isolation. 
Among others it discloses a chromium layer formed, for example, by vacuum 
evaporation deposition, which serves as a highly conductive region. This 
in turn will provide a ohmic contact to transistors. 
The two patents to Carbonel, U.S. Pat. Nos. 3,583,066 and 3,566,461, both 
relate to a method of making a magnectic circuit element. 
Finally, reference is made to a patent to Norton, U.S. Pat. No. 3,424,658, 
which also relates to a process for providing a printed circuit board on a 
metallic substrate. However, the circuit is not provided with any bumps, 
which are essential for the circuit of the present invention. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a process is provided for 
electroforming a bump circuit and a lead circuit on a tape. To this end, a 
first metal plate or base is coated with a photoresist on one surface 
thereof. The photoresist is removed from those areas where bumps are to be 
formed. Where the metal is bared, indentations are chemically etched 
partly through the metal surface to form a bump circuit. Now the 
photoresist is completely removed. The surface of the metal plate is again 
covered with a layer of photoresist which is again removed over those 
areas where a lead circuit is to be formed. The next step is chemically 
etching partly through the uncovered metal to form a lead circuit. At the 
same time the bump indentations are additionally ethced to make them 
deeper than the lead indentations. 
Now a second metal is plated over the exposed surface of the first metal, 
the second metal being of the type which resists a chemical etch for the 
base metal. Thus the first metal may consist of copper and the second of 
gold. The layer of gold may now be followed by a layer of nickel, a layer 
of copper, and a second layer of gold, all plated over the original layer 
of gold. Now the copper plate may be removed by chemical etching. 
The original layer of photoresist may remain and may form the necessary 
tape. Alternatively, the photoresist is removed and a new layer of 
photoresist is applied over the entire surface of the bump and lead 
circuit. The photoresist may be of the type which is capable of being 
melted by the thermal bonding process of the tape to an integrated circuit 
chip structure. Alternatively, the photoresist may be removed over those 
areas through which openings are to be provided, such as a window 
surrounded by the bump circuit. 
The novel features that are considered characteristic of this invention are 
set forth with particularity in the appended claims. The invention itself, 
however, both as to its organization and method of operation, as well as 
additional objects and advantages thereof, will best be understood from 
the following description when read in connection with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings and particularly to FIGS. 1-5, there is shown 
a series of fragmentary, sectional views in FIGS. 1-4 illustrating 
successive process steps for preparing an inverse bump circuit and lead 
circuit, while FIG. 5 illustrates a plan view of a portion of such a lead 
and bump circuit. FIG. 1 illustrates a metal plate 10 which may, for 
example, consist of copper. The metal plate 10 serves as a temporary base 
which is subsequently removed by etching. Therefore, the plate 10 may 
consist of any metal which can be etched while another metal is eventually 
plated over the indentations made in the plate 10, the other metal 
resisting the chemical etch for the plate 10. 
The plate 10 is now covered with a layer of photoresist 11 on one surface, 
say the top surface as shown in FIG. 2, while the bottom surface may also 
be covered with a layer of photoresist 12. The photoresist layer 11 is now 
illuminated with suitable light through a mask so that certain portions of 
the photoresist may be removed. This will now uncover the copper plate 10 
in certain areas 14 and 15 corresponding to the desired pattern of the 
bumps. 
Subsequently as shown in FIG. 3, the copper plate 10 is etched with a 
suitable chemical etching solution. The metal is exposed to the chemical 
etching only long enough to form indentations as shown at 16 and 17 partly 
through the uncovered metal surface at 14 and 15. 
During the next step in the process, the photoresist layer 11 is entirely 
removed by a suitable solvent. Subsequently, a new layer of photoresist 20 
is applied again over the entire surface of the metal plate 10. The 
photoresist layer 20 is again illuminated by a light source through a 
suitable mask. Subsequently, the areas of the photoresist corresponding to 
the desired lead circuit are removed. This, of course, will include the 
areas 14 and 15 on which the indentations 16 and 17 have been made for the 
bump circuit. The layer 12 of photoresist may remain to protect the lower 
surface of the copper plate 10. 
The plate 10 is again subjected to a chemical etching process to etch 
partly through the uncovered metal to form the lead circuit. Since the 
lead circuit surrounds the bump circuit, it will be evident that the bumps 
to be formed at 16 and 17 will be additionally etched. This in turn 
insures that the indentations for the bumps, such as 16 and 17, are etched 
deeper than the identations, such as 21 and 22, shown in FIG. 5 forming 
the lead circuit. It will now be evident that the structure, shown in plan 
view in FIG. 5, forms a matrix for electroforming a bump circuit and a 
lead circuit thereover. 
How this is accomplished and how a tape is formed about the bump and lead 
circuits will now be explained in connection with FIGS. 6-10. It should be 
noted that FIGS. 6-10 are taken at right angles to the sections of FIGS. 
1-4, as shown at the line 6--6 of FIG. 5. 
Referring now specifically to FIG. 6, the next process step is to plate a 
second metal layer 25 over the areas of plate 10 into which indentations 
have been etched, including the indentation 16 corresponding to one of the 
bumps. This metal layer 25 should be of a metal which resists the chemical 
etch to be applied subsequently to the copper layer 10. Hence the metal 
layer 25, which is plated over the copper plate 10, preferably consists of 
gold. 
Subsequently, a metal layer 26, such as nickel, is plated over the gold 
layer 25, to be followed with a copper plate 27 and another gold plate 
layer 28. The copper layer 27 serves to improve conductivity of the 
structure because the conductivity of copper is better than that of gold. 
Generally, the height of a bump in a bump circuit may be on the order of 
three mils while that of the lead circuit may be somewhat less, such as 
one-and-one-half to two mils. The temporary base plate 10 may be somewhat 
thicker and may be on the order of ten mils. The plating may be effected 
in a suitable plating bath. 
The next process step is illustrated in FIG. 7. First the photoresist layer 
12 on the reverse side of the copper plate 10 is removed by a suitable 
solvent. Subsequently, the copper layer 10 is removed by chemical etching. 
The resulting structure is shown in FIG. 7. The structure comprises of the 
successive layers 25, 26, 27 and 28, consisting respectively, for example, 
of gold, nickel, copper and gold. A bump 30 is formed in the structure and 
the entire structure is tightly held by the photoresist layer 20, which 
will be evident from the manner in which the lead circuit is formed. This 
may then be utilized as the finished product. 
Alternatively, referring back to the structure of FIG. 6, the remaining 
photoresist 20 may now be removed as shown in FIG. 8 before the copper 
sheet 20 is removed. 
Subsequently, as shown in FIG. 9, a new layer of photoresist 32 which may 
consist of polyimide is applied over the entire structure of FIG. 8, 
including the surface of the gold layer 28. The layer of photoresist 32 
now serves as the plastic carrier. The new layer 32 of photoresist may now 
be treated as previously described by illuminating it and developing it to 
uncover the metal layer 10 in the area 35 (see FIG. 5) to form a window 
central of the bumps for the application of heat to the carrier to 
thermally bond the bumps to a semiconductor device, such as an integrated 
circuit chip. This combination may again be treated by etching to etch 
away the necessary window and to remove the temporary base layer 10. The 
resulting finished structure is illustrated in FIG. 10 (the window 35 not 
being shown). 
As another alternative, it may not be necessary to remove the photoresist 
over the areas where windows are to be formed. In this case, the 
photoresist should be of a type which can be burned away or evaporated 
upon the application of the heat ram. This step forms the final 
application of the finished carrier to thermally bond the bumps to the 
contact pads of the intergrated circuit chip. 
There has thus been disclosed a process for electroforming an inverse bump 
circuit and an inverse lead circuit and for securing it to a flexible 
plastic tape. The bump circuit is made by chemically etching indentations 
partly through the uncovered metal surface of a temporary base. This is 
followed by a second etching step for the lead circuit which also partly 
etches through the uncovered metal surface. At that time the bumps for the 
bump circuit are made deeper so that the finished bumps or pads have a 
surface higher than that of the lead circuit. The resulting indentations 
are then plated with successive layers of metal to provide the bump 
circuit and the lead circuit. The temporary base is etched away and the 
bump and lead circuits may either remain on the original photoresist or a 
new photoresist may be applied over the entire surface of the structure 
from the lead side, leaving the bumps uncovered. The necessary windows may 
either be made by illuminating and developing the photoresist or else the 
photoresist may be of the type which can be melted away by the application 
of the heat ram which bonds the carrier to suitable semiconductor devices 
or circuit chips. 
Although there have been described above specific arrangements of isolated 
bump circuitry on tape utilizing electroforming in accordance with the 
invention for the purpose of illustrating the manner in which the 
invention may be used to advantage, it will be appreciated that the 
invention is not limited thereto. Accordingly, any and all modifications, 
variations or equivalent arrangements which may occur to those skilled in 
the art should be considered to be within the scope of the invention as 
defined in the appended claims.