Re-work method for removing extraneous metal from cermic substrates

A re-work method for removing extraneous plated metal layer areas from the surface of a ceramic substrate containing refractory metal areas wherein the substrate is heated at a temperature for a time sufficient to strengthen the bond between the plated metal layer and the refractory metal areas while simultaneously reducing the strength of the bond between the extraneous plated metal layer overlying the ceramic and the ceramic, immersing the substrate in a liquid, and applying localized ultrasonic energy in close proximity to the extraneous metal areas to be removed.

TECHNICAL FIELD 
This invention relates to a method for repairing metallurgy, more 
particularly to a method for the removal of extraneous, plated metal areas 
on a ceramic substrate. 
BACKGROUND ART 
In the fabrication of multi-layer ceramic substrates for use in 
semiconductor packages of the type described in U.S. Pat. No. 4,245,273, a 
mixture of ceramic particles, a resin binder, and a solvent for the binder 
is doctor bladed into thin sheets and then dried. The resultant green 
ceramic sheets are punched to form via holes, the via holes filled and 
circuit patterns imprinted with a conductive paste, the sheets assembled 
into a laminated structure and the resultant substrate sintered to burn 
away the binder and solvent and fuse the ceramic particles. After the 
sintering operation, metallurgy patterns are formed on the top and bottom 
surface to make contact with and support suitable I/O connections. These 
connections are used to make electrical connections to semiconductor 
devices, normally on the top surface, and connections to a supporting 
board, card, or other support on the bottom surface. The internal metal of 
the substrate must withstand the high temperature sintering operation. 
This normally requires the use of a refractory metal. However, these 
metals cannot be conveniently joined to I/O and device elements by solder 
and brazing techniques. What is commonly done is depositing additional 
metallurgy patterns of metals that are solderable and compatible with 
brazing operations over the refractory metal vias and patterns. 
However, during the sintering operation, green ceramic substrate shrinks 
substantially, usually on the order of 15 to 20%. Unfortunately, the 
shrinkage is not always uniform resulting in a distorted pattern of 
refractory metal vias and other patterns on the sintered substrate 
surface. When the geometry of the refractory metal pattern is small, as it 
must be in high performance semiconductor substrates, and the substrate is 
of substantial size, the subsequent metallurgy pattern cannot be deposited 
by conventional evaporation through a mask or blanket evaporation followed 
by subtractive etching because it requires alignment of a mask with the 
underlying refractory metallurgy pattern. The distortion occurring during 
sintering thus precludes mask alignment to the pattern. 
In order to apply the necessary non-refractory metal layers over the 
refractory metal areas on a substrate, the metals must be deposited by 
electroless and immersion plating or by electroplating techniques. 
Electroplating has not been generally used because this type of deposition 
requires establishing electrical contact to the specific areas to be 
plated in order to make them the cathode. Making this electrical contact 
is not always possible because some of the pad areas may be electrically 
floating. Electroless plating is a form of chemical plating which involves 
reduction of a metal salt to the metal with the simultaneous oxidation of 
a chemical compound called a reducing agent. To prevent or at least 
minimize the tendency for the oxidation-reduction reaction to take place 
throughout the plating solution, electroless plating solutions are 
formulated so that the concentration of the metal salt and/or reducing 
agent and Ph are such that the metal reduction does not occur readily. 
This being so, the areas to be plated would not be plated either. This 
problem is overcome by the use of catalysts which localize the plating 
reaction to the desired surfaces only. In the case of the ceramic 
substrates, the refractory metal surface areas are catalyzed to promote 
the oxidation-reduction reaction, whereas the ceramic material areas are 
not. The metal is thus selectively deposited only on and over the 
refractory metal areas. 
During the electroless plating operation, there frequently occurs 
extraneous metal deposition that does not overlie the refractory metal 
areas. This extraneous plating is unacceptable since electrical shorting 
will occur between metal pads that must be electrically isolated. In order 
to remove these extraneous metal areas of non-refractory metal, the entire 
metallurgy layer over the refractory metal is removed and the electroless 
deposition process repeated. This rework is time-consuming and quite 
expensive since a plurality of non-refractory metals are normally 
deposited over the refractory metal. Also, it has been found that the 
number of times that the rework operation can be repeated is low, 
sometimes only one time, before the entire substrate is degraded to the 
point that it must be discarded. At this point in time, the multi-layer 
ceramic substrate is nearly complete and represents a relatively large 
investment. 
What is needed in the packaging industry is a simple and inexpensive 
process for selectively removing extraneous metal that does not overlie 
the refractory metal layer area. The rework process must not degrade the 
substrate or the refractory metal areas and must not involve use of 
masking for the reasons previously discussed. 
The prior art, U.S. Pat. No. 3,698,941, discloses a method of applying 
contacts to a semiconductor body wherein the semiconductor surface, in 
which the surface is partly covered by an insulating layer, has deposited 
a metal layer applied to the entire surface. The metal layer is heated to 
increase the adherence of the metal layer to the semiconductor material 
and is subsequently subjected to acoustic high-frequency vibrations to 
remove the metal layer on portions overlying the insulating layer. 
DISCLOSURE OF THE INVENTION 
In accordance with the present invention, we provide a re-work process for 
selective removal of extraneous metal areas from the surface of the 
ceramic substrate containing refractory metallurgy areas in which the 
substrate is heated to a temperature sufficient to strengthen the bond 
between the refractory metal areas and the overlying metal while reducing 
the strength of the bond between the ceramic and metal, immersing the 
substrate in a liquid, and applying localized ultrasonic energy in close 
proximity to the metal areas to be removed.

BEST MODE FOR CARRYING OUT THE INVENTION 
Ceramic substrates used for packaging semiconductor devices, more 
particularly, multi-layer ceramic substrates, utilize electro, electroless 
and immersion metal plating on screened refractory features to complete 
the top and bottom surface metallurgy patterns. As a result of such 
factors as improper rinsing, plating bath hyperactivity, excessive 
handling, and fused ceramic repairs, undesirable extraneous metal can be 
deposited on the ceramic surface. Should this material grow so large as to 
bridge between features or to violate spacing requirements, the substrate 
must be rejected. One such defect between two features is sufficient to 
reject a substrate containing over 20,000 features. When a bridge is 
detected, the substrate must be reworked, that is, the metal overlying the 
refractory metal removed and replaced. More specifically, the rework 
process involves stripping off the entire deposited metallurgy layer, 
commonly nickel and gold, and replating. The replating process requires 
many steps taking up to 30 hours of time. 
The process of our invention utilizing ultrasonics provides an effective 
and economical way of removing the metal deposited on the ceramic. The 
process includes two essential steps. First, the metal film overlying the 
refractory metal must be subjected to a heat treatment which strengthens 
the refractory-metal adhesion, typically nickel to molybdenum, while 
weakening the metal-to-ceramic bond. This is achieved through diffusion at 
a temperature in excess of 600.degree. C., depending on the metals 
preferably in a hydrogen atmosphere. Secondly, the substrate is placed in 
a liquid, for example water, and an ultrasonic horn is positioned to 
direct its energy to the poorly adhering film on the ceramic until it is 
lifted off. 
Referring now to FIGS. 1-3 of the drawings, FIG. 1 illustrates the metal 
film 10, as for example a nickel layer, applied by electroless deposition 
overlying a refractory metal via 12 and the top surface of the ceramic 
substrate 14. The refractory metal areas can be screened paste areas that 
are deposited on the top surface of a ceramic surface. The area of metal 
film 10 overlying and in contact with the surface of ceramic substrate 14 
is extraneous metal which will be removed by the process of the invention. 
In FIG. 2 there is depicted the interface condition of the substrate 
indicated by wavy line 16. The metal film 10 overlying ceramic substrate 
14 has been loosened by the heat treatment previously described. Although 
not indicated on the drawing, the bond between the film 10 and the 
refractory metal via 12 has been strengthened significantly. FIG. 3 
depicts the condition of the substrate following the application of 
ultrasonic energy in a liquid environment. Note that the portion of metal 
film 10 overlying ceramic substrate 14 has been completely removed. 
FIGS. 3 and 4 illustrate and explain the criticality of the step of heating 
the substrate prior to the application of ultrasonic energy to the areas 
to be removed. When the ceramic substrate with a layer of metal, in 
particular nickel, overlies a refractory metal pad or area and the 
substrate is heated, the bond between the refractory metal pad and the 
overlying nickel layer is strengthened due to diffusion. However, during 
this same heatup phase, the ceramic imposes a constraint on the thermal 
expansion of the nickel film which results in a gradually increasing 
compressive stress in the film with increasing temperature. FIG. 4 
illustrates the difference in coefficient of expansions over the various 
temperature ranges. However, during the cool-down phase, the stress state 
in the nickel film reverses due to thermal contraction mis-match with the 
ceramic. When this stress exceeds that need to overcome the adhesion of 
the film to the ceramic, the nickel film delaminates from the ceramic. 
While a similar stress buildup would occur in the nickel film overlying 
the molybdenum features, this stress cannot overcome the strong 
diffusional bond between the two so that this interface will remain 
intact. 
The selective removal of the non-adhering metal film from the ceramic does 
not lend itself to a clear theoretical analysis although its effectiveness 
has been well established. A reasonable explanation of the mechanism 
involved is illustrated in FIG. 4. The ultrasonic horn provides a 
localized, intense source of ultrasonic waves at the horn tip. When held 
close to the substrate surface in a coupling medium such as water, the 
ultrasonic energy waves 22 incident to the nickel film adhering to the 
molybdenum features are harmlessly transmitted to the substrate and 
dissipated. The waves 24 incident on the loose nickel film on the ceramic 
are mostly absorbed in this film causing the film to vibrate at the 
frequency of the ultrasonic field about the points of strong adhesion on 
the neighboring molybdenum features. This rapid vibration eventually 
causes the film to break off from its points of adhesion due to fatigue, 
and once freed, is removed from the substrate surface. Although the above 
explanation assumes a direct coupling of the ultrasonic field to the 
substrate surface, the mechanism may also involve the production of shock 
waves by collapsing bubbles under the horn tip in the action of these 
shock waves on the loosely adhering metal film on the ceramic leading to 
their removal. 
The ease of removal depends on the number of variables, principal among 
them being (1) the nature of the metal film, (2) the surface texture of 
the ceramic, (3) interdiffusion conditions, i.e. temperature in the 
ambient, (4) horn design, (5) horn power, (6) horn-to-substrate spacing, 
(7) the nature of the coupling fluid, and (8) the temperature of the 
coupling fluid. 
It is often observed in electroless nickel plating that the thickness of 
the nickel layer in the bridging regions is significantly less than the 
thickness of the nickel layer deposited on the refractory metal features. 
This is so because the bridging layer grows from adventitious catalytic 
points far less numerous than the catalytic points present on the 
refractory metal features. When the thickness of the bridging nickel film 
is of the order of the real roughness of the ceramic surface or less, 
which typically ranges from 1-2 .mu.m, the bridging film removal by this 
technique becomes difficult if not altogether impractical. We believe that 
the reason for this is as follows: the thicker nickel deposits (&gt;2 .mu.m), 
the diffusion heat treatment imparts to the nickel film sufficient lateral 
cohesion to enable the entire film to act in concert to pull away from the 
ceramic surface under the influence of the differential thermal stress 
induced during the cooling phase of the heat treatment. In thinner 
deposits, however, the required degree of cohesion will be lacking and the 
film will girdle around asperities on the ceramic surface. Under these 
circumstances, the differential thermal contraction between the nickel and 
the ceramic only causes them to embrace these asperities even more 
tightly, making their removal by the action of the ultrasonic horn much 
more difficult. A similar difficulty does not arise with the stray 
deposits of the immersion gold because it does not form a continuous layer 
but only deposits on stray deposits of reduced nickel salts that 
occasionally seep out of the pores in the nickel coated refractory 
features. 
The process of our invention can be used to remove any electro or 
electrolessly plated layer having a coefficient of expansion significantly 
different from the underlying ceramic material. Typically, such metals are 
nickel, copper, tin, gold, lead and alloys thereof. The metal layer can 
also be a composite layer. The ceramic is typically alumina, various 
mixtures of glasses, mullite, and the like. Refractory metals include 
tungsten, tantalum and molybdenum. 
The heating step, prior to application of the sonic energy, can be to any 
suitable temperature that will achieve the aforedescribed result. 
The temperature is dependent on the metal. The lower limit of the useful 
range is the lowest temperature that produces a diffusion bond between the 
overlying metal and the refractory metal. The upper range is usually 
determined by what temperature will cause degradation of the metals and 
associated elements on the substrate. For a nickel layer overlying a 
refractory metal the range is from 600.degree. to 1000.degree. C., more 
preferably from 700.degree. to 900.degree. C. For a gold layer the heating 
temperature is greater than 400.degree., more preferably from 400.degree. 
to 700.degree. C., most preferably from 550.degree. to 600.degree. C. The 
duration of the heating step is dependent on the various conditions such 
as the type of ceramic and its surface texture, the nature and thickness 
of the metal to be removed, temperature, etc. Typically the time will vary 
from 1 to 10 minutes, although the heating can be prolonged, if desired. 
Again an important consideration is preventing degradation of the layers 
with excessively high temperatures. The frequency of the ultrasonic energy 
is ordinarily in the range of 10 to 20 KHZ, preferably of the order of 20 
KHZ. The time of exposure will also depend on various conditions such as 
the nature and thickness of the metal film, the thickness of the coupling 
medium, temperature, etc. Excessive times of exposure to the action of 
ultrasonic horn is to be avoided as it might damage the integrity of the 
refractory metal-to-ceramic bond. The coupling medium is most preferably 
water and at or about room temperature. However, any other suitable fluid 
can be used at whatever temperature the removal process works well. 
The general requirement of the coupling medium is that it is a fluid with 
high cavitation efficiency, i.e. a liquid having a low vapor pressure. 
Other suitable coupling mediums include various oils and hydrocarbons. 
Water is the most preferred because its use does not require cleaning 
procedures after usage. 
The ultrasonic energy is applied by placing an ultrasonic horn in close 
proximity to the metal areas to be removed. The horn should be placed as 
close as possible, without contacting the substrate, but preferably not 
exceeding 0.075 cm. The strength of the ultrasonic energy can be measured 
by the power supplied to power supply. The power is normally in the range 
of 150 to 600 watts/cm.sup.2. The time of application will vary. In 
general the time will not normally exceed 10 minutes. 
EXAMPLE 
A multilayer substrate having an array of closely spaced molybdenum circuit 
features on its surface had formed an unwanted bridging nickel layer 
connecting these features at several locations. The thickness of the 
electroless nickel layer was about 6.5 .mu.m on the refractory metal 
features and about 4 .mu.m in the bridged regions on the ceramic surface. 
After a diffusion heat treatment at about 850.degree. C. the bridging 
nickel layer was removed by subjecting this region of the substrate to the 
action of an ultrasonic horn held at a distance of about 0.025 cm from its 
surface while immersed in water. The horn used was rated at a power level 
of 300 watts/cm.sup.2 and the removal time was approximately 2 minutes. 
Another substrate with a bridging nickel thickness of approximately 1 .mu.m 
was subject to the ultrasonic horn action after a similar diffusion 
treatment. Under similar conditions of horn power and position as 
described previously, the bridged metal layer could not be removed even 
after a 30 minute exposure. The attempt to remove the bridging was stopped 
because of the commencement of cavitation damage to the substrate.