Method for removing trace metal contaminants from organic dielectrics

A technique is described for the removal of trace metal contaminants from organic dielectrics such as polyimide. Pulsed ultraviolet radiation is used to remove the contaminants from the dielectric, regardless of their chemical nature, by the process of ablation. The process allows prepatterned bulk metal features to be simultaneously exposed to the pulsed radiation and yet remain unaffected.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of removing metal contaminants 
from organic dielectrics in general, and in particular, from polyimide 
which is typically used in multilevel thin-film interconnect structures 
for packaging applications. 
2. Description of the Prior Art 
Polyimide has gained increasing importance as a dielectric material in 
thin-film multilayer interconnect structures. Such structures are used in 
high-speed, high-density packaging, especially for main-frame computer 
applications. In these applications, multiple layers of interconnect 
metallizations are separated by alternating layers of polyimide whose 
function is to serve as the electrical isolation between the metal 
features. 
More specifically, a typical substrate used for the above applications 
generally consists of a ceramic wafer with a spun-on film of polyimide on 
top of which fine line interconnections consisting of metallization 
features of Cu, Au, or Al are then patterned. However during the 
fabrication process, a typical step involves the deposition of reactive 
metals such as Ti:W, Cr, Ni, or Pd, on the ceramic-polyimide substrate. 
These metals serve as diffusion barriers, adhesion layers, or catalysts. 
They however have to be etched away at a later stage in the fabrication 
process, and although conventional etching techniques such as wet etching 
and plasma processing can etch away the bulk of these metals, these 
processes are unable to remove trace metal contaminants that adhere to the 
surface of the polyimide. Even trace quantities of these metals can 
severely limit the dielectric performance of the polyimide. It is 
therefore crucial to remove the contaminants and restore the dielectric 
performance of the polyimide to levels that are acceptable for packaging 
applications. 
A typical method mentioned in the literature to address this type of 
problem is that of plasma etching. The technique of plasma etching serves 
to etch vias in polyimide for multilevel metallization systems. While the 
method can remove bulk polyimide, it is unsuited for problems that require 
only a minimal removal of polyimide and a selective removal of metal 
contaminants in it. For example, U.S. Pat. No. 4,357,203 (issued Nov. 2, 
1982 to Joseph Zelez and assigned to RCA Corporation) discloses a process 
for removing the residual film remaining after oxygen plasma etching of 
polyimide by a second plasma etching utilizing a mixture of argon and 
hydrogen. While effective in removing residual polyimide under masked 
conditions, the '203 process would not serve to remove polyimide in the 
presence of gold metallizations, as it would sputter the gold, causing 
further contamination of the polyimide. If a mask were used to cover the 
gold in order to prevent sputtering, both the complexity and the cost of 
this process would be significantly increased. 
Another patent, U.S. Pat. No. 4,445,966 (issued May 1, 1984 to Robert J. 
Carlson and Daniel W. Youngner and assigned to Honeywell Inc.) discusses a 
method of plasma etching of films containing chromium. The '966 patent 
discloses the use of fluorine containing compounds and deals strictly with 
the removal of chromium from silicon substrates. The surfaces of these 
substrates are entirely different from sensitive polyimide surfaces and 
therefore the '966 technique dies not teach or suggest a solution to the 
problem of removal of chromium or any other metal contaminants from 
polyimide. 
Excimer laser bombardment has been used for the removal of non-bound or 
free-standing particles from solid surfaces. (A. C. Tam, W. Zapka, and W. 
Ziemlich, "Efficient laser cleaning of small particulates using pulsed 
laser irradiation synchronized with liquid-film deposition, "SPIE, 1598, 
13-18, 1991.) The technique applied by these authors uses excimer laser 
bombardment of a surface in conjunction with a liquid jet such as that of 
water. This process differs from the present invention in that it involves 
a wet process (not a dry one), and requires a complex experimental set-up 
involving a specially designed pulsed liquid film deposition system. In 
addition, the process used by these authors specifically addresses the 
removal of airborne particles and not embedded particles as in the present 
invention. In a patent by Karl Asch, Joachim Keyser, Klaus Meissner, and 
Werner Zapka, issued Dec. 25, 1990 and assigned to International Business 
Machines Corporation (U.S. Pat. No. 4,980,536), airborne particulates of 
extremely small dimensions (100-1000 nanometers) were removed from the 
type of silicon membrane masks that are used primarily in electron beam 
lithography. The '536 technique uses a mask to selectively expose desired 
areas of the substrate. In addition, the '536 technique involves the 
removal of airborne particles (not embedded contaminants) and addresses an 
application that is quite specific to lithographic masks. This is quite 
different from the approach of the present invention that teaches a 
technique to selectively target a single material on a multi-component 
substrate, based upon the differences in ablation thresholds of each 
component, thereby excluding the need for a mask. 
Excimer laser ablation of organic polymers, in particular that of polyimide 
has drawn tremendous interest for over a decade, resulting in numerous 
studies. (R. Srinivasan and W. J. Leigh "Ablative photodecomposition: 
action of far-ultraviolet (193 nm) laser radiation on poly(ethylene 
terephthalate) films," J. Am. Chem. Soc. 104, 6784-6785, 1982.) (J. E. 
Andrew, P. E. Dyer, D. Forster, and P. H. Key, "Direct etching of 
polymeric materials using a XeCl laser," Appl. Phys. Lett. 43, 717-719, 
1983.) (R. Srinivasan and B. Braren, "Ablative photodecomposition of 
polymer films by pulsed far-ultraviolet (193 nm) laser radiation: 
dependence of etch depth on experimental conditions," J. Polym. Sci. 22, 
2601-2609, 1984.) (G. Koren and J. T. C. Yeh "Emission spectra, surface 
quality, and mechanism of excimer laser etching of polyimide films," Appl. 
Phys. Lett. 44, 1112-1114, 1984.) (J. H. Brannon, J. R. Lankard, A. I. 
Baise, F. Burns, and J. Kaufman, "Excimer laser etching of polyimide," J. 
Appl. Phys. 58, 2036-2043, 1985.) (R. Srinivasan, B. Braren, and R. W. 
Dreyfus, "Ultraviolet laser ablation of polyimide films," J. Appl. Phys. 
61, 372-376, 1987.) The technological importance of excimer laser ablation 
of polyimide has been realized through applications such as the production 
of via-holes, (F. Bachmann, "Large scale application for excimer-lasers: 
via-hole-drilling by photo-ablation," SPIE 1377, 18-29, 1990.) 
micropatterning of surfaces, (J. H. Brannon, "Micropatterning of surfaces 
by excimer laser projection," J. Vac. Sci. Technol. B 7, 1064-1071, 1989.) 
and patterned electroless plating. (H. Niino and A. Yabe, "Positively 
charged surface potential of polymer films after excimer laser ablation: 
application to selective-area electroless plating on the ablated films," 
Appl. Phys. Lett. 60, 2697-2699, 1992.) While demonstrated for 
applications requiring the bulk removal (several microns) of polyimide, 
there are no known efforts to use excimer laser ablation for surface 
cleaning of polyimide and the removal of metal contaminants from it. In 
yet another invention, U.S. Pat. No. 4,882,200, (issued to Liu and Grubb 
on Nov. 21, 1989 and assigned to General Electric Company), an excimer 
laser is employed to pattern electroless plating activator material from 
polymer and other substrates. In the ' 200 process, the activator is 
intentionally deposited on the substrate. Hence it never becomes embedded 
in it. This is in contrast to the present invention where the contaminants 
to be removed are embedded in and are part of the substrate. In addition, 
the '200 technique is not and cannot be used as claimed to selectively 
remove trace metal contaminants without selectively exposing the polymer. 
In the present invention, the surface of the contaminated organic 
dielectric is completely, not selectively, exposed. 
It is therefore an object of this invention to selectively remove trace 
metal contaminants from an organic dielectric using excimer laser ablation 
in a maskless process not requiring selective exposure of the substrate. 
Yet another object is to provide process windows that will achieve this 
result with a minimal removal of the dielectric material. Another object 
is to leave pre-patterned thin-film metallization features on the surface 
of the organic dielectric unaffected and intact to enable further 
processing steps. Another object is to scale up this process for large 
area applications using, for example, step and repeat processing. 
SUMMARY OF THE INVENTION 
The present invention provides a maskless method by which trace metal 
contaminants can be selectively removed from organic dielectrics in 
general, and from polyimide in particular, while keeping intact the 
pre-patterned, fine-line thin-film metallization features on the surface. 
This method makes use of maskless excimer laser ablation and process 
selectivity is achieved through precisely controlled laser fluences. This 
invention makes it possible to restore the dielectric properties of an 
organic dielectric to levels required for high performance dielectric 
substrates, typically used in advanced packaging technologies. A process 
window for this surface cleaning technique has been established using a 
preferred excimer laser wavelength of 193 nm and at an additional laser 
wavelength of 248 nm.

DETAILED DESCRIPTION OF THE INVENTION 
In advanced microelectronic packaging, one is often faced with a very 
general situation of having fine line patterned metallizations on top of 
an isolating dielectric. The metallization features are designed to 
conduct high frequency electrical signals to and from different 
microelectronic systems, for example from IC chips. The isolation between 
these metal features is crucial for the functionality of the assembled 
microelectronic system. So called Tape-Automated-Bonding (TAB) tapes and 
multi-chip module substrates may serve as examples. 
The metallizations are produced by various thin film manufacturing 
processes, such as microlithography, sputtering, electroplating, and 
others. As an undesired side effect of these processes, the surface of the 
dielectric can become contaminated with trace metals, which will then 
degrade its electrical isolation resistance. These trace metal 
contaminants cannot generally be removed by standard chemical procedures, 
without affecting the pre-patterned metal features. 
The concept of this invention, as schematically depicted in FIG. 1, is to 
use short pulses of excimer laser radiation 2 to ablate the contaminated 
surface layer 4. The pre-patterned metal features 6 are not affected by 
these laser pulses because they reflect the laser radiation at the 
selected wavelength and have a higher ablation threshold than the 
contaminated surface 4 of the organic dielectric 8 on wafer 10. The metal 
features 6, the contaminated surface 4, the dielectric 8, and the wafer 10 
make up substrate 12. 
FIG. 2 depicts the basic set-up for this process. The excimer laser 14 
emits pulses of ultraviolet radiation 2, which pass through a simple 
optical system 16 and a transparent quartz window 18 before striking the 
substrate 12. The optical system 16 serves to image or concentrate the 
laser pulses on an area of the substrate 12, sized such that the incident 
laser fluence is sufficient to ablate the contaminated surface 4 of a 
dielectric 8 on wafer 10 and is yet low enough to leave intact and 
unchanged the metal features 6. It is obvious to those skilled in the art 
that the optical system 16 may be varied depending on the properties of 
the organic dielectric 8, the excimer laser 14, or other process 
subtleties. 
To ensure the proper removal of the ablation products, the substrate is 
located in a vacuum chamber 20, which can be evacuated with a connected 
vacuum pump 22, or additionally can be back-filled with a specific gas 
ambient 24 through port 26. 
The spot or image of the excimer laser radiation 2 produced on the 
substrate 12 by the optical system 16 may or may not cover the entire 
contaminated surface 4 to be cleaned. In the latter case, the substrate 12 
is stepped or translated once a region is cleaned, in order to then 
successively expose a new contaminated region to the laser beam. With such 
a step-and-repeat process, cleaning of the entire substrate surface can be 
achieved. 
EXAMPLE I: (193 nm) 
The substrate consisted of 15 .mu.m thick polyimide layers on ceramic 
substrates. Isolated gold features were fabricated on top of the 
polyimide. The polyimide surface was contaminated with Cr which could not 
be satisfactorily removed by wet etching. Polyimide could for instance, 
also be replaced by other organic dielectrics, for example, 
polymethylmethacrylate (PMMA) or Parylene. 
A Questek laser (model 2560 v.beta.) was used in this example. Excimer 
laser radiation at 193 nm was obtained with a prescribed mixture of 
fluorine with argon, using neon as the buffer gas. The central portion of 
the 2.5 cm.times.1 cm excimer beam was selected by a slit and then 
concentrated on the substrate using a S1 UV grade lens. The focal length 
of the lens was appropriately selected to achieve different laser spot 
sizes on the substrate, thereby resulting in different incident fluences. 
A sufficiently high range of fluences was produced by power locking the 
laser at various initial powers. Furthermore, for achieving the low 
fluence regime (between 20 and 70 mJ/cm.sup.2), either a 50% or a 17% 
transmitting beam-splitter was employed. In this and all the following 
experiments, the repetition rate of the laser was maintained at 1 Hz to 
minimize heating of the polyimide surface. The ablations were conducted in 
vacuum to avoid debris and soot formation that can deposit on the 
substrate, and in particular on the gold pads. Different levels of vacuum 
could be achieved by the use of either a roughing pump alone or a 
turbomolecular pump in conjunction with it. As an alternative to the 
vacuum environment, an atmosphere of He at pressures in the range of 10 to 
760 Torr could be employed. 
Following ablation at any given fluence, surface analysis was performed 
using SIMS. This analytical technique was specifically chosen because of 
its very high sensitivity. X-ray photoelectron spectroscopy (XPS) was used 
to calibrate the initial levels of Cr contamination on these substrates. 
However, due to the limited detection capability of XPS to roughly 1% 
levels of Cr, the more sensitive SIMS technique was applied. FIG. 3 shows 
the surface analysis of a selected area of contaminated polyimide, before 
and after the above described treatment with excimer laser radiation at 
193 nm. FIG. 3(a) shows the results of SIMS analysis of a contaminated 
polyimide surface. FIG. 3(b) shows the results of a similar analysis after 
exposure of this surface to 80 pulses of excimer laser radiation at 193 
nm, at an incident fluence of 40 mJ/cm.sup.2. This figure clearly reveals 
the complete removal of Cr on the initially contaminated surface of 
polyimide. 
FIG. 4 summarizes the results of 18 tests that were performed at three 
different incident laser fluences. This figure shows the secondary ion 
yield of Cr as a function of laser pulses, for fluences of 22, 40, and 100 
mJ/cm.sup.2 at a wavelength of 193 nm. Also indicated in this figure is 
the isolation resistance measured on the contaminated polyimide surface 
(number of laser pulses=0), which corresponds to a value of approximately 
10.sup.5 .OMEGA.. In addition, it is observed that at and below 
contamination levels corresponding to a secondary ion yield of Cr of 
10.sup.2, measured isolation resistances were greater than 10.sup.11 
.OMEGA.. This result is depicted by the solid line in FIG. 4. 
At 193 nm, ablations were carried out in a whole range of incident laser 
fluences from 20 mJ/cm.sup.2, following which ablation rates were 
calculated. However, an upper limit of 500 mJ/cm.sup.2 was placed on the 
incident fluence in order to prevent any damage to the Au features and 
thereby insure that further processing steps are not affected. At each 
fluence, ablation depths were measured as a function of the number of 
pulses using a Sloan Dektak 3030 stylus profilometer. The ablation rate 
was calculated from the slope of the least squares fit of the plot of 
ablation depth versus the number of pulses. At an incident fluence of 40 
mJ/cm.sup.2, the ablation rate at 193 nm was 250 .ANG./pulse, while at an 
incident fluence of 100 mJ/cm.sup.2, the ablation rate was 515 
.ANG./pulse. These values clearly indicate that there is no significant 
removal of the polyimide under these conditions. For those skilled in the 
art, it will be clear that the number of laser pulses should be kept as 
small as possible, as this will serve to both minimize ablation depths and 
reduce processing times. 
In most cases, it is preferable to minimize the ablation depth of the 
organic dielectric to be cleaned. In such cases, our preferred embodiment 
suggests exposing the contaminated polyimide surface to up to 10 pulses of 
excimer laser radiation at 193 nm, at an incident fluence in the range of 
22 to 100 mJ/cm.sup.2. 
EXAMPLE II: (248 nm) 
Results similar to those described at 193 nm and shown in FIGS. 3(a) and 
3(b) were obtained at 248 nm. Shown in FIG. 5(a) are the results of 
surface analysis, using SIMS, of yet another sample of contaminated 
polyimide. FIG. 5(b) depicts the results after exposing the contaminated 
sample to 40 pulses of excimer laser radiation at 248 nm, at an incident 
fluence of 80 mJ/cm.sup.2. Once again it can be seen that the removal of 
Cr contaminants can also be achieved using 248 nm radiation, although 
slightly higher fluences, than at 193 nm, are required. 
The advantages of this invention are several. Most importantly, it employs 
a maskless process which will reduce processing costs significantly. The 
invention provides a method for removing trace interface contamination 
which is far superior to the wet etching process. In addition, this 
process offers the distinct advantage of being independent of the chemical 
nature of the contaminant, unlike plasma processing, as it is based on 
ablating the contaminated material and not on a chemical reaction on the 
surface. The laser ablation technique is free of chemical processing steps 
and the vacuum requirements are simple. The method is highly selective 
between different materials on the same substrate, merely by a variation 
of incident fluence. As the method depends upon the absorption of 
ultraviolet radiation by the material being ablated, the process is very 
well controlled by a precise control of the incident fluence. The high 
reflectivity of metals such as gold or aluminum, at excimer laser 
wavelengths, makes them poor absorbers of the excimer laser radiation used 
in this invention. The allows for the selective ablation of the organic 
dielectrics with their trace metal contaminants, at fluences that would 
not damage or ablate these patterned metal features. This gives the 
excimer laser cleaning method a special advantage over the conventional 
chemical processes. 
Although the invention has been described in terms of a preferred 
embodiment, it will be obvious to those skilled in the art that many 
alterations and modifications may be made without departing from the 
invention. For example, the parameter range of incident fluence and number 
of laser pulses at a selected wavelength of the laser radiation is 
relatively wide and is determined by a number of factors such as the 
organic dielectric, its absorption coefficient at a selected wavelength, 
the type of contaminant, and the type of bulk metal. Accordingly, it is 
intended that all such alterations and modifications be included within 
the spirit and scope of the invention.