Selective metal etching in metal/polymer structures

A differential material removal process wherein a selected material can be rapidly removed without adverse impact to surrounding layers of different materials. Ultraviolet radiation is used to selectively remove metal without adversely harming adjacent polymer layers, in a metal-polymer multilayer structure. The wavelength (100-400 nm) of the ultraviolet radiation and the energy fluence per pulse are selected so that the removal rate of metal due to thermal processes is significantly greater than the removal rate of the polymer by ablative photodecomposition. This can occur at an energy fluence per pulse level greater than that at which the etch rate of the polymer begins to level off. For example, copper of a thickness less than 5 microns is rapidly etched in one or two pulses while adjacent polyimide layers are substantially unetched by the application of ultraviolet pulses of wave-lengths 248-351 nm, at energy fluences per pulse in excess of approximately 3 or 4 J/cm.sup.2.

DESCRIPTION 
1. Field of the Invention 
This invention relates to selective etching of metal lines by ultraviolet 
laser radiation, and more particularly to a technique for selectively 
removing metal in a metal-polymer environment without adversely harming or 
otherwise etching the polymer, by the incidence of laser pulses having 
wavelengths in the range 100-400 nm and energy fluences sufficient to 
produce a differential etch rate such that the incident laser pulses will 
rapidly etch the metal while only insubstantially etching the polymer. 
2. Background Art 
It is well known to use lasers for many purposes in the microelectronics 
and packaging industries, where lasers have been used for such diverse 
applications as etching, film deposition, repair of open circuits and 
short circuits, machining and trimming of component values, and heating of 
solder connections. Reference is made to the following articles and 
patents as illustrative examples of these uses of lasers: 
P. W. Cook et al, IBM Technical Disclosure Bulletin, Vol. 17, No. 1, p. 
242, June 1974. 
J. A. Brunner et al, IBM Technical Disclosure Bulletin, Vol. 22, No. 12, p. 
5319, May 1980. 
G. T. Sincerbox et al, IBM Technical Disclosure Bulletin, Vol. 23, No. 4, 
p. 1684, September 1980. 
U.S. Pat. Nos. 4,172,741; 4,258,078; and 4,448,636. 
In these applications, the laser pulses are primarily used as heat sources, 
in order to melt, ablate, or otherwise alter a material through a thermal 
mechanism. However, the use of a thermal mechanism can often lead to 
damage to the surrounding layers or underlayers, which is particularly 
pronounced when the laser pulses are not well focused and where the 
surrounding layers are very heat-sensitive. In this latter situation, 
surrounding dielectric or protective layers, such as polymers, can have 
their mechanical and chemical properties seriously altered by these 
thermal effects. At visible wavelengths the absorption coefficients of 
these polymer layers are very small and the polymers are essentially 
transparent to this radiation. In such cases the structure beneath the 
protective or dielectric layers can also be affected and is sometimes 
damaged. 
A newly discovered laser technique, termed ablative photodecomposition 
(APD), does not totally rely on a thermal mechanism for etching polymeric 
materials. Instead, photochemical, thermal, and other effects are the 
contributing mechanisms by which polymeric materials are rapidly and 
cleanly etched without damage to surrounding layers of the same, or 
different materials. APD relies on the incidence of ultraviolet radiation 
having wavelengths less than 400 nm, and a sufficiently large energy 
fluence per pulse that the threshold for ablative photodecomposition is 
overcome. 
The concept of APD has been described in the following references: 
R. Srinivasan, J. Vac. Sci. Technol., B1 (4) p. 923, October-December 1983. 
R. Srinivasan et al, J. Polymer Science: Polymer Chemistry Edition, Vol. 
22, 2601-2609 (1974). 
APD is a process in which ultraviolet radiation of wavelengths less than 
400 nm causes new effects in different materials. These materials absorb a 
very high percentage of this radiation in a very thin surface layer of the 
material, wherein the absorption of the radiation is confined to a very 
small volume of the material. This absorption occurs very rapidly and 
produces material fragments which explode or "ablate" from the surface of 
the ablated materials, leaving behind a localized etched region. 
Application of further pulses of radiation will cause additional etching. 
As noted, this effect requires the UV radiation to have an energy fluence 
which exceeds a particular threshold. This threshold depends on the 
wavelength of the incident radiation and on the material being ablated. 
Also, the pulse width and duty cycle of the radiation must be such that 
the incident energy is delivered to the material more quickly than it can 
be lost through normal thermal relaxation processes. Generally, lasers of 
the excimer-type provide very useful sources of radiation in the desired 
wavelength range, and can be used to produce pulsed radiation of 
sufficient energy fluence per pulse to cause ablative photodecomposition. 
In particular, some commercially available excimer lasers suitable for 
this purpose include the argon fluoride excimer laser (193 nm), the 
krypton fluoride excimer laser (248 nm), the xenon chloride excimer laser 
(308 nm), and the xenon flouride excimer laser (351 nm). 
Generally, the threshold energy fluence per pulse required for ablative 
photodecomposition increases as the wavelength increases, for a given 
material. Thus, for a given material, the required energy fluence per 
pulse is greater for 351 nm radiation than for 193 nm radiation. However, 
the required optics are usually less complicated and costly for higher 
wavelength radiation. For example, the optics required for radiation at 
351 nm wavelength are less complicated and less expensive than those 
required for 193 nm wavelength radiation. Of course, these considerations 
regarding the necessary optics and the type of available sources that can 
be used are well known to those skilled in the laser and optics 
technologies. 
UV pulse radiation has been applied by many people for many uses in order 
to provide significant processing advantages. Some of the uses of this 
effect include the deposition of metal (U.S. Pat. No. 4,451,503), the 
photoetching of polyesters (U.S. Pat. No. 4,417,948), the deposition of 
metals on polyester by irradiation of selected areas with UV, followed by 
a preplating treatment and then electroless plating (U.S. Pat. No. 
4,440,801), photoetching of polyimides (co-pending application Ser. No. 
561,445, filed Dec. 14, 1983, and U.S. Pat. No. 4,508,749), surgical and 
dental procedures wherein organic tissue is irradiated with UV (co-pending 
application Ser. No. 448,123, filed Dec. 9, 1982), conversion of SiO to 
SiO.sub.2 (co-pending application Ser. No. 448,127, filed Dec. 9, 1982, 
and now abandoned), and dry photolithography in which resist-type 
materials are patterned by irradiation with UV wavelengths (U.S. Pat. No. 
4,414,059). 
Thus, while pulsed UV irradiation has been applied to the deposition and 
etching (APD) of many materials, these procedures have involved energy 
fluences per pulse roughly between about 10 mJ/cm.sup.2 and 350 
mJ/cm.sup.2. The reason for this may be that most polymer-type materials 
appear to be ablatively photodecomposed most efficiently at energy 
fluences of about 350 mJ/cm.sup.2 or less. APD has not been applied to 
differential etching of metal lines in a metal-polymer environment where 
the polymer cannot be adversely etched (i.e., excessively etched) during 
etching of the metal lines. One reason for this may have been the concern 
that, at the fluence levels generally used, it would not be possible to 
protect the polymer layers from excessive etching without the use of a 
mask, while the metal lines were being etched. Stated another way, it 
would have been expected that etching of metal would cause an excessive 
amount of APD of the surrounding polymer layers. For multilevel thin film 
structures using very thin polymer films (for example, 5-10 microns of 
polyimide), this problem becomes more severe since these thin polymer 
films in the vicinity of or under the metal may be severely damaged during 
metal etching. This damage can be in the form of bubbling, exploding, and 
charring of the polymers. 
The aforementioned damage to polymer layers during the etching of metals is 
primarily caused by the use of laser beam spots which do not perfectly 
match the shape of the metal to be etched, thereby exposing the polymer 
layers to radiation. Another cause of polymer damage in systems using 
lasers emitting light in the visible to infrared wavelength ranges is that 
the light absorption coefficient of the polymers is very small at these 
wavelengths. This means that the focused laser beam will go through 
microns or tens of microns of polymer before it is attenuated to a level 
which causes little damage. Therefore, the damaged layer can be microns 
deep in the polyimide, thereby rendering it ineffective as a protective 
dielectric layer. 
Ultraviolet wavelength pulses, on the other hand, are readily absorbed in 
most polymers due to their high absorption coefficients at these 
wavelengths. However, at the conventionally used energy fluences, 
considerable amounts of polymer would be ablated along with the metal 
lines. Since laser processing is attractive for many reasons, it is a 
primary object of the present invention to provide a technique wherein 
laser etching can be safely used in an environment including multilayer 
structures of metals and polymers. To this end, it has been discovered 
that, if the energy fluence per pulse is increased to an amount at least 
an order of magnitude beyond that conventionally used, the etch rates of 
polymers will be very small with respect to the etch rates of metals, 
thereby allowing selective etching of metals and minimum harm to the 
surrounding polymer layers. 
Thus, it is another object of the present invention to provide a 
differential etch technique for selectively etching metal in metal-polymer 
structures. 
It is another object of the present invention to provide a differential 
etching technique that can be applied to multilayer structures of metals 
and polymers, to provide selective etching of a variety of metals without 
adversely etching nearby thin polymer layers. 
It is another object of the present invention to provide a technique for 
dry photoprocessing of multilayer structures, such as those which are 
commonly used in microelectronics and packaging. 
It is another object of the present invention to provide a differential 
etching technique that can be used for selective etching of one material 
in a structure comprised of a plurality of materials, where the materials 
are those commonly employed in microelectronic structures and packaging. 
It is another object of the present invention to provide an improved 
technique for ablation of metals on polymers, where the metals can be 
selectively removed without severe damage to the polymers. 
It is another object of the present invention to provide a selective 
etching technique which can be applied to copper-polyimide structures in 
order to selectively etch copper while not adversely etching polyimide. 
DISCLOSURE OF THE INVENTION 
This invention is a technique for selective etching of metals in an 
environment including both metal layers and polymer layers, in a manner to 
completely etch the metal layers without substantial etching of the 
polymer layers. Ultraviolet radiation in the wavelength range 100-400 nm 
is applied to structures that include both metals and polymers, where the 
energy fluence per pulse of the laser radiation is sufficiently high that 
the metal etches more rapidly than the polymers, and perferably at least 
about twice as fast. 
As an example, a few microns of copper can be etched approximately twice as 
fast as polyimide can be ablated, for irradiation with laser pulses in the 
wavelength range 100-400 nm and energy fluence per laser pulse of 
approximately 4J/cm.sup.2. However, for energy fluences per pulse in 
excess of about 1J/cm.sup.2, differential etching will occur, but to a 
lesser degree. Thus, the invention is directed to a technique in which far 
UV radiation of wavelengths 100-400 nm is used to ablate (etch) metals in 
the presence of polymers, and especially thin polymer layers, wherein the 
metal layers are completely etched while the degree of etching to the 
polymer layers is minimal even though the polymer layers are exposed to 
the laser radiation. 
Examples of metal/polymer structures are those that include metals such as 
copper, Au, Ti, W, Cr, Ni, sandwich metallurgies such as Cr-Cu-Cr and 
Cr-Cu-Ni-Au-Cr, and polymers such as polyimide, resist materials, mylarIM, 
PMMA, etc. Metals which are highly reflective (i.e.,.sqroot.60%) at these 
wavelengths generally have to be doped to reduce their reflectivity below 
this amount. An example, is A1 doped with about 4% Cu, and other alloys. 
These and other objects, features, and advantages will be apparent from the 
following more particular description of the preferred embodiments.

BEST MODE FOR CARRYING OUT THE INVENTION 
This invention relates to a differential etching technique that can be used 
to provide selective etching of a layer in a multilayer structure, without 
adversely etching other layers in the structure. More specifically, it is 
based on the discovery that, at sufficiently high energy fluences, the 
rate of etching of polymers via APD will be significantly less than the 
rate of etching of metals, for incident UV laser pulses in the wavelength 
range 100-400 nm. This allows one to etch metals completely while not 
excessively etching an exposed polymer layer. Thus, the thickness, 
integrity, mechanical and chemical properties of adjacent polymer layers 
is preserved. 
In the practice of this invention, laser pulses are utilized for etching 
selected layers in a multilayer structure. The wavelength, energy fluence 
per pulse, pulse width, and duty cycle of the laser pulses are chosen so 
that polymer layers in the multilayer structure will undergo ablative 
photodecomposition (of only a limited amount)--the mechanism of etching of 
the metal layers is not important and is generally a thermal mechanism. 
The pulse width and duty cycle of the radiation is chosen so that APD of 
the polymer will occur. Thus, a sufficient amount of energy is delivered 
that the threshold for APD is exceeded in a time period that is short with 
respect to thermal diffusion lengths in the polymer. Pulse widths of less 
than about 100 ns have been found to be satisfactory. 
Since the polymer layer often separates metal layers, the polymer layer 
must not be transparent to the incident radiation in order to prevent this 
radiation from being transmitted to an underlying metal layer. However, 
the polymer must not be excessively etched by absorption of this 
radiation. Therefore, the wavelength range and energy fluence per pulse 
are critical to have selective absorption with minimal etching of the 
polymer. This requirement insures that the thickness and integrity of the 
polymer layer are preserved, while at the same time protecting underlying 
metal layers from being etched when an overlying metal layer is 
selectively removed. 
Still further, in the practice of the present invention it has been 
discovered that a minimum number of laser pulses, and preferably only one, 
can be used to provide the desired selective etching of many thicknesses 
of the metal lines. This also minimizes damage to surrounding or 
underlying metal and/or polymer layers which are not to be etched. 
In the practice of this invention, the characteristics of the laser 
radiation are chosen so that the metal etch rate is at least about twice 
as fast as the polymer etch rate, for exposure to the same radiation. Of 
course, even these large differentials may not be sufficient to protect 
the polymer layer if that layer is very much thinner than the metal layer. 
On the other hand, if the polymer layer is much thicker than the metal 
layer, successful selective etching of metal is possible even at much 
lower energy fluences where the etch rates of polymers and metals are 
substantially the same. As will be apparent to those skilled in the art, 
the amount of energy fluence/pulse will have to be increased as the 
thickness of the metal layer increases if it is desired to completely etch 
the metal layer with a minimum number of laser pulses. 
These concepts will become more apparent by reference to FIGS. 1-3, which 
illustrate the application of this process to a multilayer structure 
comprising a plurality of metal layers separated by a polymer layer. This 
structure is intended to be representative of the many different types of 
multilayer structures found in the microelectronics industry, and in 
particular in various electronic packages. In FIG. 1, the structure to be 
selectively etched includes a substrate 10, a layer 12 of polymer (such as 
polyimide), a first metal layer 14, another layer 16 of polymer, and a 
further metal layer comprised of portions 18A, 18B, and 18C. It is desired 
to selectively etch portion 18A without adversely (i.e., excessively) 
etching metal portions 18B, 18C, and underlying metal 14. In order to do 
so, the incident laser pulses must not substantially etch polymers 12 and 
16. However, this is difficult to accomplish because the input laser beam 
often is not perfectly matched to the size of the metal region to be 
etched, and the polymer layer often does not have a sufficiently high 
absorption coefficient for the incoming laser pulses. By operating with 
selected wavelengths and energy fluences per pulse, however, the desired 
result is achieved, as will be explained with reference to FIGS. 2 and 3. 
In FIG. 2, ultraviolet light represented by arrows 20 impinges upon the 
multilayer structure, and more particularly on metal 18A. 
The ultraviolet radiation 20 can be directed through a mask (not shown) 
onto the multilayer structure, or a focussed light beam can be directed 
across the surface of metal 18A, in order to etch it. Further, it is 
preferrable that the entire depth of metal 18A be etched by a single laser 
pulse (or very few pulses), rather than requiring several pulses for the 
complete etching. This provides high efficiency etching and also minimizes 
any possible damage and/or etching to surrounding polymer regions 12 and 
16. Still further, polymer regions 12 and 16 must have a high absorption 
coefficient for the incoming radiation to prevent this radiation from 
being transmitted to laterally adjacent metal portions 18B and 18C, and to 
underlying metal portion 14. 
FIG. 3 shows the result that is achieved when the incoming ultraviolet 
radiation has a properly selected wavelength and energy fluence per pulse. 
In this figure, metal portion 18A has been completely etched, there being 
no damage to metal portions 14, 18B, and 18C. Polymer regions 12 and 16 
have been minimally etched by the radiation and still provide a protective 
and/or dielectric function. These functions are maintained even if polymer 
regions 12 and 16 are very thin or very narrow. 
The ultraviolet radiation 20 has a wavelength and energy fluence per pulse 
such that the etch rates of metal and polymer are dissimilar, and 
preferrably dissimilar by a factor of at least 2. This is accomplished if 
the wavelength and energy fluence per pulse are chosen such that the metal 
layer is rapidly etched, or ablated, while the polymer layer undergoes 
only a small amount of ablative photocomposition (i.e., the removal rate 
of the polymer is small and approximately constant). For some polymers, 
this minimum energy fluence is approximately 3-6 J/cm.sup.2, per pulse. 
This minimum fluence depends on the metal thickness and metal absorption. 
Thicker metal films will require higher energy fluences in order to be 
etched with a very small amount of pulses, as explained previously. The 
wavelength range for the ultraviolet radiation is 100-400 nm, but the 
particular wavelength chosen is matched to the relative rates of etching 
of the metal and the polymer. The polymer must etch slowly by APD, while 
the metal must etch rapidly by any mechanism, and generally a thermal 
mechanism. 
The foregoing will be more clear with reference to FIGS. 4 and 5, and the 
examples to be described hereinafter. 
FIG. 4 plots the etch depth per pulse, in angstroms, for polyimide which is 
etched with radiation of 248 nm and 308 nm, as a function of the energy 
fluence per pulse. While this polyimide is spin applied, similar results 
(within about 10%) have been found when Kapton (a trademark of E. I. 
Dupont de Nemours) is used. 
As is apparent from FIG. 4, the etch rate of polyimide at these wavelengths 
saturates at approximately 3-5 J/cm.sup.2 and remains relatively constant 
thereafter. On the other hand, a metal such as copper (of a few microns 
thickness) will be rapidly removed by laser pulses having energy fluences 
greater than this amount. 
This type of saturation may exist for longer wavelengths and for shorter 
wavelengths than those illustrated in FIG. 4. It is anticipated that 
polyimide will saturate at a higher fluence if the wavelength is increased 
beyond 308 nm, and will saturate at a lesser fluence for wavelengths less 
than 248 nm. For example, the saturation level for 351 nm laser pulses is 
higher than that for 308 nm radiation, but only slightly higher. 
FIG. 5 is a plot of etch depth per pulse versus energy fluence per pulse 
for ablative photodecomposition of polymethyl methyacrylate (PMMA), at 248 
nm. This plot also shows a saturation effect in the etch rate of the 
polymer, where the saturation occurs in the approximate range of 4-8 
J/cm.sup.2. This saturation effect is even more pronounced when the energy 
fluence for ablative photocomposition is approximately 24 J/cm.sup.2. Of 
course, the removal rate of a metal at these high fluences is extremely 
large, and several microns of metal can be removed in a single laser pulse 
at these fluence levels. 
In contrast with the leveling-off of the etch rate of PMMA illustrated in 
FIG. 5, this leveling-off is not observed for ablative photodecomposition 
of PMMA at 308 nm. Thus, in the practice of this invention it is important 
to choose both the wavelength and energy fluence such that the metal is 
rapidly removed, while the polymer is only very slightly etched by an 
ablative process. In order to have a large tolerance window when etching, 
the energy fluence can be set at an amount where the APD of the polymer 
has leveled-off. 
As a specific example, a multilayer structure comprising polyimide and 
copper has been selectively etched to remove a copper layer without undue 
etching or harm to the polyimide layers. For laser radiation of 
wavelengths 248 nm, 308 nm, and 351 nm, and energy fluences per pulse in 
excess of 3J/cm.sup.2, selective etching can be accomplished. Four microns 
of sputtered copper film on cured polyimide can be etched in about two 
laser pulses at a fluence of approximately 4J/cm.sup.2 at 248 nm and 351 
nm. The polyimide removal rate at such fluences is very small, and is 
approximately 1 micron/pulse at 351 nm. Using a fluence of 4J/cm.sup.2 and 
a wavelength of 248 nm, a sputtered copper film of 4 microns thickness is 
etched in 1-2 pulses, while 7 pulses at the same energy fluence and 
wavelength are required to etch 5 microns of polyimide. Thus, the relative 
rates of etching of the copper and polyimide are very dissimilar and 
selective etching of a multilayer structure can be done without harm to 
surrounding or underlying metal layers. 
Another advantage is that the reflectivity of many metals is substantially 
reduced in the wavelength range used in this invention. For example, 
copper has a reflectivity in the visible and infrared regions which is 
greater than 90%. The reflectivity of copper drops to less than 50% in the 
wavelength range used in the present invention. Thus, there is an improved 
coupling of the incident energy into the metal film to be removed at these 
short wavelengths. This, in combination with the high absorption 
coefficient of polymers at these wavelenghs, leads to almost total 
absorption of the ultraviolet radiation in an extremely thin surface 
region of the polymer, thereby leaving the bulk of the polymer unetched 
and otherwise unharmed. As an example, a thin surface region of polyimide 
less than 1 micron would be affected by an ultraviolet pulse in this range 
of wavelengths, while several microns of copper would be totally removed 
by this single laser pulse. 
Thus, in the practice of this invention, selected regions of a metal, such 
as copper, Cu-doped Al, titanium, tungsten, etc. can be removed rapidly 
with one or two laser pulses, while the same laser pulses will affect only 
a very thin surface region of the polymer, if the wavelength and energy 
fluence per pulse of the ultraviolet radiation are suitably chosen. In 
accordance with the principles of this invention, this choice is such that 
a considerable depth of metal is removed rapidly while the polymer is 
ablatively photodecomposed by only a minimum amount. Thermal effects are 
not necessarily a dominant mechanism in the ablation of the polymer, but 
are probably the dominant effect in the metal removal process. 
It will be appreciated by those of skill in the art that ablative 
photodecomposition of polymers will occur at low energy thresholds, 
approximating 10-30 mJ/cm.sup.2. However, at these energy fluence levels, 
no metal removal is observed. Thus, this is not an appropriate regime for 
a selective etching operation. Instead, the energy fluence per pulse has 
to be sufficiently high that the metal removal rate is rapid, while the 
polymer etch rate due to ablative photodecomposition is small (and 
preferably has leveled-off) with respect to the metal removal rate. 
While the invention has been described with respect to particular 
embodiments thereof, it will be apparent to those of skill in the art that 
variations can be made therein without departing from the spirit and scope 
of the present invention. For example, metals and polymers other than 
those specifically illustrated herein can be envisioned for specified 
wavelengths and energy fluences in order to provide selective removal of 
these metals from multilayer structures. However, such metal and polymer 
combinations will be etched in accordance with the principles of the 
present invention.