Method of optical inspection

A method for determining the configuration of a resin portion of an article comprising exposing the article to a light source at a first wavelength and measuring emission of light at a second, different wavelength; wherein the resin contains a moiety containing a structure of the formula: ##STR1## or a moiety which is a reaction product of said moiety in a quantity sufficient to impart fluorescent properties to said resin.

FIELD OF THE INVENTION 
This invention relates to methods of optically inspecting articles and in 
particular articles containing thin films of benzocyclobutene-containing 
resins or reaction products thereof. 
BACKGROUND OF THE INVENTION 
There are a wide variety of resins which may be used as dielectric layers 
in electronic applications or as thin films in other applications. It is 
advantageous to be able to determine the configuration of said resins 
during or after the processing steps used to fabricate a finished article 
containing the resin. By configuration it is meant such things as the 
shape, thickness, uniformity of thickness, presence or absence of voids, 
contamination, mounds of excess material, presence or absence of resin, 
concentration of the fluorescing species or inclusions in the resin and 
the like. 
A useful method for finding voids, contaminants and excess material is 
called optical inspection which may be used to find such defects in 
dielectric layers in microelectronic devices. It is beneficial to use the 
fluorescent properties of the resin for such inspections. Not all resins 
are inherently fluorescent at useful wavelengths. 
U.S. Pat. No. 5,040,047 discloses a method for enhancing the fluorescence 
of resins by adding fluorescent dyes such as perylene to the polymer. This 
adds steps to the process for making the microelectronic articles and the 
presence of the dye changes the properties of the resin leading to 
potential deterioration of the properties or requiring a change in the 
formulation or procedure for using the resin. 
SUMMARY OF THE INVENTION 
The invention is a method for determining the configuration of a resin 
portion of an article comprising exposing the article to a light source at 
a first wavelength and measuring emission of light at a second, different 
wavelength; wherein the resin contains a moiety containing a 
benzocyclobutene structure of the formula: 
##STR2## 
or a moiety which is a reaction product of said moiety in a quantity 
sufficient to impart fluorescent properties to said resin. 
A feature of the invention is the use of a resin containing a moiety 
containing a structure of the formula: 
##STR3## 
or a moiety which is a reaction product of said moiety in a quantity 
sufficient to impart fluorescent properties to said resin. 
An advantage of the invention is that since the resin which contains the 
benzocyclobutene moiety or reaction product thereof is inherently 
fluorescent one does not have to add a dye to the resin to obtain such 
fluorescence, thus saving process steps and avoiding potential 
deterioration of resin properties. When the resin which contains the 
benzocyclobutene moiety or reaction product thereof does not already 
inherently fluoresce and does not contain a moiety which quenches the 
fluorescence, the addition of said moiety which is chemically bound to or 
alloyed into the resin avoids potential property changes encountered with 
additive formulations. 
An additional advantage is that many resins containing the benzocyclobutene 
moiety or reaction product thereof are transparent and therefore difficult 
to see with optical inspection equipment operating in the visible light 
spectrum. In the near ultraviolet spectrum the resin will absorb light 
making the resin less transparent and permitting properties such as 
thickness and concentration to be measured. 
DETAILED DESCRIPTION OF THE INVENTION 
Resins useful in the method of the invention are disclosed in numerous 
places. Generally, resins containing reacted or unreacted moieties of the 
formula: 
##STR4## 
are useful. The 
##STR5## 
moiety may react to form useful structures by reacting as shown with 
itself to form moieties, nominally, of the structure shown: 
##STR6## 
or by reacting with a dienophile such as an ethylenic unsaturation as 
shown to form moieties, nominally, of the structure shown: 
##STR7## 
The structures may inherently cause fluorescence or may spontaneously 
change to form other structures which cause fluorescence. For example, 
thermal or oxidative degradation or both may lead to dehydrogenation at 
benzylic sites and the formation of conjugation extended beyond the 
benzene rings. 
Not all reacted benzocyclobutene moieties will form resins which inherently 
fluoresce. Such non-fluorescing resins are not within the scope of this 
invention. Exemplary inventive resins are disclosed in the following 
references. 
U.S. Pat. No. 4,708,994 discloses the incorporation of reactive 
arylcyclobutene groups by means of alkylating or acylating aromatic groups 
of resins with a molecule containing a reactive arylcyclobutene moiety to 
provide pendant arylcyclobutene moieties randomly along the polymer 
backbone. Such resins are then crosslinked and cured during a subsequent 
heating step and shown to be more solvent and heat resistant. 
U.S. Pat. Nos. 4,795,827 and 4,825,001 disclose cyclobutarene ketoaniline 
monomers that may be used for attaching cyclobutarene groups to polymers 
or other molecules having amino-reactive functionalities. 
U.S. Pat. No. 5,198,527 discloses a carbonate polymer prepared from one or 
more multi-hydric compounds and having an average degree of polymerization 
of at least about 2 based on a multi-hydric compound and having terminal 
benzocyclobutene moieties. Other embodiments of the invention include such 
carbonate polymers having an average degree of polymerization of from 
about 2 to about 100 and carbonate polymers having polymerized therein 
from about 0.01 to about 1 mole of terminal benzocyclobutene-containing 
compound per mole of multi-hydric compound. 
U.S. Pat. No. 4,540,763 discloses polymers of poly(arylcyclobutenes) and 
how they may be made. U.S. Pat. No. 4,812,588 discloses polymers of 
poly(arylcyclobutenes) bridged by an organopolysiloxane group and how they 
may be made. 
U.S. Pat. No. 5,034,485 discloses a polymeric composition produced by the 
reaction of, for example, styrene in a free radical polymerization 
reaction which is initiated by a cyclobutarene peroxide, wherein the 
cyclobutarene fragments are incorporated into the polystyrene polymer. A 
further polymeric product can be produced from the polymeric composition 
of the free radical polymerization by ring opening polymerization of the 
cyclobutarene moiety to produce branched, crosslinked or a mixture of 
branched and crosslinked polymers. 
U.S. Pat. No. 4,724,260 discloses polymeric compositions comprising, in 
polymerized form, a monomer containing a polymerizable arylcyclobutene 
moiety, and a polymerizable unsaturated alkyl moiety; wherein the monomer 
is polymerized by subjecting it to conditions sufficient to polymerize the 
unsaturated alkyl moiety. A related disclosure is U.S. patent application 
Ser. No. 872,334, filed Jun. 9, 1986, now U.S. Pat. No. 5,360,296 which 
exemplifies copolymers of vinylbenzocyclobutene and styrene. 
U.S. Pat. No. 4,698,394 discloses a solid random copolymer comprising from 
about 99.99 to about 80 mole percent of a monoalkenyl arene monomer and 
from about 0.01 to about 20 mole percent, based on total moles of 
incorporated monoalkenyl arene monomers and olefinic benzocyclobutene 
monomers, of an olefinic benzocyclobutene of the formula: 
##STR8## 
where R.sub.1 is hydrogen or CH.sub.3 and R.sub.2 is (CH.sub.2).sub.n 
where n is 0 to 6. Copolymers of vinylbenzocyclobutene and styrene are 
exemplified. 
U.S. Pat. No. 5,077,367 discloses that syndiotactic homopolymers of an 
arylcyclobutene functional monomer and syndiotactic copolymers of an 
arylcyclobutene functional monomer and a vinylaromatic monomer are 
prepared by polymerizing the monomers in the presence of a catalytic 
amount of a suitable coordination catalyst such as the reaction product of 
polymethylaluminoxane and a transition metal compound. 
U.S. Pat. No. 5,185,391 discloses polymers formed by a side ring opening of 
an arylcyclobutene moiety such as polymers of 
##STR9## 
1,3-bis(2-bicyclo[4.2.0]octa-1,3,5-trien-3-ylethenyl)-1,1,3,3-tetramethyld 
i 
siloxane (hereinafter DVS), available as a partially polymerized solution 
in mesitylene from The Dow Chemical Company as Cyclotene.RTM. 3022 and an 
amount of an antioxidant such as a compound of the formula: 
##STR10## 
sufficient to inhibit oxidation of the polymer. 
European Patent Publication 227124 discloses various copolymers of, for 
example, vinyl benzocyclobutenes and other additional polymerizable 
materials. These copolymers may be heated to react the benzocyclobutene 
moieties and crosslink the copolymers. 
Monomers of the formulae: 
##STR11## 
or polymers and copolymers thereof are preferrred. 
Monomers of the formulae: 
##STR12## 
or polymers and copolymers thereof are more preferrred. 
Homopolymers of a monomer of the formula 
##STR13## 
are most preferred. 
A number of resins containing either the structure 
##STR14## 
or the reaction product of said structure show fluorescence. 
In general, resins containing either the structure 
##STR15## 
or the reaction product of said structure show strong fluorescence. 
The functionality attached to the structure may also quench the 
fluorescence. Exemplary quenching moieties are ketones and hydroxy 
functionalities. The property of fluorescence of said structures is most 
useful when incorporated into resins which do not contain other moieties 
fluorescing with the quantitative efficiencies of the benzocyclobutene 
moieties and when said resins do not contain a quenching moiety. 
As one aspect of determining the configuration of a resin, one may 
determine the quantity or concentration of the structure: 
##STR16## 
or the reaction product of said structure incorporated into the resin. 
Said structure may be copolymerized into the resin or may be blended into 
the resin as an alloy or may be inhomogeneously mixed with the resin. 
The strength of fluorescence may be a good indicator of the quantity or 
concentration of the structure: 
##STR17## 
or the reaction product of said structure incorporated into the resin. 
Surface defects and thickness of coatings of the resin of the invention 
over another resin may be measured using the fluorescence. For example, a 
polycarbonate containing terminal groups of the structure: 
##STR18## 
or the reaction product of said structure as described in U.S. Pat. No. 
5,198,527 may be coated onto or coextruded with polycarbonates that do not 
contain said structure. One may determine which side of the article 
contains the coated or coextruded layer and the thickness or absence of 
the coated or coextruded layer. This is most beneficial when both resins 
are optically clear in visible light. 
A polycarbonate containing terminal groups of the structure 
##STR19## 
or the reaction product of said structure as described in U.S. Pat. No. 
5,198,527 may show a shift in fluorescence. The resin fluorescence at a 
molar concentration of 0.03/1 benzocyclobutene to bisphenol A has two 
large maxima at about 370 and 400 nm. The resin fluorescence at a molar 
concentration of 1/1 benzocyclobutene to bisphenol A has a single large 
maximum at about 420 nm. One may take this possibility into account when 
determining benzocyclobutene concentration in a polycarbonate or other 
resin and make empirical concentration standards against which to measure 
inspected samples in the range of interest. 
A fluorescence spectrometer may be used to screen resins for suitability 
for fluorescent optical inspection. The fluorescence spectrometer 
comprises of a broad band light source, a pair of monochrometers, a sample 
cell and a detector. The light from the source is filtered through the 
first monochrometer to a wavelength of interest. The filtered light is 
then directed onto the resin sample. Fluorescent light emitted by the 
sample is measured at various wavelengths using the second monochrometer. 
One may measure the ratio of illumination intensity of the source and 
emitted from the sample to obtain a more meaningful ratio of the intensity 
of the fluorescence to the source. Not only may one determine if the resin 
is suitable for fluorescent optical inspection using this method, but one 
may also optimize the incident light and fluorescent light wavelengths to 
obtain the most sensitive measure of resin configuration. 
Fluorescent optical inspection may be carried out by illuminating a part 
with a specific wavelength of light and inspecting the part at a different 
wavelength. The illuminating wavelength is filtered from the light emitted 
to the detector. A simplified embodiment comprises a mercury lamp which 
illuminates the part at a wavelength of 365 nm. The emitted light is 
detected with a stereoscope and a camera. The part is placed in the field 
of view of the stereoscope and the part is illuminated with the lamp from 
the side. The crown glass optics of the stereoscope filter out the 
reflected UV light from the lamp, but allow the longer wavelength 
fluorescent light from the part to be observed. The camera may be used to 
record the emitted light. 
One may use conventional, commercially available, optical inspection 
devices for such optical inspection. Typically these devices use a bright 
line source, a broad band pass filter and some form of machine vision. The 
source of illumination may be a laser or atomic line emission source. The 
use of a line source eliminates the need to prefilter the light to get 
only the wavelength of interest. The emission filter system is chosen to 
block only the illuminating light wavelengths, while allowing fluorescent 
light of any wavelength to pass to the detector. The detector may be an 
array type so that computer logic can be applied to pass or reject parts. 
Preferably the illuminating and fluorescent light wavelengths are both in 
the near UV. Preferred illuminating wavelengths are 337.5, 356.4 or 408 nm 
of a Kr.sup.+ laser; 351.1 or 383.8 of an Ar.sup.+ laser; 325 or 442 nm of 
a He/Cd laser or 254 or 365 nm of a mercury vapor lamp. 
Fluorescence is the absorption of light at one wavelength and the 
re-emission of that energy at a different, longer wavelength. Fluorescent 
illumination provides an advantage in that one may illuminate with one 
wavelength and detect at a different wavelength of light. Fluorescence 
more easily distinguishes between resin and inclusions such as metal 
circuitry which do not fluoresce. If the incident and reflected light have 
the same wavelength, the only parameter distinguishing portions of the 
article being seen is the reflectance. Reflectance of metal inclusions may 
vary depending on the amount of oxide on the surface. 
There are a number of commercial automated systems available for optical 
inspection of laminate boards and multichip modules. Some commercial 
sources are Optrotech Inc. of Billerica, Mass. or Orbot Inc. of Santa Ana, 
Calif. Photometrics Ltd. of Tuscon, Ariz. produces the "Star I CCD.TM." 
imaging system. This system may be attached to a stereo microscope for 
magnification. The image produced may be stored on a personal computer and 
enhanced with commercially available software programs with features such 
as color enhancement, line and edge signal filtration, contrast 
adjustment, image magnification and the ability to produce images of the 
difference between two objects. An example of such a program is IPLab 
Spectrum.TM. software from Signal Analytics Corp. of Vienna, Va. 
Automated optical inspection may be divided into three basic steps: image 
acquisition, logic and processing effect data. Image acquisition--the 
creation of an accurate digital replica of a circuit pattern is most 
important. The article to be inspected is illuminated by a light source 
and the emitted light is detected usually by a camera which can convert 
the image to digital format. 
U.S. Pat. No. 4,152,723 provides a detailed description of an optical 
inspection method for circuit boards. A beam of light energy scans, in a 
predetermined pattern, a surface of the board comprising a pattern of a 
metallic conductor disposed on an insulating substrate. The beam has an 
energy level high enough to excite detectable fluorescence in the surface 
of the insulating substrate. The fluorescence is selectably detected by 
means sensitive to the wavelength of the fluorescence and is converted to 
a binary signal which indicates whether the beam is incident on the 
fluorescing substrate or on the non-fluorescing metallic conductors. The 
binary signal is then synchronized with the scanning of the beam such that 
a binary image representation of the board's surface is generated. 
Image processing options expand the ability to detect and identify defects 
in thin films. A digital system may be used to compare a desired image, 
generated using a CAD image or an actual functioning (golden board) with 
the board being inspected. The system may be programmed to show only the 
differences between the two images and to distinguish between the types of 
defects. Films could be rejected automatically based on the number of 
areas with too little or too much resin. 
With a Fourier transform of the difference between the two images, the type 
of defect could be identified. For more sensitive processing than 
differentiation, the Fourier transformed image can be examined for spatial 
frequencies characteristic of defects. After filtering the transform for 
defects of interest, an inverse transform would consist of an image of 
only the defects. 
The types of configurations one may determine using the fluorescent optical 
inspection method of the invention are thickness of the resin, coating 
uniformity, particulate contamination, pin holes, gels, bubbles, clearing 
of vias, cracking, blisters, wrinkles and delamination. 
The thicker the resin, the more strongly it emits and thus one may 
determine the thickness or uniformity of thickness. The more concentrated 
the resin is in terms of the 
##STR20## 
moiety or reaction product thereof, the more strongly it will emit and 
thus the concentration of that moiety may be determined. Contaminant 
particles show as dark spots on the picture. Pin holes show as dark spots 
surrounded by a bright ring because of totally internally reflected light 
that is emitted at the edges of the pin hole. Particles of excess resin or 
other lumps of excess resin in the film or on the surface of the film show 
as light spots in the picture. Cracks show as dark lines with bright 
edges. Gels, blisters and wrinkles show as brighter areas. Delaminations 
show as darkened areas.

ILLUSTRATIVE EMBODIMENTS 
The following examples are given to illustrate the invention and should not 
be interpreted as limiting it in any way. Unless stated otherwise, all 
parts and percentages are given by weight. 
EXAMPLE 1 
Cyclotene.RTM. 3022 resin is spin coated on four inch silicon wafers to 
give a 5-10 micrometer thick coating after evaporation of solvent and 
complete curing of the resin. The coatings are illuminated by a mercury 
vapor lamp with the beam passing through a monochrometer at a wavelength 
of 365 nm. Polaroid.RTM. pictures of the image seen through a stereoscope 
are taken. The film is sensitive to the fluorescing wavelengths. 
Dust particles show as dark spots on the picture. Pinholes show as dark 
spots surrounded by a bright ring because totally internally reflected 
light is emitted at the edges of the pin hole. Particles of excess resin 
or other lumps of excess resin on the surface of the film show as light 
spots in the picture. 
Homopolymers of monomers of the formulae: 
##STR21## 
yield similar results. 
EXAMPLE 2 
A high density multichip module with Cyclotene 3022.RTM. resin used as the 
interlayer dielectric is photographed using both visible white light and 
UV light from a 365 nm source. The metal structures show up clearly in 
both photographs. It is difficult to differentiate the different layers of 
metal in the visible light photograph. The resin is transparent to visible 
wavelengths. In the near UV, since the resin fluoresces, the thicker the 
dielectric layer, the more strongly emitting the portion of the image. 
This permits differentiation between different layers of metal circuitry. 
EXAMPLE 3 
A series of wafers are prepared. One half of the area of some silicon 
wafers is sputter coated with aluminum metal. The wafers are then coated 
with a resin formulation containing a partially polymerized resin of the 
monomer of the formula: 
##STR22## 
a photocrosslinking agent, an antioxidant and a solvent. The solvent is 
evaporated to leave a resin layer, nominally, 10-12 micrometers thick. The 
resin layer is photopatterned by exposing and developing it with a 
development solvent and thermally curing it. The resin layer is patterned 
with vias or through holes 1, 2, 5, 10, 15, 25, 35, 50, 75 and 100 
micrometers across in both round and square shapes. 
The wafers are held under a mercury UV lamp at arms length and visually 
inspected. Some of the resin is not competely removed from the smaller 
vias by the development solvent. Residual resin is apparent even when less 
than one micrometer thick. Residual resin 0.1-0.3 micrometers thick is 
visible in the bottom of the smaller vias. Cracks, streaks and other 
defects are observed on some wafers made using a nonoptimized process. 
The wafers are inspected in a light-tight box to exclude room light. Images 
are taken at 39.times. and 115.times. magnification using a stereo 
microscope with a T adaptor and a Star I CCD.TM. imaging system. The image 
is transferred via an IEEE-488 interface to a Macintosh IIci PC loaded 
with IP Spectrum.TM. software. The image is stored on a PC or mainframe 
computer. The image may be processed by contrast adjustment, digital 
filtering, color enhancement or any combination thereof. Any necessary 
hard copy of the image may be made with an existing compatible printer. 
Images are taken while the wafers are illuminated with diffuse white light, 
illuminated with a line filtered 365 nm mercury UV lamp or illuminated 
with the mercury lamp with the box open to let in room light. Diffuse 
white light shows shadows of vias down to 25 micrometers and hints of vias 
at 15 micrometers. One is not able to determine whether the smaller vias 
are open. The 100 and 75 micrometer vias appear to be open. It is 
difficult to tell whether the 50 micrometer vias are open. 
When illuminated with the line filtered 365 nm UV lamp open vias appear as 
dark areas in the otherwise light emitting resin. The 50, 75 and 100 
micrometer vias appear to be open. The 25 and 35 micrometer vias are 
apparent but it is not clear whether they are open due to the low 
magnification. 
With the one side of the box open, one is able to see via features as small 
as two micrometers. The mercury lamp is at an angle. One may obtain 
clearer images of smaller features by moving the lamp to a position 
perpendicular to the feature of interest.