Emission spectra and etching of polymers and graphite irradiated by excimer lasersG. Koren and J. T. C. Yeh Citation: Journal of Applied Physics 56, 2120 (1984); doi: 10.1063/1.334211 View online: http://dx.doi.org/10.1063/1.334211 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/56/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Neutral and ion emissions accompanying pulsed excimer laser irradiation of polytetrafluoroethylene J. Appl. Phys. 74, 4729 (1993); 10.1063/1.354341 Emission spectroscopy during excimer laser ablation of graphite Appl. Phys. Lett. 57, 2178 (1990); 10.1063/1.103927 Periodic surface structures in the excimer laser ablative etching of polymers Appl. Phys. Lett. 57, 765 (1990); 10.1063/1.103414 Excimer laser etching of polymers J. Appl. Phys. 59, 3861 (1986); 10.1063/1.336728 Emission spectra, surface quality, and mechanism of excimer laser etching of polyimide films Appl. Phys. Lett. 44, 1112 (1984); 10.1063/1.94661

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Emission spectra and etching of polymers and graphite irradiated by excimer lasers

G. Koren Physics Department, Technion-Israel Institute o/Technology, Haifa, 32000, Israel

J. T. C. Yeh IBM T. J. Watson Research Center. Yorktown Heights. New York, 10598

(Received 9 January 1984; accepted for publication 22 May 1984)

Emission spectra in the visible and ultraviolet range, scanning electron microscopy of various polymers [polyimide, Mylar, and polymethyl-methacrylate (PMMA)], and graphite exposed to 193-,248-, and 351-nm laser radiation were used to investigate the laser etching process. Measurements were performed under vacuum and in air and He environments. At low laser fluence, which depends on the sample and laser wavelength, an unresolved continuum emission was observed. At higher fluences clear e2, e, and in some cases also eN emissions were found in addition to the continuum. At still higher fluences ionic carbon emissions became dominant. It was found that laser-produced plasma processes are responsible for most of the observed emissions. Rough and smooth etched surfaces were obtained at different laser fluences depending on the linear absorption coefficient of the sample at the laser wavelength. In polyimide, a rough­to-smooth surface transition versus the laser fluence was found to occur exactly when the Cz and C emissions appeared on top of the continuum. It is suggested that the laser etching mechanism is mostly a statistical thermodynamic process, but without complete energy randomization.

I. INTRODUCTION

Recently, 193-nm radiation from 12-ns ArF excimer laser pulses has been shown to produce clean material remo­val (etching) of polymer films 1-3 and clean and smooth inci­sions in biological tissues4 by a process termed ablative pho­todecomposition (APD). In these reports it was demonstrated that the 193-nm laser APD yields smooth and sharp boundaries of the irradiated areas, in contrast to long­er wavelength (green) etching of cutting, where the damaged samples show roughening and pits from exploding bubbles. In the polymers, APD etching was observed from a thresh­old ftuence of about 10 mJ/cmz and up to several tens of JI cmz. Gas chromatography-mass spectrometry was also per­formed on the ablated materials and various molecular frag­ments were found, of which benzene was the most common. 2

In general, it is clear that APD by lasers has a great potential as a dry, self-developing etching process for organic poly­mers.3 In the past few months it became apparent that other excimer laser wavelengths can also produce direct etching of polymeric material, and papers using the 308-om XeCllaser, the 248-om KrF laser, and the 351-nm XeF laser have al.­ready been published. ~.6 Moreover, it was demonstrated by the authors6 that not only the etching itself, but also smooth­ly etched sur/aces, can be obtained in polyimide films regard­less of the 1aserwavelength (193, 248, and 351 nm), provided a sufficiently high laser fluence is used. It was also shown in that paper that the absorbed energy density in the surface a</J (where a is the linear absorption coefficient and </J is the laser fiuence), rather than the laser fluence itself, is the important parameter controllng the laser etching process. Hence the 193-om radiation is not unqiue in its ability to produce a good dry etching, and other laser wavelengths can do so, provided a</J exceeds a given threshold value.6 In the present

article a systematic study of the APD dependence on the laser wavelength, the laser fluence, and the sample proper­ties was performed by measuring temporally-resolved emis­sion spectra from the ablating fragments and by scanning electron microscopy of the etched surfaces.

U. EXPERIMENT

A schematic diagram of the experimental system is giv­en in Fig. 1. The laser used for the irradiation was a Lambda­Physik EMG 201-E excimer laser which emitted 0.I-O.8-J pulses of 15-ns duration at the ArF, KrF, and XeF wave­lengths. It was operated at 1-IO-Hz repetition rate during the measurements. The fluences quoted hereafter, however, were measured by a Gen-Tec calorimeter (model ED-5OD) at 1 Hz. The laser beam was focused onto the sample with a cylindrical quartz lens (15.6-cm focal length) or a spherical quartz lens (15- or 25-cm focal lengths) and produced a bright emission region which extended to a distance of a few millimeters from the target. This region was imaged perpen­dicularly to the laser beam by a quartz lens to produce a 1 : 1 image on the entrance slit of either of two 1!4-m Jarrell-Ash monochromators: one was connected to a photomultiplier (EMI 9789) and a boxcar integrator, and the other to an intensified optical multichannel analyzer with temporal re­solution (OMA-2 Princeton Applied Research 1215, 1216, 1211, and 1254 units). TheOMA was equipped with a silicon detector which had a scintillator coating on it that enabled UV measurements down to 2000 A. Due to the large back­ground reading of the OMA, a background spectrum was recorded for each measurement with the laser beam blocked, and subtracted from the measured spectrum when the laser irradiated the target.

2120 J. Appl. Phys. 56 (7),1 October 1984 0021-8979/84/19212~7$O2.40 @ 1984 American Institute of Physics 2120 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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ArF 193 nm KrF 248 nm

XeF 351 nm

EXCIMER LASER

OPTICAL

MULTICHANNEL

ANALYZER

MONOCHROMATOR

POLYMER FILM

,--~~;;a-- TARGET

MONOCHROMATOR

FIG. 1. Schematic diagram of the experimental system.

The polymer samples were commercial films or plates ranging in thickness between 25 and 1000 /-Lm. They were mounted in a plane perpendicular to the laser beam either in a stainless-steel vacuum chamber or in the open air. When using the vacuum cell, the experiments were performed un­der vacuum or with air or He environments, generaliy with these gases flowing during the measurements to remove the ablating materials from the cell. When recording spectra with the boxcar integrator, the laser was operated at 5-10 Hz and the samples were translated in their plane back and forth at a rate of 24 mm/min to avoid perforation and heating of the polymers. Measurements with the OMA were done at 1-Hz laser repetition rate. All spectra shown in the present study are uncorrected for the detection system's spectral re­sponse.

m. RESULTS AND DISCUSSION

Typical emission spectra from polyimide using the OMA are shown in Fig. 2. The emission bands are due to the C2 Swann system (A 3ilg _X,3il,.. at 5635-5470 A, 5165-5098 A, 4737-4679 A, and 4382~365 A), the CN violet system (B2~_X2~ at 4216-4152 A, 3883-3851 A, and 359{}-3584 A), and the atomic C emission at 2478.5 A which appears here at 4957 A in second order.7

,8 In the inset to Fig, 2 the peak at 38~3900 A was resolved by using a higher resolution grating in the monochromator. The bands that constitute this peak were easily identified as CN bands. Simi­lar higher resolution spectra of other emission peaks were also measured and gave a positive identification of those peaks as belonging to ~ and CN molecules.

To find how different environments affect the emission spectra, measurements were carried out in He, air, and vacu­um under exactly the same experimental setup. The results are given in Fig. 3, where one can see that tbe intensity of the emission spectrum is weak in vacuum and is significantly enhanced in air and He. The shape of the spectra, however, is

2121 J. Appl. Phys., Vol. 56, No.7. 1 October 1984

7

? 6

'c :::J

.e S z 0 u; II)

:i w

1'. : I , I

,

POLYIMIDE

: I 248 nm , :- 2.7 J/cm2

I I I

/\ ,. \ I ,

I , \

• I

• • -.. -"'-" \ '---

0~ ______ L-____ ~ ____ -L ______ L-__ ~

3500 4000 4500 5000 5500 6000 >..(A)

FIG. 2. Emission spectra from polyimide irradiated by 1.2 J/cm2 at 193 nm (solid line) and 2.7 I/cm2 at 248 nm (dashed line). The spectra were mea­sured with He flow of 700 Torr, zero delay relative to the laser pulse, and with 10-msand I-jts gates at 193 and 248 nm, respectively. Each spectrum is recorded with only ten laser shots and the ratio of the ordinate scales of the 193-nm curve and 248-nm curve is 1 : 10.

independent of the vacuum or gas environments, as is clearly demonstrated in Fig. 4. Note that except for the strong ~ Swann bands emission, the narrow peak at 3510 A is due to scattered light from the XeF laser.

The temporal and spatial behaviors of the ~ emission at 4700 A from polyimide irradiated by the 193-nm laser with a He flow of 40 Torr are shown in the oscillograms of Fig. 5. The photomultiplier in this experiment was connect­ed directly to an oscilloscope and the monochromator was translated with a micrometer back and forth parallel to the direction of the laser beam in such a way that different cross sections of the emitting region were 1 : 1 imaged on the 50-/-Lm-wide entrance slit of the monochromator. While trans­lating the monochromator it is easy to identify two limiting regions where the intensity of the signals approaches zero. One is very close to the target (or occurs when the target itself is imaged on the slit) and the other is far away from the target when the emitting species have already lost their excitation. Hence it is possible to obtain the temporal behavior of the emission as a function of the distance D of the emitting re-

II)

I-

5

Z 4 ::>

ai a:: 3 <t

~ 2 II) II)

:i

KAPTON 351 nm

3J/cm2

w /UNDER VACUUM

O~~~~~~~~~~~~~~~~~~~d 3500 4000 4500 5000 5500 6000

A(A)

FIG. 3. Emission spectra from polyimide irradiated by 3 J/cm2 at 351 nm. The spectra were recorded under vacuum and in an atomosphere of air and He, with zero delay and 5-jts gate.

G. Koren and J. T. C. Yeh 2121

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'iii 5 351 nm

!:: 3J/em2 z ...... AIR ::> 4 a:i ---He a; -VACUUM ~ 3 z 0 Vi 2 (/l '. ~ w

O~------L------L------~----~------~ 3500 4000 4500 5000 5500 6000

A(~)

FIG. 4. Spectra of Fig. 3 normalized at the 5150-A peak.

gion from the target. The distance D is given as a variable parameter in Fig. 5. Similar results were observed in the other emission bands of ~ and also in the C and CN emis­sions.

It is obvious from Fig. 5 that the emission has two dis-tinct components: a fast one which appears simultaneously with the laser excitation (taking into account the - 30-ns response time of the photomultiplier), and a slower one whose propagation speed is about 1 mm/250 ns or 4 X l(f cm s -I [see Fig. 5(c)]. Such behavior is typical of laser-pro-

, (0)

~

~ 0 l-

V)

Z ILl

1 I-Z

z Q V) V)

::E ILl

0

1 0=1 mm

0 250 500 TIME (ns)

FIG. 5. C,-4700-A emission oscillograms of polyimide irradiated by I J/ cm2 at 193 run with the distance D from the target as a parameter. He flow at 40 Torr was used and the gains are 50mV /div. in (a) and (b), and 20mV /div. in (e).

2122 J. Appl. Phys., Vol. 56, No.7, 1 October 1984

duced plasma and was described previously as the "shell" (fast) and the "core" (slow) emission components.9 It is also clear from Fig. 5 that at D = 1 rom the emission lasts for about 500 ns after the laser pulse (at larger distances D the emissions last for a few j.ls), a time that is much longer than the excitation time and the emitting electronic state's life­time. This, together with the emission enhancement in the presence of air and He (see Fig. 3) is also typical of electron impact excitation and plasma recombination. The He (or air) serves to cool the hot electrons by collisions leading to a more efficient electron impact excitation and plasma recom­bination, leading also to plasma confinement near the target which enhances the emission from this region.9

From the preceding discussion it follows that the emis­sion mechanism is due to plasma excitation and recombina­tion processes. Chemiluminescence, however, can also yield emissions with the above-noted characteristics. To check whether this is a possible source, emission spectra from pure graphite were recorded. Figure 6 shows that using the 248-nm irradiation in a He environment on graphite yields essen­tially the same spectrum as that obtained in polyimide (the dashed line in Fig. 2), except for the CN emissions which are obviously missing in graphite. Hence, since chemilumines­cence is unlikely in graphite in a He environment, it is sug­gested that this emission mechanism is not significant in po­lyimide as well. Moreover, with increasing laser fluence one finds a dominantly ionic emission spectrum from graphite, as shown in Fig. 7. The two strong peaks at 4267 A and 5130-5150 A are due to en emissions, and the weaker peak at 4647-4651 A is due to Cm emissions8 Spectra similar to that shown in Fig. 7 were also found in the polymers, and different degrees of carbon ionicity were obtained depending on the sample, the exciting laser wavelength, and. the tluence. Thus, in conclusion, plasma is created in the laser etching process and this plasma gives rise to the observed emissions.

So far, the spectra shown were obtained with the inten­sified OMA whose sensitivity is about three orders of magni­tude smaller than that of a typical photomultiplier. As long as the emission intensity is strong it is much easier to use the OMA. However, to observe photofragments larger than~,

7 GRAPHITE

in 248nm I- 6

2.5 J/em2 z ::> 5 IN He ai a;

4 <t

Z 0 3 CJ) (/l

::E 2 w

o~==---L----~------~-----L----~ 3500 4000 4500 5000 5500 6000

~(A)

FIG. 6. Emission spectrum of graphite irradiated by 2.5 J/cm2 at 248 nm under a He flow of 600 Torr. A Pyrex filter was inserved in front of the entrance slit of the monochromator, and 200-ns delay time and 2-fLs gate were used.

G. Koren and J. T. C. Yeh 2122

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7

~ 6 z ~ 5 IIi ~ 4

~ 3 iii en 2 :::E w

GRAPHITE

351 nm IOJ/cm2

IN He

°3~5~O=O~--4~00Q~-----4~~------5000~-----5-000L-----6~OOO ACA)

FIG. 7. Emission spectrum of graphite irradiated by 10 J/cm2 at 351 nm with no delay time. 400-ns gate and 50-Torr He flow.

CN, and C one has to use low laser fluences. The emissions are expected to be in the UV, where one needs a better sensi­tivity and signal-to-noise ratio. Hence, the use of a photo­multiplier and a boxcar integrator is necessary. A UV emis­sion spectrum from polyimide irradiated by 0.5 J/cm2 at 193 nm using such a setup is shown in Fig. 8. One can easily identify the sharp CI emission line at 2478.5 A, which is due to recombination of singly-ionized carbon ions with elec­trons. The broad emission band that degrades to the red with its head at 2600 A can be attributed to benzene or other aromatics that emit in this region.7 The other UV degrading band whose head is at 2340 A is as yet unidentified. It is possible, however, that the aromatic fragments, including benzene, absorb strongly between 2340 and 2600 A, result­ing in self-absorption rather than emission in this region. This would mean that the sharp rise at 2340 A in Fig. 8 is not due to a real emission-band head. Preliminary attempts to resolve the benzene band were unsuccessful, probably be­cause of the polyatomic nature and high temperature of the emitting species.

To investigate the temporal behavior of the emission spectra with different gates and delay times from the time of the laser excitation, polymethyl-methacrylate (PMMA) was chosen as a representative case. The experiments were per­formed in air using 300 mJ/cm2 of the 193-nm laser radi-

6r-----~------.-----~-------!!!. KAPTON '§ 5 500 m]/cm2

.Q 4 ~ 1-4.4fLs

z o (ij CJ)

~ w

O~----~------~~----~----~ 2000 2500

>.01.) 3000

FIG. 8. UV -emission spectrum oflCapton irradiated by 0.5 J/cm2 at 193 nm with 3.4-ps gate.

2123 J. Appl. Phys., Vol. 56, No.7. 1 October 1984

ation and with the photomultiplier and boxcar setup. A por­tion of the spectra is shown in Fig. 9 with three different gates and delay times. Since PMMA does not originally con­tain nitrogen, the clearly observed violet bands of CN are indicative of a secondary reaction in the air in which CN is created. Close to the laser pulse and up to 0.5 ps, after it (trace a), a strong and broad continuum radiation is emitted at all wavelengths. The strong CN peak at around 3870 A is only starting to develop and its structure is not at all re­solved, which is typical of a hot-gas emission spectrum. At a later time (trace b) the continuum emission is greatly re­duced, the CN emissions are dearly seen as strong peaks above the continuum level, and their structure is better re­solved. The tendency to reduce the continuum and increase the resolution continues also at an even later time (trace c), but the total emission intensity decreases due to the loss of excitation and cooling of the emitting species. The ~ Swann emissions, of which only the peak at 4360 A is shown in Fig. 9, and which are independent of secondary reactions in the air, also show a similar temporal behavior. From measure­ments in other polymers it appears that this temporal behav­ior is typical in all of them.

Systematic spectral measurements in polyimide using the OMA again with a constant gate of 50 ns were performed as a function of the delay time from the laser pulse. The results show the following:

(i) the continuum emission is strongest during the time of the laser pulse (short-lived continuum),

(ii) there is also a weaker continuum emission which lasts for a few hundreds ofns (long-lived continuum), and

(iii) the C, C2, and CN emissions, when present, do exist from the time of the laser pulse, but are sometimes buried under the strong continuum, especially at the shortest delay time. These results were obtained at moderate laser fluences (where C2 Swann emissions are observed) and their laser wavelength dependence will be discussed in detail1ater. The short-lived continuum (i) is familiar from laser-produced plasma measurements9

,IO and is originated in the free (e1ec­tron)-to-bound transitions. This is a real continuum in con­trast to the long-lived continuum (ii) which seems to origi­nate in hot polyatomic fragments and was simply unresolved under the present experimental conditions.

As shown in Fig. 9, CN emissions are observed from the PMMA laser-produced plasma in air even though PMMA does not contain nitrogen. Figure 10 shows emission spectra from Mylar (which also does not contain nitrogen) and polyi­mide (which contains nitrogen) but in the presence of He under exactly the same experimental conditions. One imme­diately observes the CN violet emission peaks from polyi­mide and their complete absence in Mylar. The C2 emission spectra are sim.ilar in both polymers. One can therefore con­clude, on the basis of the data presented so far, that in all the polymers used in the present study, as well as in graphite, the C2 Swann emissions can always be observed regardless of the laser wavelength and gas environment of the experiments. The CN emissions from the materials that do not contain nitrogen can be observed only in the presence of air using the 193- and 248-nm radiations. In polyimide, this can be ob-

G. Koren and J. T. C. Yah 2123

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3500 3700 3900

>.(A)

PMMA 300 mJ/cm2

(0) 0-0.5 fLs

(b) 0.4-1.4fLs

(c) 1.4-7.4fLs

4100 4300

FIG. 9. Emission spectrafromPMMA irradiated by 0.3 J/cm2 at 193 nm in air as a function of the different gating times as noted in the figure for the three traces.

tained also in He and vacuum environments, but even in this nitrogen-containing polymer, no CN emission is observed using the 351-nm radiation (see Fig. 3). The larger emission intensity from one material (the Mylar in Fig. 10) compared to the other (polyimide) can be due to a combination of rea­sons. The material with the greater absorption coefficient, the greater volatility, the lower heat capacity, and the lower ~hermal diffusivity will generally produce the brighter (more mtense) laser-produced plasma emission.

We turn now to investigate surface morphological changes produced by the laser etching process in the poly­mers, using different laser fluences at the three available la­ser wavelengths. With increasing laser fiuence, an interest­ing structure transition between roughly etched surfaces and smoothly etched surfaces was found in polyimide at aU exci­tation wavelengths. The transition can be seen in the scan­ning electron microscope pictures of Fig. 11, where the XeF laser was used with different fluences. The corresponding emission spectra below, at, and above the transition are shown in Fig. 12. Below the transition a continuum emission and strong laser-light scattering are observed, whereas above the transition the C2 Swann bands emission is clearly seen

5

(f) 193 nrn

~ 4 1.4 J/crn2 z ;:, IN He a:i Il:: 3 oCt

z 2 2 (f) (f)

:i w

4500 5000 5500 6000

A(A) FIG. 10. Emission spectra from Mylar and polyimide (Kapton-H) irradiat­ed by 1.4J/cm2 at 193 nm in I atm He under exactly the same experimental geometry. Zero delay time and 100~s gate were used.

2124 J. Appl. Phys., Vol. 56, No.7, 1 October 1984

and the laser-light scattering is strongly reduced. Similar re­su~ts, with different transition fluences ¢J, (A I, were obtained usmg KrF and ArF lasers. The linear absorption coefficient alA I of polyimidell is shown in Fig. 13 and its values at the laser wavelengths together with ¢J, (A) and the products alA l¢J, (A I are given in Table I. One can easily see that a(A l¢J, (A ) values for all the three wavelengths are almost the same within the experimental error which is estimated as ± 10%. This result unifies the description ofthe rough-to-

smooth transition for the different wavelengths and means that above a threshold value of about 5 X 104 J/cm3 of ab­so~bedenergy per unit volume in the surface of the polyi­mlde film, a smooth etched surface is obtained. Note, how­ever, that the alA )¢J, (A )-energy-absorbed-per-unit-volume ~~sideration is good only as a first approximation, since mSlde the film the fluence decreases exponentially with the depth. Nevertheless, this approximation holds and even im­proves when large material removal rates occur since the thin ablating layers are subject to the same a¢J, during the laser pulse.

At moderate fiuences [for example, ¢J (A. ) > 5 X 1 Q4 / alA I J/cm2 in polyimide], a systematic examination of the emis­sion spectra as a function of the laser excitation wavelength shows simiJ.arities in the C2 Swann emission and differences in the CN violet emissions (see Figs. 2, 3, 6, and 1.0). To explain this behavior we note the following:

(i) it is most likely that the 193- and 248-nm radiations on the one hand, and the 351-nm radiation on the other hand are absorbed by different electronic states II (see also Fig. 13),

(ii) the fragmentation or bond breaking occurs either in a direct photochemical predissociation or after electronic­to-vibrational. energy tnmsfer processes, and

(iii) the 193-nm radhttion of 6A-eV photon energy is sufficient to dissociate the C2 molecules, but not the eN molecules (the bonding energies are 6.21 and 7.76eV, respec­tiVe!yI2).

Hence the similarities in the spectra can be explained as due to statistical vibrational energy distributions prior to abla­tion, and the differences can be accounted for by the addi­tional C2 photodissociation in the 193-nm irradiation and also by the incomplete energy randomization prior to abla­tion. Specifically, the fol.lowing mechanism for APD can be considered. In the 351-nm excitation, the absorbed energy is statistically distributed in the polyimide molecules, leading to dissociation of the weakest bonds and therefore rel.easing C2 rather than CN molecules. In 193-nm excitation, it seems as if the creation of the CN molecules is enhanced relative to that of the C2 moJ.ecules (see Fig. 2). This, however, is not so since the 193-nm radiation, in contrast to the 248-nm radi­ation, simply depletes the C2 mol.ecules by photofragmenta­tion. The differences in the relative intensities in the C2

Swann e~ni.ssions following the 193- and 248-nm irradiations can be explained as being due to an additional emission mechanism of excitation from above of the C 2 molecuJ.es that are created by collisional association of the plentiful carbon atoms in the 1.93-nm irradiation (see Figs. 2 and 8). This emission mechanism is resonantly enhanced in ~ since the ground state of the C2 molecule is readily dissociated by the 193-nm photons to produce 2C(3p), which recombine to

G. Koren and J. T. C. Yeh 2124

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1 I

(c) O.5fLm

FIG. 11. Scanning elect.ron micrographs of the polyimide surface irradiated by the 351-nm XeF laser at different fluences: (a) 1.3 J/em2, (b) 1.6 J/cm2, and (c) 1.9 J/cm2

• All pictures were taken with the same magnification. The white stacks in (b) and (c) are carbon and large particles deposits which were not fully ablated.

7 '

6 ~

~ 'c 5 ::>

-e 2 4 z 2 3 (J) (J)

::E 2 w

{

0 3500

POLYIMIOE

351nm

1.75 J/em2 -_ .... ," .... 50 I 2 .-f I. J em ~-':!"- _

2 :., 1.25J/em ... /

4000 4500 5000 ).(}!.l

..................

5500 6000

FIG. 12. Emission spectra from polyimide irradiated by the 351-nm laser with 700-Torr He flow, zero delay, 5-Jls gate, and the following fluences and surface qualities: 1.75 J/cm2 and smooth (dotted line), 1.25 J/cm2 and rough (dashed line), and 1.5 J/cm2 and starting to be smooth (solid line).

-t 0 0 4 II II N'C J§rC,N-@-O 'C c

l

.. 3 II II E o 0 u n

10 POLYIMIDE 2

2 ~

~

oU-~~~~~~~~~ .. ~~~~~~~-U 3000 4000 5000 6000 2000

X(AI FIG. 13. Wavelength dependence of the linear absorption coefficient of po­lyimide as measured in Ref. 11 and calibrated by us. The arrows show the absorption coefficients at the three excimer laser wavelengths which were used in the present study.

2125 J. Appl. Phys., Vol. 56, No.7, 1 October 1984

C2e fig), which emits the Swann bandsY The fact that one observes CN emissions in the 193- and 248-nm excitation (see Fig. 2) is indicative of absorption and ofCN bond-break­ing in the CN-containing imide groups (see Fig. 13), and also of non thermal localization of the residual absorbed energy in this part of the molecule. It is interesting, at this point, to make an analogy between the present APD process and the infrared multi photon dissociation process in molecules. Though the pumping mechanism is obviously different, once electronic-to-vibrational energy transfer occurs in the pres­ent process, the two systems show basically the same fea­tures; namely a partial statistical redistribution of the ab­sorbed energy in the molecules 13 and some non thermal population distributions. 14

To explain the rough-to-smooth transition at 5 X 10" JI cm3 in polyimide, we note that this absorbed energy density in the surface is the same for all excitation wavelengths (see Table I). It should, therefore, lead to the same vibrational temperature increase of the polyimide surface. Hence, the rough-to-smooth structural transition occurs at this tem­perature and it is tempting to assume that it is related to melting and smooth resolidification. At the present time, however, we have no further support to this interpretation. An alternative explanation of this transition may be based on the fact that the C2 Swann emissions are starting to show up on top of the continuum emission exactly when the transi-

TABLE I. Linear absorption coefficients a and rough-to-smooth threshold transition fluences ;. of polyimide films irradiated by the ArF, KrF, and XeF laser wavelengths. Also shown are the corresponding energies ab­sorbed per unit volume a;,.

A .... , (nm)

193 248 351

a 1000(cm-')

45 26

3

;, a;, (J/cm2) 10"(J/cm3

)

O.lO±O.OI 4.5 ± 0.5 0.20±O.02 5.2 ± 0.5 1.50±0.15 4.5±O.5

G. Koren and J. T. C. Yah 2125

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tion occurs (see Fig. 12). This continuum emission, which last for about 1 Ji-S (and should not be confused with the short-lived free-to-bound continuum) may come from ejec­tion of large fragments, whereas the diatomic emission cer­tainly indicates ablation of small fragments. Hence it is pos­sible that rough and smooth surfaces are created by large and small fragment ablation, respectively.

Surface morphology of Mylar and PMMA after the la­ser etching process was also investigated. Scanning electron micrographs were taken and the fonowing results were found: Mylar could never be etched smoothly regardless of the laser fluence, and when using the 248- and 193-nm radia­tions, wavy surface structures of about 5 Ji-m were always observed. This behavior is correlated with the amorphous and crystalline areas in Mylar, which were also observed in Ref. 1. PMMA could be etched smoothly using the 193-nm radiation. The surface structures were much smaller than 0.5 Ji-m and appeared as if melting had occurred. With 248-nm radiation, pits from bubbles were observed in PMMA and smooth etching could not be obtained by increasing the laser fluence. Using the 351-nm irradiation at various fluences, only very rough surface etching could be obtained, which would better be described as damaged surfaces. No further study of the etching ofPMMA and Mylar (consider­ing their different absorption coefficient at the different laser wavelengths) was performed.

IV. CONCLUSIONS

By a systematic investigation of the emission spectra from polymers irradiated by excimer lasers it was found that a laser-produced plasma is created in the ablating materials in the ablative photodecomposition process. It was demon­strated that the 193-nm laser wavelength is not unique in its ability to produce good etching (smooth, with no flow of boundaries and with reasonable etch rates) of potyimide, and that the 248- and 351-nm laser wavelengths can do so as well, provided a sufficiently high laser fluence is used. The ab­sorbed photon energy per unit volume in the surface of the polymer [a(A. )4J (A. )], rather than the laser fluence itself, was shown to be the important parameter that controls the laser etching process. As for the etching mechanism, the similari­ties in the emission spectra from all the polymers used, re­gardless of the laser wavelength, were interpreted as a result

2126 J. Appl. Phys .• Vol. 56. No.7. 1 October 1984

of a statistical energy randomization in the film surface prior to ablation. The differences in the emission spectra of poly i­mide using the different laser wavelengths were observed mostly in the violet CN emissions. Again, the 351-nm data are consistent with absorbed energy randomization that leads to dissociation of the weakest bonds, thus releasing C2

rather than CN molecules. In the 193- and 248-nm excita­tion a significant amount of CN molecules was released, thus indicating a direct bond breaking, absorption, and nonther­mal localization of the absorbed energy in the CN-contain­ing imide group followed by ablation. Finally, we should note that further investigation is needed into the many open problems of the etching mechanism, energy dissipation, fragment absorption with further fragmentation, the laser­produced plasma absorption by inverse bremsstrahlung, and so on. The use oflaser pulses of different durations and pulse probe measurement techniques will probably be most useful in elucidating these problems.

ACKNOWLEDGMENTS

The authors wish to thank J. E. Rothenberg, Y. Tom­kiewicz, R. Srinivasan, J. J. Wynne, and P. Avouris for their help and interest in this work and for many useful discus­sions. The skillful technical assistance of B. Braren and J. Donelon is greatly appreciated.

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6G. Koren and J.T. C. Yeh. Appl. Phys. Lett. 44, 1112 (1984). 7R. W. B. Pearse and A. G. Gaydon, The Identification 0/ Molecular Spec­tra (Wiley, New York, 1976).

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"H. Ishida, S. T. Wellinghoff, E. Baer, and 1. K. Koenig, Macromolecules 13, 826 (1980). Absolute absorption at 351 nm was measured by us.

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G. Koren and J. T. C. Yah 2126

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