Nature of ‘‘incubation pulses’’ in the ultraviolet laser ablation of polymethylmethacrylateR. Srinivasan, Bodil Braren, and Kelly G. Casey Citation: Journal of Applied Physics 68, 1842 (1990); doi: 10.1063/1.346620 View online: http://dx.doi.org/10.1063/1.346620 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/68/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in On the role of chemical reactions in initiating ultraviolet laser ablation in poly(methyl methacrylate) J. Appl. Phys. 101, 103113 (2007); 10.1063/1.2740340 Incubation: Subthreshold ablation of poly-(methyl methacrylate) and the nature of the decomposition pathways J. Appl. Phys. 88, 2975 (2000); 10.1063/1.1288501 Ablation of polymethyl methacrylate films by pulsed (ns) ultraviolet and infrared (9.17 μm) lasers: A comparativestudy by ultrafast imaging J. Appl. Phys. 73, 2743 (1993); 10.1063/1.353048 Influence of pulse width on ultraviolet laser ablation of poly(methyl methacrylate) Appl. Phys. Lett. 53, 1233 (1988); 10.1063/1.100023 Time resolved transmission studies of poly(methyl methacrylate) films during ultraviolet laser ablativephotodecomposition J. Appl. Phys. 61, 2090 (1987); 10.1063/1.338015

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Nature of "incubation pulses" in the uitraviolet laser ablation of polymethyl methacrylate

R< Srinivasan, BodH Braren, and Kelly G. Casey IBM 1: J. Watson Research Center. Yorktown Heights. New York 10598

(Received 9 March 1990; accepted for publication 3 May 1990)

The term "incubation pulses" is applied to the pulses of ultraviolet laser radiation which are directed initially at a polymer surface and which etch less depth per pulse than subsequent pulses which remove identical depths of material at each pulse. This incubation effect which is particularly strong when the polymer absorbs moderately at the laser wavelength has been studied in polymethyl methacrylate at 193 and 248 nm< The transformation of the surface during incubation pulses has been followed by 1) photographing the blast wave that is produced by the products that emerge from the surface, using a fast ( < 1 ns) dye laser that is electronically triggered to light the ablation plume after a set delay, and 2) by treating the exposed surface with a solvent to dissolve out any photolyzed material that is left in the exposed area. The results show that ablation, as shown by the ejection of the products at high velocity, occurs even during the incubation period but the surface is not etched at all or only partially< The etch pit is filled with chemically transformed material which is removed by dipping the sample in a solvent. In the photochemical model ofSutc1iffe and Srinivasan [J. App!. Phys< 60, 3315 (1986) J, if UV laser ablation is seen as a volume explosion, then incubation can be attributed to insufficient production of the gaseous products that are needed to build up sufficient pressure to eject the large fragments into which the solid is largely broken up.

I. INTRODUCTION

Although the ablation and etching of polymers by pulsed ultraviolet radiation has been studied over the past eight years, most of the results concern only the depth of the etching per pulse as a function of the laser parameters and the absorption oftbe polymer, ThiS/is certainly the easiest of the measurements that can be made although there are many pitfalls even in this exercise. It is also tedmologically the most useful, which is no insignificant consideration. There is a plethora of theories which claim to reproduce the etch curves from fundamental principles but only under rigorous boundary conditions.

polymer using 248-nm pulses of 16-m; and 300-fs FWHM.4 Their results will be discussed in detail in later sections of this article.

One of the features of UV laser ablation that is readily seen but has attracted little systematic study is the phenome. non of "incubation pulses," a term that was first introduced in 19861 although it was recognized earlier. 2 It is seen readily i.n weak absorbers at a gi.ven UV wavelength and results in the need for a certain number of pulses to condition the poly­mer surface before the etch depth/pulse at a given fluence (that is greater than the flucnce threshold) settles to a con­stant value. The first few pulses may not give an etch pit at aU and succeeding pulses may etch less than the steady-state value. This phenomenon is of critical importance in practical applications of UV laser ablation in which the etch depth must be controlled precisely, e,g<, in the shaping of the sur­face of the cornea for refractive surgery<

Up till now the only analysis of the incubation phenome­non in a polymer has been conducted by Kuper and Stake3

who used UV and infrared spectroscopy to study the changes in the absorption spectra of films of polymethyl methacrylate ( = PMMA) when exposed to 248-nm laser pulses at a auenee that was well below the threshold for etching. They also compared the incubation data in the same

I!. EXPERIMENT

In Fig. 1, the experimental arrangement that was used is shown schematically< The excimer laser (Lambda-Physik EMG 201) that was used in the ablation produced 193 or 248 nm pulses (20 ns FWHM) depending upon the gas fill. A second excimer laser (Lambda-Physik LPX 120i) pro­duced 308-um pulses which were used to pump optically a dye laser (Lambda-Physik FL 40(0) which produced pulses

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1842 J < App!. Phys< 68 {4}. 15 August 1990 0021-8979/90/161842-06$03,00 @ 1990 American Institute of Physics 1842

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of visible (596 om) radiation of < 1 os FWHM. The two excimer lasers were electronically coupled by a pulse gener­ator (Philips Pulse Generator PM 5716) which al10wed a variable time delay between the UV ablation pulse and the visible probe pulse. The time delay was monitored by a Tek­tronix 2467 Oscilloscope which was attached to a biplanar photetube (ITT F4000). To photograph the blast wave and the solid debris, a standard photographic camera (Hasselb­lad 500C; Rodenstock Heligon Lens, f = 25 mm) was used. The magnification at the film plane could be varied from 13 X to 21 X. A band-pass filter centered at 600 ± 40-om limited radiation that was sensed by the camera to the wave­length of the dye laser.

Samples of PMMA (Rehm and Haas, 0.7 nm thick) were cleaned superficially with alcohol before use. The repe­tition rate of the UV laser was kept at less than 1 Hz to avoid heating of the surface when multiple pulses were used. Ex­periments were conducted exclusively in an air atmosphere. A fresh surface of the polymer was used for each series of pulses. The depths of the etched holes were measured with a Tenor Alpha Stepper. A mixture of methyl isobutyl ketone and isopropyl alcohol (3: 1) was used as the solvent to wash the samples after UV exposure.

III. RESULTS In Fig. 2, results on the etching ofPMMA with 1-100

pulses of 193-nm laser radiation are shown, At a fluence of 0.16 J/cm2 which was wen above the threshold fluence for the etching of this polymer at this wavelength,5,6 the incuba­tion effect was dearly seen until about pulse 20. On the same figure the results of dipping the laser-exposed sample in a solvent are also displayed. The solvent removed an addi-

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; triangles: 0.16 J/cm2•

1843 J, AppL Phys .• Vol. 68, No.4, 15 August 1990

tional depth of the material over the depth obtained in the absence of this treatment. This additional depth that was removed decreased steadily with the pulse number and was 5-8% after pulse twenty. At a fiuence of 0.67 J/cm2

, the incubation effect was not seen to any significant extent, but the effect of a solvent dip on the first few pulses was remark­ably high.

Figures 3(a) and 3(b) show the incubation effect in the etching with 248-nm pulses at fluences which are in the lin­ear portion of the etch curve. 5

•6 At a fluence of 0.9 J/cm2

,

there was no measurable depth to the hole until about pulse No. 16. Instead there was a slight elevation of the surface at the point onaser exposure. Dipping in a solvent showed that there was depolymerized material in the exposed area which could be leached out [Figs. 3 (a) and 3 (b) insets]. It took about 40 pulses before the system settled to a constant etch depth/pulse without solvent treatment and 60 pulses before solvent treatment removed a minimum (5%-8%) of addi­tional material. At a greater fiuence of 2.1 J/cm2

, the first and second pulses together actually caused an elevation in the surface that was measured at > 4 pm! Liu, Cole, Philipp, and Guida 7 have also described this initial elevation in the surface of PMMA on exposure to UV laser pulses. With subsequent pulses as well as with solvent treatment, the sys­tem showed the same trends as those seen at the lower fluence. The inset shows the changes in the surface during the first five pUlses.

Fast imaging of the ablation by 248-nm pulses at various pulse numbers are shown in the sequence of photographs in Fig. 4. Qualitatively, these pictures allow the visualization of the front of the blast wave," the density of the particulate matter, and the directionality both of the blast wave and of the solids, Quantitatively, the time-dependent behavior of the blast wave can be followed precisely but that of the solid matter, only with considerable uncertainty. At a fluence of 2.1 J/cm2

, even at the very first pulse, a blast wave can be seen faintly at 211m (not shown) 0 No solid matter appeared. At the next pulse, there was both a blast wave [Fig. 4 (a-I) ] and an opaque stream of matter which appeared almost to­gether. The former travelled at supersonic velocity (about 5 X 104 em/s) while the latter, at its leading edge, had a ve­locity of about 2 X 104 em/so The spreading of the blast wave contrasted with the strongly normal direction oftne opaque stream. From the subsonic velocity of the latter, it was judged to be made up ()f solid particles.') This tapered off in about 2-3 ItS [Fig. 4(a) part 3]. The third pulseis specially interesting because the data on the etch depth (Fig. 3) showed that it caused a change in the etch depth from - 4.2 to + 1.0 pm. It is not surprising that the photographs of this pulse [Fig. 4(b) parts 1--6] show a long stream of particu­late matter which lasted well into tens of microseconds. Sub­sequent pulses, when viewed photographically showed very similar behavior and the results from the tenth pulse (at which point the etch depthjor a single pulse reached a con­stant value at this fluence) are shown for comparison [Figs. 4(c) parts 1-3].

The results at 248 urn at a lower fluence (0.9 J/cm2)

followed the pattern seen above at a larger fluence but the changes, as is to be expected, were more gradual. There was

Srinivasan, 8raren. and Casey 1843

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FIG. 3. Average etch depth/pulse for the ablation ofPMMA by 248-nm pulses. (a) Fluence 2.1 J/em"; (b) 0.9 J/cm'. The filted and open data points show the values before and after washing with a solvent. (a) Inset: Actual depth of etchcd surface during the first fivc pulses. The first two pulses lead to an elevation in the surface. (b) Inset: Trace of profile of surface after the 14th pulse. The surface was elevated before washing but depressed after the wash.

no sign of a plume until the thirteenth pulse but the surface rose up as noted above. In the next few pulses this elevation was etched away gradually and it was accompanied by a weak blast wave as well as a stream of solid. Since dipping the film in a solvent results in a net hole, it would seem that the first twelve pulses broke up the polymer into oligomers of lower molecular weight and gases and the product mass swelled up from out of the surface. In the absence of treat~ ment with a solvent, a hole was not evident until about the 16th pulse but ablation of material can be clearly seen [Figs. 5(a)-5(c)]. When the system passed the incubation stage, it produced a plume pattern that was entirely similar to that seen at the larger fluence [Fig. 4(c) J except that the images were less intense presumably because the amount of material etched/pulse was a fourth of that etched at the larger fluence.

At the laser wavelength of 193 nm which etched far less material per pulse than at 248 nm5

, the ejection of solid mate­rial was seen only at the largest fluence that was used. But the blast wave can be seen from the very first pulse at 0.16 J/cm2

, the smallest fluence that was used. The etch depth was negligible amounting to a total of 0.68 f.tm for the first ten pulses. It has been discussed by us elsewhere that the blast wave from the UV ablation ofPMMA does not foHow the time dependence that follow from theories about explo­sions.s We have chosen to display the present results simply using a linear relationship between time (measured from the start of the UV pulse) and the displacement of the blast wave

1844 J. Appt. Phys., Vol. 68, No.4, 15 August 1990

from the surface. These results are shown in Fig. 6 as a func­tion of the delay in the probe beam. The velocity decreased with time as would be expected for an expansion into an air atmosphere. 1O When 193-nm pulses were used, the initial velocities were nearly double the value of the values obtained with 248-nm pulses. In Fig. 7, the velocity of the blast wave at a fixed point in time after the firing of the UV pulse is plotted as a function of the fluence. With increasing number of pulses, the velocity of the blast wave increased in a rela~ tionship that was similar to the etch depth/pulse.

Figure 8 shows a scanning-electron microphotograph of the surface of a PMMA sample which had been exposed to 10 laser pulses at 248 nm, 1.3 J/cm2

• Although under these conditions fast imaging showed the same pattern as in Fig. 3 (b) and 3 (c), it is surprising to see that the exposed area is covered with a friable mass and only a tiny opening is evi­dent. This material is presumed to be the solvent extractable photoproduct that is produced during the incubation pulses.

IV. DISCUSSION

The phenomenon of incubation pulses is encountered only when the polymer sample has an absorptivity of less than 4000/cm at the laser wavelength and it is particularly significant at fluences near the threshold for ablation for the material. Our original reference I to incubation pulses was in the context of the dynamic model that we had proposed which stated that the volume explosion that leads to ablation

Srinivasan, Braren, and Casey 1844

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delay from the start of the UV pulse.

1845 J. Appl. Phys., Vol. 68, No.4, 15 August 1990

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Srinivasan, Braren, and Casey 1845

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FIG, g, Scanning Electron Microphotograph of PMMA surface after 10 pulses at 248 nm, 1,3 J/cm2

, The straight lines are marks made by the Alpha Steppa, The bar represents 100 r~m,

was caused by the buildup of fragments of photo de compos i­tion of the polymer in the irradiated volume. A flux thresh­old had to be reached before the photon flux was capable of causing bond break and the flux above this threshold had to reach a second threshold before ablation can occur. The pos­sibility to accumulate fragments not only during a single pulse but from pulse to pulse was an important consequence of this model. Several important features of incubation pulses which arc consistent with the dynamic model have been established in the present work. First, during the incu­bation pulses, the decomposition of the polymer to give oli­gomers and volatile products to a far greater depth than was seen by a depth measurement on the "dry" surface is seen by the increase in solubility of the material in the exposed vol­ume in relation to the unexposed material. This effect is striking during the first few pulses when 193-nm radiation at a fluence of 0.66 J/cm2 is used. The irradiated volume that the laser pulses produced at 248 nm was fined with a volu­minous, friable mass which was soluble although the etch depth was a negative value. Secondly, ablation which we define as the forcible ejection of the products as gases, va­pors, and solid polymer is seen even during the incubation pulses although a net depression is not found on the etched surface. The etch depth/pulse is not a sensitive index of the onset of ablation. This has been pointed out already in the laser ablation of polyimide from similar considerations. II Thirdly, the dynamics of the ablation plume can be inter­preted to mean that there is more gas production at 193 nm than at 248 nm and, at a given wavelength, at high fluences than at low fiuences. More efficient photofragmentaion will allow a greater fraction of the pulse energy to drive the abla-

1846 J, Appl. Phys,. Vol. 68, No, 4, 15 August 1990

tion which will result in greater velocities of the blast wave. This appears to be a function of the pulse number. The first few pulses seem to break up the polymer (as shown by the solubility) but not create the same fraction of gaseous prod­ucts as pulses following the incubation.

Kuper and Stuke,4 who have also studied the nature of incubation pulses in the UV ablation of PMMA, irradiated films of the material with UV radiation at various power densities and analyzed their UV and infrared spectra. They detected the buildup of new absorptions in these regions which they concluded was due to the formation of photopro­ducts in the exposed areas. They have attributed the phe­nomenon of incubation to the initial formation of these prod­ucts and the resulting changes in the UV spectrum of the polymer. Methyl formate was one of the products that was detected by them when they used a laser fluence of 0.035-0.040 J/cm2 at 248 nrn. This value for the fiuence is well below the threshold for ablation but they reported that qual­itatively the same spectral changes were induced by expo­sure of PMMA to 248-nm radiation of higher fluences. It is difficult to extrapolate the results of experiments that were conducted at fluences that were wen below the ftuence threshold to fluences that lead to ablation.

It is not surprising that Kuper and Stuke detected pho­toproducts which have been described before in the low-in­tensity photolysis of PMMA with 253.7-nm radiation 12 such as methyl formate. In our earlier work on the ablation of PMMA at 248 nms, in which we carried out the exposure in a closed system, trapped the volatile products in liquid nitro­gen, and analyzed them by gas chromatography coupled to a mass spectrometer, we did not detect methyl formate. Since we did detect and estimate the formation of < 1 % of MMA which is isomeric to methyl formate, we would put an upper limit of 0.2% for the yield of the latter based on the amount ofPMMA that was ablated. The work of Estler and NogarI3 is the only study ofPMMA ablation in which the formation of methyl formate under supposedly ablation conditions is reported. However, closer study of their article shows that they found this compound only when they used a wave­length of 240 nm which was produced by anti-Stokes shift from a 266-nm laser beam. It is noteworthy that when they detected methyl formate by mass spectrometry, they were using a ftuence of 0.050 J/cm2 which corroborates the results of Kuper and Stuke.4 But Estler and Nogal' provide no evi­dence that they were ablating the polymer under these con­ditions. There is plenty of evidence that photolysis of PMMA with 248-nm radiation (pulsed laser or stationary lamp) can lead to a variety of products including methyl formate at ftuences below the ablation threshold. There is no evidence at present for or against the suggestion that at fluences above the threshold for ablation the same products and/or similar products cause an increase in the absorption coefficient of the surface of PMMA and thereby cause the incubation phenomenon.

'E. Sutcliffe and R. Srinivasan, J, Appl. Phys, 60, 3315 (1986), 2B, Braren and R. Srinivasan, unpublished work,

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's. Kuper and M. Stuke, App!. Phys. A 49, 211 (1 9S9). .. s. Kuper and M. Stuke, App\. Phys. B 44, 199 (1988). 'R. Srinivasan, B. Braren, D. E. Seeger, and R. W. Dreyfus, Macromole­cules 19, 916 (1986).

6R. Srinivasan, B. BraHm, L. Hadel, R. W. Dreyfus, and D. E. Seeger, J. Opt. Soc. 3, 785 (1986).

'Y. S. Lin, H. S. Cole, H. R. Philipp, and R. Guida, Proe. SPIE 774, 133 (1987).

1847 J. Appl. Phys., Vol. 68, No.4, 15 August 1990

"R. Srinivasan, B. Braren, K. G. Casey, and M. Yeh, App!. Phys. Lett. 55, 2791 (1989).

"P. E. Dyer and R. Srinivasan, J. Appl. Phys. 66, 2608 (1989). lOp. E. Dyer and J. Sidhu, J. App!. Phys. 64, 4657 (1988). \ I R. Srinivasan, K. G. Casey, and B. Braren, Chemtronics 4, 153 (1989). 12 A. Gupta, R. Liang, F. D. Tsay, and J. Moacanin, Macromolecules 13,

1696 (1980). DR. C. Estler and N. S. Nogar, Appl.Phys. Lett. 49,1175 (1986).

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