Pulsed laser ablative deposition of thin metal films

Applied Surface Science 36 (1989) 15%-163 North-Holland, Amsterdam

157

~:%ILSED LASER ABLATD/E D E P O S I T I O N O F T H I N M E T A L F I L M S P. M O G Y O R O S I , 7,'. SZORI~NYI, K. BALI, Zs. T O T H and I. HEVESI Research Group on Laser Physic:; tq'the Hungarian Academy of Sciences. D6m tdr 9., H-6720 Szegec~ Hungary

Received 2 June 1988; accepted for publication 11 July 1988

Different mechamsms of pulsed laser ablation and ablative deposit;.ov,of thin metal films at various fluences are discus:ed. The window for clear ablation, i.e. the flt~encerange in which the film is completely removed without any damage of the supporting sub.c,trate and the window for best quality printing are determined.

1. In~'oducfion The interaction of intense laser radiation with solid surfaces has extensively been studied for many years, in part, as a result of possible applications in mJcromechanical and microeh~ctronic processing. In particular a vast body of' literature exists on laser-induced non-chemical material removal using different (sometimes confusing) ter,~as as ablation [1,2], etching [3]., evaporation [4], particle emission [5], and pla~ma formation/generation [6]. Throughout this paper we will use the term ablation defined as material removal from a solid surface by a laser pulse or a ttz,in of pu~.ses without any chemical reac6.,*n on the surface itself. Although the possibility of pulsed laser ablation of tb;..a metal films has bee,n known since the beginmng of the 70's [7-i41 and found inventive applic~,tions [1], there is no consistent and clear description bas,ed o~ comprel~ensive studies in a broad fluenc~ range. Very recently Bohandy and cc~workers [13,14] reported that thin metal film ablation from a support by h i l l eriergy laser pulses and the deposition of the ablated material onto a substrate in close proximity to ',he film is a challenging alternative technique for patterrfing microelectronic devices and masks. In order to optimize this printing method it is essential to understand the pr~ceding laser ablation. :in this paper we ~ v e a complete picture of the rather d~verse processes involved i~ ~ e t a l film ablation and cooncornitant deposition i:.y single pulses of fluences ranging from 0 to 70 J / c m 2. We compare the results of a systematic experimental study and temperature calculations. After revision of existing '3169-.4332/~/$o5.59 ~.5 Elsevier Science Pubfishers B.V. (North-Holland Fhysics P~ablishing DbAsioiD

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models [10,14] we present a model which we believe to be appropriate for pulsed laser induced metal film ablation/deposition.

2. Experimental de~iRs We have systematically "~tudied the response of thin vanadium, chromium, titanium, germanium and tin films on quartz or glass substrates to spatially homogeneous single pulses from a Q-switched ruby (~ ==694 nm, 20 ns at FWHI~:) and a XeC1 excimer laser (?~ = 308 nm, 8 ns at FWHM). The beam of the ruby laser was homogenized by a glass rod of 3 mm diameter while the homogeneous middle par~ of the excimer laser beam was cut out by a pin-hole and imaged onto the sample surface. The samples were held in the image plane with the help of an auto'~ocus system using adjust-lasers ( H e - N e for ruby and dye for the excimer). The spot sizes on the sample surface were 3 and 0.15 mm for the ruby and 30 ~ m for the excimcr lasers, respectively. The laser fluence was tuned in steps of -- 20 m J / c m 2 by a set of calibrated filters. In this contribution we focus on results obtained on vanadium films of thicknesses between 30 and 200 nm vacuum-evaporated onto glass and quartz substrates and covered by blank glass or quartz plates (for ruby and excimer processing, respectively) in order to pick up the material removed from the supporting substrate during processing, i.e. for producing metal prints. Note that this glass plate served as protection against unwanted metal deposition onto the optics as well. Tbe samples were irradiated both from the substrate (back side: BS illumination) and the metal film (through the coveting plate; front side: FS illurrfination), both in air and vacuum ( p < 10 --~ mbar). We performed temperature calculations for ruby laser irradiation. The absorption coefficient, a (at 694 rim), was calculated from measured transmittance and reflectance of the virgin films. The transmittance of the ablated holes and the metal prints/copies was measured by a H e - N e laser and a photodiode.

3. Results and discussion Optical rnic';ographs of holes in a vanadium film (A) and corresponding deposits on the covering glass plate (B) obtained after processing with single pulses of different fluences from the ruby laser are shown in fig. 1. Ablation starts at surprisingly low fluences: Oth ----60-80 m J / c m 2. There exists a fluence window for ,=lear ablation in which the metal film is completely removed without any damage of the underlying substrate. Both visual and microscope examinatior,, suggest that ablation is due to tipping off in this fluence window, since metal chips remain both on the substrate and the covering glass (see

P. Mog~'orbsi et aL / A blative deposition of thin metal films

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Fig. 1. Optical micrographs of ablated holes (A) and corresponding deposit~ (Bt. 68 nm V nn glass; frnnt side (left) and back side (right) illumination; 30 < • < 2 0 ~ ~rd/cm2. fig. 1), the e d g e s a r e well d e f i n e d a n d s h a r p w i t h o u t traces o f m e l t i n g . F o r e x c l u d i n g p o s s i b l e effects o f t h i n i n t e r m e d i a t e layers ( a d s o r b e d g a s o r c o n t a m i n a n t s ) we c o m p a r e d t h e a b l a t i o n b e h a v i o u r o f ~everal v a n a d i u m

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P. Mogyorbsi et aL / Ablative deposition of thin metal films

film-substrate series using wet chemically, ultrasonically mad glow-discharge cleaned glasses of different quality, and quartz, as substrates. We observed clear ablation in any case w/th m/r.or shifts in @,h only. This point was further strengthened by the results on germanium ablation. The lower and upper fluence fimRs for clear ablation proved to remain the same within 10% for glass and glass covered by a 70 nm th/ck selenium layer vacuum-evaporated (in situ) prior to Ge deposition as sul~o~rates. The width of the clear ablation window appears to remain practically ~ndependent on layer thickness, d, for d~" 200 rim, the broadening being apparent for thicker films only. This obselvafion is in l/ne with the results of Andrew et al. on the dependence of patterning fluence (~,h in our terms) on film thickness for AI on g l a s s / M y l a r and Ag on Mylar [12]. At - 300-400 r r d / c m 2, i.e. in the upper part of the ablation window, malting and (partial) evaporation starts: deposits (prints) of excellent quality appear on the covering glass. Above the clear abb.tion window a moisture-like thin metal layer remains on the supporting substrate and gets darker with increasing fluence up to - g00-1000 n d / c m z (slightly depending on film thickness and substrate material). This darkening might be due to interface interaction a n d / o r possible redeposition. Further increase in fluence results in bleaching and above • = 2 - 3 J / c m 2 the processed surface clears up again indicating a second type of ablation which differs in mechanism from the previous one. As seen in fig. 1, within the clear ablation window the diameters of the holes and prints remain equal to the diameter of the processing laser spot on the surface. Increase in • results in an increase of the processed area with halo formation and badly defined

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P. Mogyorbsi et al. / Ablative deposition of thin metal films

h o l e s / p r i n t s . A t very high fluences (tens of J / c m 2) p l a s m a f o r m a t i o n a n d d a m a g e o f the s u b s t r a t e occurs. This s e q u e n c e o f events is characteristic b o t h for n l b y a n d excimer processing and for all ~ l e t a l - s u b s t r a t e pairs e x a m i n e d w i t h s o m e differences in the fluence scale. T h e m e a s u r e d d e p e n d e n c e o f t r a n s m i t t a n c e , T, o f the holes a n d prints o n p r o c e s s i n g laser fluence reflects the a b o v e d e s c r i b e d s e q u e n c e m o r e q u a n t i t a tively. Fig. 2 clearly s h o w s that there is n o difference b e t w e e n the results o f f r o n t side (FS) a n d b a c k side (BS) processing. In the ,:lear ablation "~indew the

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LASER FLUENCE, ~ [ m J / c m 2] Fig, 4. Calculated maximum temperatures in vanadium films of thicknesses ranging from 50 to 400 nm as a function of processing la~er fluence. Insert: time evolution of temperature within a V film of 150 nm thickness for • = 250 mJ/cm 2 (1.a) and 150 mJ/cm2 (2.a), (1.b, 2.b) Time scale of the laser pulses, a = 57~m-1; R = 0.44.

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~ransn-~ttance measured in the huddle of the ho~es (source) is ove~ 0.8 while the best quality prints from a 68 nm thick vanadium film possess T values of 0.55. The results depicted in fig. 3 verify that (i) the pJ-esence of the covering glass plate does not remarkably modify the ablation process and (ii) increase in film thickness results in darker prints ( T ~ < 0.25). Calculated peak temperatures (up to the melting point, Tin) c'~used by one single pulse from a ruby laser in vanadium films as a function of laser fluence are shown ha fig. 4 for film thicknesses between 50 and 400 nm. The h~sert of fig. 4 shows the time evolution of the temperature in the thermally thin film (temperature difference between the front and back sides is < 10 K). It is reassuring to see that the temperatures corresponding to the threshold fluences, Oth, are well below the melting point for all thicknesses examined, and that melting and evaporation is possible in the upper part of the ablation window only. So the results of temperature calculations - in excellent agreement with the experiments - reveal that clear ablation is due to pure mechanical effects without evaporation. Zaleckas and Koo suggest that the film removal meeharfism is of explosive nature, i.e. a positive pressure earl develop under the film if the evaporation temperature (or the decomposition/deges temperature) of the dielectric substrate is lower than the melting temperature of the metal film leading to explosion-like removal [10]. We have checked thic possibility by abla ,tmg Sn from quartz substrates, where the melting point of the metal film is well below that of the substrate. Clear ablation was possible at very low fluences wtfich excludes the dominant role of pressure under the film. On the other hand the moving solid-melt boundary model put forward by Bohandy and coworkers [14] to expiain printi.lg seems to be inappropriate, too, since our results unequivocally prove that both front and back side illumination results in printing of the san~e quality (fig. 2). -

4. Condmlom Our experhnental results together with temperature calculations reveal that iust above threshold ablation is due to thermal expansion of the heated area (strain) without evaporation resulting in absolutely clear and well dehned patterrdng. With slightly increasing fluence melting and evapocation commences, resulting in best quality printing. Further increase in • results in melting through the metal film and partial evaporation (with enhanced diffusien into the substrate) leading to uncomplete material removal. Lateral heat conduction broadens the ablated patterns. With optimized process parameters ablatiw depositior from tlfin metals deposited on separate substrates offer a promising novel technique for single step patterning of surfact.

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