SF6 decomposition and layer formation due to excimer laser photoablation of SiO2 surface at

gas–solid system

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2004 J. Phys. D: Appl. Phys. 37 3402

(http://iopscience.iop.org/0022-3727/37/24/008)

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INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 37 (2004) 3402–3408 PII: S0022-3727(04)81187-2

SF6 decomposition and layer formationdue to excimer laser photoablation of SiO2surface at gas–solid systemBatool Sajad1,2, Parviz Parvin1,3,5 and Mohamad Amin Bassam4

1 Physics Department, Amirkabir University, PO Box 15875-4413, Tehran, Iran2 Physics Department, Alzahra University, Postal Code 1993891176, Tehran, Iran3 Laser Research Center, Atomic Energy Organization of Iran (AEOI), PO Box 11365-8486,Tehran, Iran4 Excimer Laser Lab, Emam Hussain University, PO Box 16575-4347, Tehran, Iran

E-mail: parvin@aut.ac.ir

Received 25 May 2004, in final form 27 September 2004Published 2 December 2004Online at stacks.iop.org/JPhysD/37/3402doi:10.1088/0022-3727/37/24/008

AbstractIn this work, the effect of an excimer laser has been studied for presenting amethod for SF6 decomposition and simultaneous formation of a SiF2 layeron amorphous SiO2. Though the excimer laser did not establish a gas phasephotodissociation, we have shown that UV photoablation leads strongly tomolecular decomposition in the SF6–SiO2 system. Moreover, thedependence of the decomposition process on the exposure parameters suchas the wavelength and intensity as well as the gas pressure and the focalpoint distance from the gas–solid interface has been investigated.

1. Introduction

The unique properties of laser light are the basis foran overwhelming potential for scientific investigations onmaterial processing. Laser light can influence molecule–surface interactions and promote localized superficial chemicalreactions. This is being investigated for microelectronic devicefabrication in processes such as chemical etching, chemicalvapour deposition, semiconductor doping and metal alloying.Laser enhanced chemical etching studies have been carriedout primarily at the infrared (IR) and visible lines with pulsedTEA CO2 and CW Ar+ lasers [1–8]. On the other hand,progress is being made in UV photochemistry with excimerlasers [9–11]. For laser micro patterning, some types of surfacemodification and thin film deposition, the excimer laser isan almost ideal source. Moreover, its applications in microlithography for microchip production in the semiconductorindustry have developed at a rapid pace and have created newchallenges in UV laser chemistry.

Halogen radicals, which are produced by photo-dissociated precursor molecules, are known to be highlyreactive, and are therefore well suited for laser chemicalprocessing. Etching of Si surfaces using atomic fluorine is

5 Author to whom any correspondence should be addressed.

a central process in the micro structuring of semiconductormaterials. Fluorine atoms not only become dissociativelychemisorbed on silicon surfaces but also diffuse into the surfaceand form a fluorosilyl (SiFx) layer. UV photochemical dryetching of SiO2-rich glasses has been investigated mainlyusing halide radicals. Even though both ArF and KrF laserradiation have been used frequently to photo-dissociate halidemolecules like COF2 [9], CF2Cl2, CF2Br2, CF3Br, CF3I,CF3NO and CO(CF3) [12, 13], experiments carried out undersimilar conditions with Br2 and SF6 were previously reportednot to etch SiO2 significantly [14].

This work deals with the addition of SF6 to SiO2 surfacesfor decomposition of gaseous SF6 molecules using an excimerlaser to establish a fluorosilyl layer. This has been investigatedfor optimizing the process for micro fabrication purposes.We have utilized ArF (193 nm) and KrF (248 nm) lasers toenhance chemical etching reactions with silicon content. Eventhough there are a few articles on vibrational excited SF∗

6–Siinteractions [3, 4], to our knowledge, the decomposition of SF6

molecules in the UV spectral region has never been reported.

2. Experimental apparatus

The experimental set-up consists of an irradiation chamber, ahigh vacuum-mixing table, a UV coherent source, conducting

0022-3727/04/243402+07$30.00 © 2004 IOP Publishing Ltd Printed in the UK 3402

SF6 decomposition and layer formation

and focusing optics, laser pulse diagnostics and an FTIRspectrometer. An irradiation chamber, ∼100 cm3, with cross-type quartet windows, was fabricated of stainless steel. AnAR-coated MgF2 window and a Si-content substrate wereinserted opposite to each other as the input and output couplers.A couple of broadband (9–11 µm) AR-coated ZnSe windows,perpendicular to the axis along the MgF2 window, wereprovided to monitor the dominant characteristic absorptionpeaks in terms of laser energy doses. A Balzer vacuumvalve was connected to the cell in order to withstand thehigh vacuum within the cell and withstand the atmosphericpressure. The pressure attainable using a PD40 high vacuumsystem is ∼2 × 10−6 mbar. A Lambda Physik LPX 100excimer laser (0–100 mJ per pulse energy, 8 ns pulse duration,pulse repetition rate of 1–10 Hz) was used as the UVcoherent source at 193, 248 and 351 nm. A semiconductordetector (PIN, EG&G, FNT100), a Tektronix 30145 300 MHzdigital storage, 2.5 Gb s−1 oscilloscope, a Tektronix 7844400 MHz oscilloscope and a Coherent™ joule meter (FieldMaster, LM-P10 and LM-P5 LP heads) were used for therelative and absolute power measurements. Variation ofthe laser energy delivered to the cell was done by usingseveral NDF aluminized attenuators with the appropriateoptical densities. A modified FTIR spectrometer, modelBomem MB100, with 4 cm−1 resolution was utilized toregister the characteristic IR absorption peaks for analysingdata. A Perkin-Elmer IR spectrometer, model 783, wasalso used to calibrate the instrument to ensure measurementaccuracy. The surface morphology was studied using ascanning electron microscope, model XL30 Philips SEM, witha sputter coater, model BAL-TEC SCD050. The thicknessof the fluorosilyl layer was also measured using a stylusDektak™ profilometer. Energy dispersive x-ray (EDAX)microanalysis and Rutherford backscattering spectrometry(RBS) instrumentation was used along with the electronmicroscope to determine the chemical element levels to studythe composition of the layer formed.

3. Result and discussion

A series of experiments were performed with different laserparameters, such as pulse energy, power density, wavelength,repetition rate, gas cell pressure and focal point distance tosurface to investigate the UV chemisorption process throughdissociation analysis.

First, the cell was filled precisely with pure SF6 at0.3 mbar and irradiated by ArF laser at 1 Hz with variouspulse energies. With the unfocused beam, no moleculardissociation was observed even at higher pulse energies andirradiation doses. The beam was focused at a definite locationwithin the chamber, typically 3 mm from the SiO2 glasssurface. For pulse energies below 5 mJ, there was no SF6

decomposition. Subsequent chemical reactions were not seeneither; however, on increasing the energy, we observed SiO2

etching just at the onset of dissociation. This implies intensitydependence of the SF6–Si interaction, having a threshold of∼0.16 J cm−2. Data acquired from the IR spectra were usedto evaluate the gradual decrease in the dominant characteristicpeaks corresponding to the decomposition of SF6 molecules.The measurable quantities are n0 and nN, the initial molecule

Figure 1. Variation of nN/n0 ratio with number of laser shots atdifferent incident pulse energies on a logarithmic scale.

Figure 2. The obliteration of SF6 molecules versus number of lasershots at different cell pressures on a logarithmic scale.

concentration and that after N -pulse irradiation, respectively.Figure 1 shows the measured nN/n0 ratio in terms of thenumber of laser shots with various pulse energies in the range5–100 mJ per pulse, equivalent to 0.16–3.5 J cm−2 at the focalpoint. The best fit of the experimental data resembles anexponential decay, in accordance with nN = n0 e−ωN, andtherefore the parameter ω can be calculated easily. The SF6

dissociation rate per pulse, R = 1 − e−ω [15], versus energydose is nonlinearly proportional to φ1.9, which indicates that atwo-photon process may be invoked.

The accuracy of the measurements is limited by the degreeof intensity and wavelength stabilization of the laser. Empiricalresults emphasize that the dissociation rate is independent ofthe pulse repetition rate. Therefore, we have chosen a 1 Hz rateto minimize the standard deviation of the data due to the slightenergy alternation of the successive laser shots. Moreover, athigher pulse repetition rates, the SF6 molecules (at millibarpressures) undergo a rapid decomposition, in a fraction ofa second. This happens at the expense of slow recordingof the characteristic amplitudes, because of our diagnosticinstrumentation.

The effects of various gas pressures has also beeninvestigated by performing successive experiments at a

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(a)

(b)

Figure 3. (a) Typical EDAX microanalysis of an untreated SiO2 surface as a reference; (b) typical EDAX microanalysis of the treatedsurface including the formed layer.

typical pulse energy of ∼90 mJ at different SF6 pressures(0.15–2 mbar) in the irradiation chamber, where the focalpoint is kept ∼3 mm from the interface. Figure 2 showsthe exponential dependence of the ratio nN/n0 in terms ofthe number of laser shots at different gas pressures. Thedissociation rate decreases strongly at higher gas pressures

due to collisional events in the gas phase. The maximumconcentration of dissociated fragments, Nd, which is clearlyequivalent to Rn0 [15], determines the optimum workingpressure of pure SF6 to be ∼0.3 mbar.

We have investigated the wavelength dependence of theprocess by using the UV photons of KrF and XeF lasers.

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SF6 decomposition and layer formation

(a) (b)

Figure 4. Typical SEM micrographs: (a) at 0.9 mbar SF6 pressure due to ArF laser exposure after 100, 400 and 800 shots and (b) at 0.9 mbarHe pressure due to ArF laser exposure after 100, 400 and 800 shots.

As expected, there is no dissociation with the XeF laser at351 nm exposure because of the small absorption coefficientof SiO2. Despite this, we have observed the correspondingevents at 248 nm due to KrF laser irradiation; however,the dissociation rate increases significantly at 193 nm withthe ArF laser, indicating the strong wavelength dependenceof the process. This arises simply from the fact that theabsorption cross-section has significantly higher values atshorter wavelength VUV than near UV. The correspondingdissociation rates were determined to be ∼5.6 × 10−3 and∼2.5 × 10−5 due to exposure to ArF and KrF lasers,respectively.

We have examined the layers formed with EDAX andRBS systems to search for experimental evidence of fluorinepenetration and the layer composition. First, we studiedan untreated section of the SiO2 surface (as the reference)

using EDAX. The microanalysis spectrum of EDAX shows thecounts versus electron energy (kiloelectronvolts). The K-linecharacteristic peaks of Si and O were easily seen, while thatof the F peak was not detectable. The typical relative ratiosfor O, Si and F were reported to be ∼50%, 39% and 0.00%,respectively, as shown in figures 3(a). Afterwards, the lasertreated surface was examined by EDAX too. The K-linecharacteristic peak amplitude due to F obviously increases inaccordance with the fluorine abundance in the layer structure.The corresponding ratio for F denotes a remarkable increaseto 12%, while those of O and Si decrease to ∼45% and29%, respectively, as shown in figure 3(b). EDAX supportsthe evidence of fluorine penetration into the treated area.Moreover, the electron beam goes deeply into the SiO2 glass,beneath the SiFx layer, and therefore, the variation in electronenergy affects the amplitude of the characteristic Si peak. It

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Figure 5. The mean thickness of the SiF2 layer versus number ofArF laser shots at 0.3 and 1 mbar of SF6.

(a)

(b)

Figure 6. (a) Variation of nN/n0 versus number of laser shots atdifferent distances of the focal point from the SiO2 glass surface ona logarithmic scale. (b) Dissociation rates versus mean distancefrom laser focal point to surface.

indicates that the SiO2 glass substrate affects microanalysisresults.

RBS also confirms the evidence of fluorine content in thesurface treated using the proton beam (generated from a Van deGraaf accelerator) on the target. The untreated samples havebeen compared with treated ones to show the basic changeswhich arise from laser exposure. The RBS spectrum showsthe counts versus proton energy channels (kiloelectronvolts),

(b)

(c)

(a)

Figure 7. (a) IR absorption spectra of the cell filled with SF6 at0.5 mbar before irradiation, (b) after 900 shots of the ArF laser at90 mJ per pulse and (c) after 1600 shots.

identifying a fluorine peak, obviously around channel 450,corresponding to ∼1.2 MeV.

The gradual growth of fluorosilyl layers on treatedamorphous SiO2 has been investigated as well. A scanningelectron microscope with a secondary electron (SE) detectorand a backscattering electron (BSE) detector was used toobserve the morphology of the layer formation. A SiO2 surfaceusually has no electric conductance, and hence a 10–15 nmgold layer is essentially coated using a sputter coater to removeelectrostatic charges from the surface due to electron beamradiation during the microscopy. Figure 4(a) shows typicalSEM micrographs of a SiO2 surface at 0.9 mbar SF6 pressurefor various UV laser doses to demonstrate the effects ofSF6 addition on the etched SiO2 surface. The laser spoton the SiO2 surface is nearly oval in shape, with size about∼1500 µm × 500 µm, 3 mm from the focal point.

Further experiments have been performed to explain thereaction kinetics. Keeping the laser beam and SiO2 surfaceparallel and setting the laser beam focus point at variousdistances including 3 mm from the surface, with the laser beamarrangement not touching the surface, no SF6 decompositionwas observed. The UV laser simply induces gas decompositionwhen the SiO2 surface is perpendicular to the beam axis.This emphasizes that the UV laser photoablation of the surfacehas the dominant effect on SF6 decomposition. Figure 4(b)shows the ablation without layer formation when we filledthe cell with a noble gas (He or Ar) instead of SF6. Thesurface photoablation occurs in vacuum too. This indicatesthat the laser–SiO2 interaction leads to photoablation withoutany layer observation. However, in a SF6 atmosphere,the simultaneous SF6 decomposition and layer formationcontinues as long as the fluorosilyl layer covers the entireirradiated area.

In fact, the coated fluorosilyl layer strongly decreases therate of ablation of the SiO2 surface beneath, which graduallycauses the SF6 decomposition to stop. It is worth mentioning

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SF6 decomposition and layer formation

(a)

(b)

Figure 8. (a) A couple of IR induced chemisorption schemes including both direct dissociation at high intensity laser exposure above thethreshold, and the SF∗

6–Si reaction at low intensity. (b) The proposed UV induced chemisorption scheme of the SF6–Si∗ reaction due tosurface photoablation.

that a SiF2 layer, like CaF2 and MgF2, becomes hardened undercoherent UV irradiation to form a formidable layer against UVphotoablation. The thickness of the layer has been measuredusing a Dektak™ stylus profilometer. Figure 5 shows thecorresponding mean thickness versus ArF laser shots at 0.3 and1 mbar to indicate that the layer thickness increases nonlinearlyin terms of the number of pulses.

The reaction becomes noticeable at a focal point closerto the Si content surface. Figure 6(a) shows the nN/n0

ratio against the number of laser shots at different focalpoint–interface distances. The reaction rate decreases asthe distance increases mainly because the UV ablation ratereduces. Figure 6(b) shows the dissociation rate versus thedistance (r) from laser focal point to the surface. In the vicinityof the interface, the simultaneous surface excitation by laser

radiation also becomes significant such that the photoablatedevents result in a further enhancement of the reaction.

When UV coherent photons are absorbed by a surface, theelectromagnetic energy of the beam is converted to mechanical,thermal, chemical and electronic energy and resulting ejectedablative debris, in the form of a plume, which includes atoms,molecules, ions, photons, electrons and an agglomeration offragments of the irradiated materials [16, 17]. Similarly, it iswell known that skin ablation occurs with high intensity laserirradiation on SiO2 surfaces, particularly with excimer lasersat shorter wavelengths. That means many active species, A∗,such as Si and O radicals and ions and electrons, are generatednear the SiO2 surface. These species (especially low energyelectrons) have very large reaction cross-sections. Therefore,it is considered that SF6 molecules are decomposed by the

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reaction A∗ + SF6. It is also known that the SiO2 surfaceis modified by UV laser irradiation [18], and hence there isalso a probability of a reaction between the SiO2 surface madereactive by the UV irradiation and the photoablated reactivespecies generated by the reaction A∗ + SF6.

Eventually, we suggest the following scheme for excimerlaser induced chemisorption based on the dominant SF6–Si∗

correlation at the surface:

SiO2 + nhν → Si∗ + 2O∗ or (Si+ + 2O+ + 3e)

(photoablation), (1)

SF6(g) + Si∗ → SF5 + F− + Si (decomposition). (2)

Some of the chemisorbed F− ions penetrate into the Si andconstitute a fluorosilyl (SiF2) layer.

Si (or SiO2) + 2F− → SiF2 (or SiF2 + O2) (adsorption).

(3)

The adsorbed SiF2 species in turn react with each other to formthe volatile tetrafluoride silane (SiF4) components:

SiF2(ads.) + SiF2(ads.) → Si + SiF4(ads.), (4)

SiF4(ads.) → SiF4(g). (5)

The gaseous SiF4 has been detected at 1033 and 1023 cm−1.Figure 7 shows typical IR absorption spectra of the gaseouscomponents in the cell before and after UV irradiation. As thedominant SF6 characteristic peak at 948 cm−1 decreases withthe successive laser shots, in the meantime, a major pair ofincreasing peaks due to volatile SiF4 species appears to becomemore prominent at higher pressures and energies. AlthoughSF5 radicals are initially generated, the gas phase collisionscause the unstable SF5 components to recombine to createsulfur decafluoride as shown below:

SF5 + SF5 → S2F10 (cluster). (6)

We have observed the formation of S2F10 clusters, whichresemble a type of white dust, deposited throughout the walls,becoming noticeable at higher pressures.

It is worth comparing the mechanism of interaction of UVwith IR lasers on SiO2. A TEA-CO2 laser is mostly used forlaser induced SF6– Si interactions [4], while a number of stepsoccur, depending on the types of active species involved inthe reactions. At low CO2 laser power, vibrationally excitedSF∗

6 molecules are supposed to be the most important speciesfor SF∗

6–Si interactions. At higher laser power, SF6 canbe driven beyond the threshold for dissociation and thus inaddition to SF∗

6, the photodissociated fragments, particularlyfluorine radicals, can also be reactive with Si [4], as shownin figure 8(a). Despite the vibrational excitation, SF6 inducesdirect photodissociation; however, a UV beam does not leadto gas phase photodissociation. The decomposition occurswhen Si is available in the cell environment for establishing anSF6–Si∗ interaction. Figure 8(b) shows the dominant kineticscheme of the proposed UV induced chemisorption accordingto the equations above. SiF4 is taken into account as the sameby-product in both cases, whereas the formation of S2F10 dustappears to be a distinct event in the UV process instead ofproduction of gaseous SF4 due to IR exposure.

4. Conclusion

The aim of this work was an experimental study of SF6 gasdecomposition in the vicinity of a surface, a Si containingmaterial, irradiated by UV laser radiation. Excimer laserradiation does not induce gas phase dissociation; however, wehave shown that UV photoablation of a SiO2 surface leadsstrongly to molecular decomposition of SF6. The results, as afunction of the laser fluence on the SiO2 target, tend to indicatea threshold phenomenon of SF6 decomposition, and above thisthreshold, a two-photon process may be invoked. Theinfluence of wavelengths of lasers of ArF (193 nm) and KrF(248 nm) was investigated, indicating a better efficiency of ArFirradiation. The decomposition was also studied as a functionof SF6 gas pressure, and deposition of SiF2 layers onto the SiO2

surface around the irradiated region has been pointed out. Theadsorbed SiF2 produces a layer, which was observed underSEM, and the corresponding thickness was measured usinga profilometer to indicate that there is a strong correlationbetween the rate of decomposition (figure 2) and the fluorosilyllayer thickness (figure 5) at various SF6 pressures. Finally,a UV induced chemisorption scheme for SF6 decompositionhas been proposed, based on empirical results such as thecharacteristic peaks, the reaction threshold and the relativedistance between the focal point and the gas–solid interface.

We conclude that the SF∗6–Si interaction due to vibrational

multiphoton absorption at 10.6 µm of the TEA CO2 laser[4] undergoes a mechanism changeover which arises fromSF6–Si∗ (or SF6–A∗) reactions due to excimer laser irradiationat 193 and 248 nm, where A∗ denotes the photoablationspecies.

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