J. Phys. Chem. 1993,97, 11731-11737
11731
Dynamics of Laser Sputtering of the Hydroxyl Radical from Ca(OH)2, Sr(OH)2, and Sucrose Samples J. VWnueva, S. Jodeh, Subhash Desbmukb, and G . P. Reck' Department of Chemistry, Wayne State University, Detroit, Michigan 48202 Received: July I , 1993;In Final Form: August 27, I99P
The ArF laser sputtering of calcium hydroxide, strontium hydroxide, and sucrose crystals is studied by laser induced fluorescence (LIF) and LIF time of flight (TOF) techniques under vacuum conditions. Studies using LIF for these three materials, at several fluences (0.83, 1.7, and 2.2 J/cmZ) and probe distances (0.4,0.8, 1.2, 1.5, and 2.1 cm) yield an average rotational temperature of 560 f 16 K from the first six rotational states for the Q1, R1, R2, and 4 2 lines. No rotational cooling was observed as a function of distance. The translational temperature analysis by LIF-TOF indicates that the process can be represented by a fast and slow cgmponent. The fast particles tend to have high J a n d the slow particles tend to have low J. An apparent surface temperature is calculated from TOF data using a sputtering model. It appears that the high J (for N = 10 18) which constitute the fast component originate from the initial sputtering events while the low J and slow component originate from processes which continue for an extended period.
-
I. Introduction
Withtheadventoflasers,it hasbecomepossibletostudysurface chemistryeffects such as laser-assisted chemicalvapor deposition, photodeposition, and sputtering (ablation). The deposition of photon energy on a solid surface, in a short period of time, causes dramatic changes in or near the surface. These changes may include a short lived visibly bright plume following the laser pulse, production of surface debris as observed in polymers or biological tissues, and clean removal of surface material as in etching or phase transformation. An example of the latter is changing diamond into graphite.' These effects depend on the wavelength, pulse duration, laser intensity, and the material subjected to sputtering or desorption. Srinivasan and Mayne-BantonZobserved that short pulses of UV light on poly(ethy1ene terephthalate) (PET) etched the surface with little evidence of heating. Further work has provided an even clearer picture of short time and long time In contrast, long wavelength pulses cause bulkdamage with heating effects dominating in the ablation process. Dlott et a1.: using an ultrashort pulse of 80 ps with a YAG laser (532 nm), have demonstrated the presence of shock waves on poly(methy1 methacrylate) (PMMA) and a lag time of 20 ps between the arrival of the laser pulse and the observation of the plume. It is thus expected that the process of sputtering by a UV excimer laser, with a pulse width of 10ns, is a combinationof decomposition and material ejection due to thermal and photochemicalprocesses. Previously, Deshmukh et a1.6 have observed that at lower laser fluence, 200-500 mJ/cmz, where the plume was not visible there is considerable deviation from Boltzmann behavior' in the low J rotational state distribution of OH from Ca(0H)t. At high fluences the distributionis more Boltzmann-like. Non-Boltzmann NO distributions have been observed by Burgess et a1.8 Cousins and Leoneg have observed the presence of fast and slow components in time of flight (TOF) signals using an ArF laser for sputtering studies on condensed films of Clz and NO on a transparent window. Similar results have also been observed by Burgess et a1.8 with a NO film on Pt foil. When Burgess et a1.8 deconvolute their TOF signal, a temperature of 350 K is obtained. In addition they find that as molecules with higher rotational energies are probed, the peak in the TOF distribution shifts to shorter times. For example, they report a translational *Abstract published in Aduance ACS Absrracrs, October
IS, 1993.
0022-3654/93/2097-I173 1$04.00/0
temperature of 1200K for J = 3.5 and a translational temperature of 2800 K for J = 19.5. In a thermal process, the translational energy of the ejected particles should depend on the laser fluence, while in a direct photochemical process particle energy should have little fluence dependence. However, the complexities of the process including the long time laser plume interactions make correlationsdifficult. The actual sequence of events is not entirely clear but recent photographic3and video imaging1° studies have revealed much that was unexpected. The broad purpose of this paper is to add to an understanding of the mechanism of sputtering by photons. The materialschosen for study are several compounds containing the hydroxyl groups, the choice being made because we were set up to analyze OH spectra. The compounds studied are (1) Ca(0H)z films and pellets, (2) Sr(OH)* pellets, and (3) sucrose crystals. The film and pellet samples differ in structure. The pellet substrate is more densely packed due to the pressure exerted on it in the process of making the pellets. As an alternative, a sucrosecrystal was chosenbecausethe molecules in the crystal are bound together by van der Waals forces rather than ionic. In addition, the OH group is covalentlybound in sucrosewhile in the metal hydroxides, the OH exists as OH-. By using the LIF technique on the OH radicals, we had hoped to relate the dynamics of the sputtering process to the nature of the binding and bulk properties of the material. Sucrose produced much less OH but qualitatively the temperature and TOF distribution from all three materials are similar. Therefore, we conclude that the sputtering process as studied here did not depend on the binding.
II. Apparatus 1. her-Induced Fluorescence (LIF). A schematic of the LIF experiment is shown in Figure 1. The components are an excimer laser which produced the particles, an excimer pumped dye laser which functions as the probe of the plume, a multiple channel delay generator, boxcar averager, and computer. The computer (IBM XT) can be used both as a pulse generator and to control the scanning of the probe laser. The primary laser is an ArF excimer laser (Lambda Physik EMG 102 MSC). The 193-nm laser light is directed into the vacuum chamber at l e 3 Pa. Table I shows the ArF fluences used in these LIF and TOF experiments. Channel 2 on the delay generator triggers the probe laser. This trigger pulse has a variable time delay with respect to 0 1993 American Chemical Society
11732 The Journal of Physical Chemistry, Vol. 97,No. 45, 1993
Vacuum Chamber
Villanueva et al. 2. Timeof FUght(T0F). Thetimeof fight dataweregathered on the Ca(0H)Z pellet at a distance of 2.1 cm and at an ablation fluence of 0.83 J/cm2. In order to obtain a TOF s p e c ” , the probe laser was tuned to the resonant frequency of the (Mor 1-1 transitions for the Q1 band in the A22-X211 absorption band of OH. The probe laser is delayed in a range from 1.CL16.0 ps in order to scan the time. 3. Sample Prepuation. The Ca(OH)2 and Sr(OH)2 pellets were prepared from ACS reagent grade chemicals of hydrated calcium hydroxide and hydrated strontium hydroxide. These chemicals were pulverized in a mortar and pestle and then dried inanoven,at 11O0C,formorethan24h. Pellets,withadiameter of 2.0 cm and a 2 mm thickness, were made with a hydraulic press at a pressure of IO8 Pa. The Ca(OH)2 film was prepared by depositing a slurry in methanol on a glass slide. The slide was baked at 110 OC for about 15 min. The sucrose used was large crystals of “rock candy”.
III. Aodysis 1. LIF Analysis. If we assume that (1) all the emitted light is collected, (2) the transition is not saturated, (3) there are no polarization effects, and (4) no quenching occurs, then the fluorescence intensity equals the intensity of the light absorbed. If thedetector has a wide spectral resolution, then Breit’s formulall becomes equivalent to eq 1 Figwe 1. Schematic diagram of the experimental setup for LIF studies on OH radicals.
TABLE I: ArF Laser Fluencea Used power power sample fluence density sample fluence density area (cm2) (J/cm2) (MW/cmZ) area (cm2) (J/cm2) (MW/cmZ) 0.160
0.040
0.42 0.83
42 83
0.0105 0.0050
1.6 2.2
162 220
Channel 1. The time for maximum signal set for channel 2 depends on the distance between the sample surface and the probe position. In fact, we discovered that it even depends on the rotational state probed. When studying rotational state distributions a best time delay for each distance was chosen and remained fixed. This delay time is at or near the maximum in the TOF spectra. The probe laser system consists of a XeCl excimer laser (Lambda Physik 200E) which pumps a dye laser (Lambda Physik FL 2002) with sulforhodamine B. The tuning range for this dye is from about 590 to 640 nm. The light frequency is doubled with a crystal. This allows us to tune the light to a working frequency range from 305 to 3 17 nm. In this frequency range the light can be tuned to various 0-0 and 1-1 transitions in the A2Z-X211 absorption band of the OH radical. The tuned UV light has a 0.02-nm resolution, a 1-2 mJ/pulse energy depending on the laser frequency, and a beam diameter of about 0.8 mm. The probe laser beam arrives in the vacuum chamber at a fied distance from the sample surface. The probe beam leaves the chamber through another window and its energy is monitored by a photodiode located at the chamber exit. The fluorescence is detected, in a direction that is perpendicular to the two laser beams with a monochromator and photomultiplier setup. The monochromator (1/4 m Jarrel-Ash) is set at the excitation wavelength with the PMT placed at the exit slit. The output of the PMT is fed into one channel of a dual gated boxcar averager. The other channel of this boxcar receives the signal from the photodiode monitoring the probe laser intensity. The boxcar is triggered by the XeCl laser. The signals are collected and stored in the boxcar for later analysis. Care was taken to ensure that the photodiode was not saturated and was linear with laser intensity.
I(a,b,c) = K’(uf)p(ut)N,B(a,b)A(b,c) (1) where B(a,b) and A(b,c) are the Einstein coefficients for induced absorption and spontaneousemission for the transitionsindicated. The number of molecules in level a is N., K’ is an apparatus constant containing for example the detector sensitivity, optical geometry, etc., and p is the radiation density of the probe beam in the region. As a result we may write
N, = I’/B(a,b)ZJ(b,c)
(2) where 1’s I/K’p and the summation is over all transitions within the wavelength range of the detector. Thevalue of the Einstein A coefficientfor spontaneousemission for the transition of interest was obtained from Goldman and G i W 2 for the 0-0 vibrational transitions. The values for the 1-1 transitions were obtained from the work of Chidsey and Cr0s1ey.l~The excitation wavelength and frequency as well as the emission wavelengths were obtained from refs 12 and 14. Based on the resolution of the monochromator, it was assumed that all emissions occurring beyond k 4 nm from the excitation wavelength would not be observed. The rotational energies, for the 0-0 vibrational transitions, were obtained from the work of Goldman and Gillis.12 The rotational energies for the 1-1 vibrational transitions were calculated, using the spectroscopic constants provided in ref 15 for the X W state. Plotsofln(Na/(U+ 1))vsEjareusedtoobtaina temperature. 2. TOF Dah Analysis. In order to analyze the time of flight data, the data were fitted to a Maxwellian distribution function of speed centered around a flux velocity, u. The general form is16
flu) = A#” exp[-(u - ~ ) ~ / a ~ ] (3) whereflu) is the normalized speed distribution function, a2 = 2kT/m, and A. is related to the number of particles. The value of n varies depending upon the ~ituation.~ For LIF, n = 2,17 Since information is collected as a function of time, eq 3 is written as a function of time: d N = -A r4 exp[-a( l / t - u)’] dr
(4) where a = 1/a2.The simplifyingassumptions in eq 4l8*19are that the areas impinged by the laser and the detector are small with respect to their spacing and that the detector is on axis, normal
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11733
Dynamics of Laser Sputtering of the Hydroxyl Radical
I
1
I
I
I i
a
8 10 MQbOroaal
12
i4
I
I8
F'igurel. Timc-of-flightdatafor0-0Q1(4)ofOHfittedwithonemodified
Maxwcllian distribution.
-'OD1 ' I
b
"" ' 4
i
b
io
I
in
i4
1s
Tk.w=--W
Figure 4. Time-of-flight data for 0-0 Ql(4) of OH fitted with three
modified Maxwellian distributions.
I
I
-1
I
f
i
Ik*m"w
m 3 . Timc-of-flightdataforM)Q1(4)ofOHfittedwithtwomodified
Maxwellian distributions.
i
b
b
10
12
14
I
10
hbnraaaona)
Figure 5. Time-of-flight data for 0-0 Ql(14) of OH fitted with one
modified Maxwellian distribution.
to the target surface. Therefore, the TOF signal can be fitted to eq 4.20 Equation 4 was used to fit to the TOF data jathered for the OH rotational lines Q1(4), Q1(8), Ql(lO), Q1(13), Q1(14). Q1(15), Ql(17). and Ql(l8) for the 0-0 vibrational transition from Ca(OH)2 pellets at a distance of 2.1 cm and a fluence of 0.83 J/cm2. The data were fitted assuming one, two, and three different Maxwellian components. A typical term is of the form
N = Aft4 exp(-a,(o
- ut)']
(5) where Ai, ai, and ui are to be determined by fitting. The overal results are checked by the standard deviation as well as by visual analysis through the use of graphics. 3. Analysis of Curve Fitting with One, Two, and Three Maxwellhi Components. The curve fitting is always improved by additional Maxwellian curves because additional parameters have been introduced. For the states with low rotational energy, we found that there is a greater improvement in the quality of the curve fitting by using two Maxwellian curves. This is shown in Figures 2,3, and 4. Figure 2 shows the fit with one Maxwellian curve, and Figures 3 and 4 illustrate the improvement in curve fitting by having an additional Maxwellian term. In going from one term to two terms, the fit improves by 19%. Adding another Maxwellian term does not dramatically improve the quality of the fit. In contrast, there is little improvement in the fit by increasing the number of Maxwellian curves for the states with high rotational energy. Figures 5,6, and 7 show this. The TOF data are best described by using an equation with two Maxwellian componentsfor low Jand a single Maxwellian for high J. Tables I1 and 111show the best results using one and two components, respectively.
-.-,
0
2
i
b
8
io
i2
i4
is
Tlmf-)
Figure 6. Time-of-flight data for 0-0 Ql(14) of OH fitted with two modified Maxwellian distributions.
W . Results and Discussion 1. hr-Induced Fluorescence Studies. A. Distance-Dependent Studies. Table IV summarizes the results of probing the rotational distribution of the OH radical as a functionof distance.
The rotational temperatures were calculated from the slope of a best fit line. The temperatures from Table IV show that, for J < 6.5 the average of all points is 556 f 19 K. There is no trend of coolfng as the hydroxyl fragments are probed from 0.4 to 2.1 cm from the surface of the sample. In Table V it is seen that as higher rotational states are probed for the Ca(OH)2 pellet, higher temperatures are observed. It can be seen in Figures 8,9,10, and 11, which are representative, that there seems to be a rotational
11734 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
Villanueva et al.
TABLE n7: Rotational Temperature for Low J ( J = 0.5 to J = 5.5) for the Sputtering of Calcium Hydroxide in Film (I), Pellet (p), and Sucrose Crystal ( 8 ) with an ArF Laser at a Fluenee of 0.83 J/cm2 and at Several Distama (em) distance(cm) sample R1 (K) R2(K) Q1 (K) Q2 (K) 0.4 0.4 0.4 0.8 0.8 0.8 1.2 1.2 1.5 2.1 I b 10 12 14 18 n m( ~ n d ~ ) Figure 7. Time-of-flight data for 0-0 Ql(14) of OH fitted with three 2
4
8
modified Maxwellian distributions.
TABLE 11: TOF Analysis with One Maxwellian ComponenP OH line u b/s) u (s/m)2 x 10’ N x 103 1000 1810 2180 2600 2600 2950 3330 3600
Ql(4) Q1(8) Ql(l0) 4~13) Ql(14) ~105) 4 ~ 7 ) Ql(18) a
7.0 7.0 11.9 8.0 7.4 8.5 4.6 4.7
4.18 3.81 3.59 1.73 2.54 1.68 1.24 1.29
P f
256 i 32 552 f 116 457 f 93 683 i 95 581 133 464 f 63 585 i 146 545 356 438 94
S
f
P S
f S
* *
f f
740 t 150 324 f 86 2507 f 1135 331 f 83 609 f 110 302 t 81 684 f 147 719 f 151 540 f 95 438 f 130 599 f 123 361 f 90 539 f 19 366 f 99 668 f 149 293 f 83 1073 f 344 253 f 54 908 f 90
389 78 382 f 82 385 f 94 453 f 134 512 f 181 439 f 127 391 f 103 422 & 74 381 f 122
TABLE V: R o t a t i ~ Temperatures ~l from the Sputtering of Ca(OH)2 Pellets with ArF at Several Distances a d at a Fluence of 0.83 J/cm2 OH linesa at 0.4 cm (K) at 1.2 cm (K) at 2.1 cm (K) 0,Ql(1-6) 1, Ql(1-6) 0, Ql(8-18) 1, Ql(9-13) 0, Q2(7-19) 0, Rl(7-16)
704 f 150 730f119 16 700 f 4000 24 000 i 7500 16 300 f 2700 10 300 f 2200
1 400 f 420 1040f200 76 OOO f 48000 8 OOO i 1400 198 OOO f 164000 11 800 i 3100
‘0, (0’ = 0, u” = 0) OH transition lines; 1, (uf transition lines.
145 i 220 662 f 120 10 560 f 1500 6 420 i 430 11 400 i 1200 19 600 f 7100 5
1, u” = 1) OH
u = particle width distribution number; N = relativeparticle density.
TABLE IIk TOF Analysis for a Two-Component M a x w e W Curve‘ OH line UI W s ) a1 (s/m)2 x 106 N]x 103 2240 2650 2600 2900 2950 3000 3500 4250
OH line Ql(4) Q1(8) Ql(10) Ql(13) 4 ~ 4 ) Ql(15) 41~7) Ql(18) a
u2
(m/s)
1640 2000 2050 2000 2100 2400 2800 2880
2.0 2.0 2.5 1.3 1.2 0.96 0.55 0.80
2.65 2.64 3.42 2.11 2.93 2.27 1.65 1.31
a2 (s/m)2 x 106
N2 X lo3
7.0 6.0 8.0 8.1 6.0 10 7.0 2.0
2.13 1.75 1.32 4.79 6.23 2.25 1.91 7.23
a = particle width distribution number; N
’b
t d m 2 d D O J d m d m s d m a d D o T d m E rd
Figure 8. Sputtering of calcium hydroxide pellet with ArF (0.83 J/cm2)
at 2.1 cm from the sample surface for (0,O) transition.
c 1.1 662
i
1201
= relative particle density.
state where there is a transition to high rotational temperatures. These high rotational temperatures seem to originate at a rotational state with J f = 8.5 (N= 8), corresponding to a rotational energy of 1324 cm-I for the vibrational transitions of 0-0 (Figures 8 and 10). The same change in temperature (Le. slope) can be observed for the vibrational transitions involving the 1-1 states, where J” = 8.5 has a rotational energy of 1425 cm-’, Figures 9 and 11, Results of the TOF analysis show that these higher states also have a higher translational temperature. B. Fluence-Dependent Studies. The effect of different fluences on the ablated OH fragment were also studied. Table VI shows that, for this range of rotational lines, the temperatures are basically the same. Since there is no difference in rotational temperatures due to changes in fluences, the overall process can be described by an average temperature of 560 f 16 based on all points in Table VI. This average temperature falls within the
1.3
1
t d m 2 d D O 3 d o o d m s d m a d D o 7 m o E rd (-1)
Figure 9. Sputtering of calcium hydroxide pellet with ArF (0.83 J/cm2) at 2.1 cm from the sample surface for (1,l) transition.
same range as the average temperature (556 f 19 K) for the distance studies, for the same range of rotational lines. From Table VII, the difference between low J and high J is evident. For the higher rotational lines, the calculated temperature is at least 1 order of magnitude higher. These high temperatures merely reflect the uniform distribution at higher J, probably indicative of a nonthermal process. The presence of
The Journal of Physical Chemistry, Vol. 97, No. 45,1993
Dynamics of Laser Sputtering of the Hydroxyl Radical
11735
TABLE W: Rotatioapl Temperatures (K) from the Ablation of Ca(OH)2 Pellet with ArF at Several F'luencw (J/cm2) and at a Distance of 1.2 cm OH lines# at 0.83 J/cm2 (K) at 1.7 J/cm2 (K) at 2.2 J/C" (K) 0, Q1( 1-6) 1,Q1(1-6) 0, Ql(8-18) 1,Q1(9-13) 0,Q2(7-19) 0, Rl(7-16) __.__
T
-
a
662 h 180 716 i 130 18800 h 3400 26500h 5900 21100 h 4300 9200 h 800
680 & 180 760 & 140 13100 2000 111000* 1300 17500 4OOO 17100 h 2700
* *
"0,(0' = 0,v" = 0) OH transition lines; 1, (u' = 1, v" = 1) OH transition lints.
Qt(b1S. 00
a
1400 i 420 1040h200 76000 i 48000 8 0 0 0 i 1400 198000 h 164000 11800 t 3100
16700
240m
TABLE WI. Analysis of TOF Velocity Distribution. l
O
m
2
m
o
~
~
~
s
m
a
m
OH line u1 (m/s) x ,
ElU(an-1)
Figure 10. Sputtering of calcium hydroxide pellet with ArF (0.83 J/cm2) at 0.4 cm from the sample surface for (0,O) transition.
'li
OH line u2 (m/s) Ql(4) Ql(8) Ql(l0) Ql(13) Ql(14) Ql(15) Ql(17) Ql(18)
14.
I
01 (s/m)2 X
2240 2650 2600 2900 2950 3000 3500 4250
1640 2000 2050 2000 2100 2400 2800 2880
2.00 2.00 2.50 1.30 1.20 0.96 0.55
0.80 02
106 M(1) Tn/Tk(l) Tm/TI(l) TI(K) 3.95 4.67 5.13 4.12 4.03 3.66 3.24 4.74
0.534 0.470 0.435 0.517 0.526 0.564 0.611 0.464
0.445 0.392 0.363 0.431 0.439 0.470 0,509 0.387
920 1463 1520 1591 1681 1562 1962 3806
(s/m)2 X 106 M(2) Tm/Tk(2) Tm/T1(2) T,(K) 7.0 6.0 8.0 8.1 6.0 10.0 7.0 2.0
5.41 6.11 7.23 7.10 6.42 9.47 9.24 5.08
0.415 0.372 0.316 0.322 0.356 0.236 0.243 0.439
0.346 0.311 0.263 0.268 0.297 0.197 0.202 0.366
633 1051 1301 1215 1213 2387 3160 1850
Note: values of u and (I were taken from Table 111.
l d m 2 d m J d m d D o s d D o s d m m a , E IQ m-1)
-11. Sputteringofcalcium hydroxidepelletwith ArF (0.83 J/cm2) at 0.4 cm from the sample surface for (1,l) transition.
TABLE M: Rotational Temperatures (0-0 Vibrational Transitions) for N = 1 to 6, for the Ablation of Calcium Hydroxide in Film and PeUet, Strontium Hydroxide Pellet, and Sucrose Crystal with ArF Laser at a Distance of 1.2 cm and at Several Laser F'l(J/cm2) ~
laser fluence
(J/cm2) sample 2.2 1.6 1.7 1.7 0.83 0.83 0.83 0.83 0.83
Ca,f Ca,f
S,c S,c Ca,f
S,c Ca,p Sr,p S,c
R1 (K) 4 2 5 i 70 468i95 532h122 5 9 9 i 108 464i63 5 8 5 i 146
R2(K)
Q1 (K)
3 5 6 h 100 1 3 3 7 f 3 2 1 370h111 370h41 272h56 694h111 3 6 0 h 104 6 0 9 h 100 361 h 9 0 539h79 366i99 668f 149 1400 f 420 718 h 190 640 i 190
Q2(K) 441 i 105 948h400 397h99 390h86 4 3 9 i 127 391 h 103
Ca = calcium hydroxide, S = sucrose, Sr = strontium hydroxide, f = film, p = pellet, c = crystal. two slopes for the rotational distribution indicates that the OH radicals for the low rotational states are produced by a different mechanism than the radicals that have a high rotational energy. From the results obtained we concludethat there is no difference between the temperatures for different substrates which indicates that the mechanism is similar for all substrates. 2. T i of Flight. A. Velocity Distribution. The velocity distributions resulting from the time-of-flight data (Table 111) which results from a fit to a thermal distribution may be used in a model of the sputtering (ablation) process developedby Kelly and co-w~rkers.'~J~ This model is based on the assumption that laser sputtering can be more nearly described by a thermal model where there is a transient vaporization process. This transient vaporization arises due to the initial effects caused by the laser pulse.
Following these events, energy flows, causing particle emission at ambient temperature (the local temperature after energy has migrated).19 This can be viewed as a slow thermal process as distinct from processes linked to a thermal spike. In the initial processes, particles are emitted from the hot surface at number densities sufficient to cause collisions. A layer develop in which there are collisions,and mass flow develops. This layer is termed the Knudsen layer. This layer evolves into an unsteady adiabatic expansion ending in a region of free expansion. It is assumed that the detector is situated in the region of free expansion. In order to develop Table VIII, Mach numbers were calculated using the standard speed ratio. In the equation, 7 is the specifc heat ratio, 9 / 7 , for a diatomic particle assuming perfect gas law, two rotational degrees of freedom, and one degree of vibrational freedom, u is the fitted value of the mass flow, and a = m/2kT. The Mach number is
M = (2u/7)'/'u (6) The temperature of the Knudsen layer, Tk, and the temperature, Tm,including the Mach number are related by the equation (7) The ratio Tm/Ts, in Table VI11 below, was obtained from the product of (Tm/Tk)(Tk/T8).The ratio Tk/T8is the ratio of the Knudsen layer temperature to the surface temperature.1s This ratio is given by eq 8 where y is the heat capacity ratio and j is the number of internal degrees of freedom accessible at T,.The quantity,j, is taken to be 4 assuming ideal rotation and vibration.'* The ratio is
The ratio Tm/T,is used to estimate the true surfacetemperature
11736 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
TABLE M: Temperatures from h t a Treatments for Ca(OH)2 Pellets at a Distance of 2.1 cm and a Flwnce of 0.83 J/cm2
-1
3
~~
low J (1-6)
highJ(8-18)
4
-r. A 0
Villanueva et al.
1
”
3
l
a l ~ Ehop awl
m
&
m
1
Figure 12. Boltzrnann plot from data in Table IV using the relative particle densities from TOF fit. Upper curve is for fast component while lower curve is for the slower component.
with gas interactions, T,,in eq 6a of Kelly21 (9)
Thequantity, 7 , is calculated using eq 17 from ref 18 or eq 7 from ref 19 for a detector located normal to the surface 16 0.5 2 v = (Y/8)( 1 + (1 +
y))
The results of this analysiscan be seen in Table VIII. The average apparent temperature for the surface using this model is 1810 f 800 K for the fast component and 1600 f 770 K for the slow component. This modelz2results in a mass flow of particles that has a forward peaked distribution. As the angle of the probe, in reference to the angle of removal from the surface is changed, the TOF signal follows an angular dependencedescribed by w For a density 0. The value of p is given by p = (1 sensitive detector, and a diatomic species, the average Mach number, from Table VIII, is 5.6 f 1.8. A value of 36 is obtained for p. Based on Figure 4 of Kelly2*the ejection polar angle is between 20 and 30°. This is a very narrow ejection angle. B. Density Distribution Analysis. A Boltzmann plot of the data from Table 111,using the relative particle densities obtained from the TOF fits, treated as we did the LIF data, yields two rotational temperatures as shown in Figure 12. One at 6700 f 1000 K and another at 3200 f 700 K. The higher temperature corresponds to the first component (the faster) of the two component curves. The lower temperature corresponds to the second component (the slower). The slope for these components corresponds to temperatures that are higher than the average temperatures found from the velocity distribution analysis. This result is not surprising because the temperature calculation did not include the formation of a Knudsen layer; hence the temperature is an overestimate. This overestimate ranges from 2.58 for monoatomic speciesto 3.28 for specieswith many degrees of freedom.1* Taking this into consideration, the 6700 K temperature scales down to a value within the range found by the velocity distribution analysis; the apparent surface temperature is estimated to be -2OOO K. It follows that the temperature obtained from the second component is also overestimated. The temperature for this component is estimated to be 976 K. These estimates are not inconsistent with the observed rotational temperatures. Actually, using the TOF particle densities is the preferred means of obtaining the rotational distribution since it includes all times for a given state, not just at the fixed delay of the LIF method. Figures 5 and 12, and Table 111,show that the ejected particles with low rotational energy contribute more to the second
+
-
rotational state (K) 745 220 10560 & 1500
TS 1600*710 1810+800
TAaPP) -976 -2000
component of the TOF signal than particles having a high rotational energy. This means that the fast particles tend to have high J and the slow particles tend to have low J. The apparent surface temperatures in Table VI11 obtained from the TOF data for both components are not drastically different from the rotational state temperatures. This is summarized in Table IX. The rotational temperature from the high J states is quite high, as shown in Figures 8-1 1, and may even be thought of as reflecting a uniform distribution (Le. a nonthermal process). The low J states show a rotational temperaturequite similar to the apparent surface temperature. It appears that the high J and fast component originate from the initial process which produces a nonthermal rotational distribution while the low J and slow component originates from somethingclose to a thermal process. It should be remembered that when discussing thermal or equilibrium processes that provided one is not at an extreme, collisions will always lead to distributions of the form of cqs 3 or 4. Even if the initial particle release is not thermal, a similar picture will hold provided sufficient collisionsoccur and a Knudsen layer forms. In effect, a distribution of the form of eqs 3 and 4 can be expected rather generally.23 V. Conclusion The state distribution of OH in low and high rotational states was determined via LIF and LIF-TOF as a function of laser fluence, distance from the sample, and different sample structure or form. The low J levels had a low temperture -600 K. The high Jlevels had a very high Ytemperature* 10000-100000 K. There was no evidence of cooling as the hydroxyl fragments were probed from 0.4 to 2.1 cm from the surface of the sample. The results were virtually independent of the binding. The LIF-TOF spectra are best described by two components. This implies that there are two basic contributions, one fast and one slow, describing the mass flow from sputtering. The data from the LIF-TOF spectra were treated according to a model developed by Kellyand cm-workers.1p-21Anestimateofthesurface temperature was obtained by using this model’s velocity distribution analysis. In addition, the density of particles in each component was obtained and corrected for the overestimation of the temperature. The fast component has a temperature of 2000 K and the slow component a temperature of 976 K. These estimates are consistentwith theobserved rotational temperatures. The apparent surface temperatures obtained from the LIFTOF data for both components are not drastically different from the rotational temperatures. The rotational temperature from the high J states is quite high and may even be thought of as reflecting a uniform distribution (nonthermal). The low Jstates show a rotational temperature quitesimilar to the apparent surface temperature. It appears that the high J and fast component originate from the initial processes while the low J and slow component originate from a close to thermal process. N
VI.
summary
It is difficult to make totally general statements about the sputtering process because the results can be strongly dependent upon the material, wavelength, and fluence. At present our understanding may be summarized as follows: When a pulse of laser light strikes a material, there is a threshold fluence below which little material is removed and not much light is emitted. Above the threshold for the particular material, etching proceeds with a plume of material produced that has identifiable atomic
Dynamics of Laser Sputtering of the Hydroxyl Radical and molecular species present. Material may come off in other forms that are not spectroscopically identified. See for example Srini~asan.~ Some of the emitted material is produced in excited electronic states and can be characterized by the emission. Most of the atomic and molecular species are in their ground electronic state. In the plume's early stages the densities are high enough that multiple collisions occur. LIF on the ground state material can yield rotational and vibrational temperatures, but the distributions are not strictly thermal. TOF studies can be used to determine the translational temperature. What appears to happen is that the short laser pulse produces a burst of material that appears to have a high translational and rotational temperature. Thistemperature increaseswith laser fluence. Residual energy in the material diffuses and produces slower particles over a longer period of time. These particles have both a lower translational and rotational temperature. More precise details await further studies on these systems using LIF,2S TOF,26 and video imaging techniques.'O References d Notes (1) Deshmukh,S.; Hitchcock, L. M.;Rothe, E. W.; Reck, G. P. Diamond Relat. Mater., in press. (2) Srinivasan,R.; Mayne-Bandon, V.Appl. Phys. Lett. 1982,41,576578. (3) Srinivasan, R. 1. Appl. Phys. 1993,73,2743-2750. (4) Srinivasan, R. J. Appl. Phys. 1992, 72, 1651-1653. (5) Dlott, D. D.; Kim, H.;Postlewiate, J.; Zyung, T. Appl. Phys. Len. 1989,54, 2274-2276. (6) Deshmukh, S.;Rothe, E.; Reck, G. J. Phys. Chem. 1991, 95 (21). 838-387. (7) Bemstein, R. B. Chemical Dynamics via Molecular and Laser Techniques;Oxford University Prm: New York, 1982;Chapter 3.
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(8) Burgess, D., Jr.; Cavanagh, R.R.; King, D. S. J. Chem. Phys. 1988, 88,6556-6569. (9) Cousins, L.;Leone, S. J. Mater. Res. 1988, 3, 1158-1168. (10) Fried, D.; Kushida, T.;Reck, G. P.; Kim,0.-S.;Rothe,E. W .Appl. Specrrosc. 1993, 47, 1046-1057. (11) Kinscy, J. L. Annu. Reu. Phys. Chem. 1977.28, 349-372. (12) Goldman, A.; Gillis, J. R. j . Quant. Spectrosc. Radial. Transfer 1981,25, 111-135. (13) Chidsey, I. L.; Crosely, D. R. J. Quant. Spectrosc. Radiat. Transfer 1980,23, 187-199. (14) Dieke,G.H.;Crosswhite,H. M. J . Quant.Spectrosc.Radiat. Transfer 1%2,2,97-199. (15) Spectroscopic Data; Heteronuclcar Diatomic Molecules; Suchard, S. N., Ed.; Plenum: New York, 1988;Vol. 1. (16) Lubman, D. M.; Rettner, C. T.; a r e , R. N. J. Phys. Chem. 1982, 86, 1129-1135. (1 7) Kittel, C. Elementary Solid State Physics: A Short Course; John Wiley and Sons: New York, 1962;Chapter 1. (18) Kelly, R.; Dreyfus, R. W. Surf. Sci. 1988, 198,263-276. (19) Kelly, R.; Dreyfus, R. W. Nucl. Insrrum. Methods Phys. Res. 1988, 832,341-348. (20) Matthias, E.;Dreyfus, R. W. Photoacoustic, Photothermal and Photochemical Processes at Surfaces and in Thin Films: Heas. P.. Ed.: Springer-Verlag Sci. Topics in Cirrent Physics; Springer: New York, 1989; Vol. 47,pp 89-128. (21) Kelly, R. J. Chem. Phys. 1990,92,5047-5056. (22) Kelly, R. Nucl. Instrum. Methods Phys. Res. 1990,B46,441447. (23) Kelly, R.; Miotello, A. In Pulsed Laser Deposition of Thin Film; Chrisey, D. B., Hubler, G. K., Eds.; Wiley: New York, in preas. (24) Fried, D.; Reck, G. P.; Kushida, T.; Rothe, E. W. J. Appl. Phys. 1991, 70,2337-2342;Fried, D.; Kushida, T.; Reck, G. P.; Rothe, E. W. J. Appl. PhyS. 1992, 72, 1 1 13-1 125. (25) Fried, D.; Kushida, T.; Reck, G. P.; Rothe, E. W. J. Appl. Phys. 1993, 77,7810-7817. (26) Fried, D.; Jodeh, S.; Reck, G. P.; Rothe, E. W.; Kushida, T. J. Appl. Phys, in press.
