From processing of cosmic ices to optical communications

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Nuclear Instruments and Methods ia Physics Research B 116 (1996) l- 12

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Beam Interactions with Materials 8 Atoms

ELSEVIER

From processing of cosmic ices to optical communications Walter L. Brown AT & T Bell Laboratories,

*

Murray Hill, NJ 07974-0636,

USA

Abstract In the low temperatures of space, frozen layers of water, ammonia and methane are subject to chemical and physical

alteration by bombardment with energetic ions, electrons and photons. In the lithographic definition of submicron silicon integrated circuits, the optical elements of the lithographic system are damaged by light at high intensities. In glass fiber communication systems optical grating for wavelength selectivity can be formed by UV irradiation. This small subset of radiation effects in insulators is discussed as illustrative of the range of influence of this field in current science and technology.

Radiation effects in insulators include an enormous diversity of phenomena produced by bombardment of insulating materials by particles and quanta that span the range from UV photons to electrons and ions with MeVs (or even hundreds of MeV) of energy. This paper skips around in this huge field, touching on only a few of the materials and phenomena that are included in the 8th Radiation Effects in Insulators Conference. It starts with a topic of interest to the solar system, where the author began in this field, and ends with discussion of areas of substantial technological importance to silicon integrated circuits and to optical communications.

2. The enigma of water ice Water ice is an ubiquitous material in space, on the surfaces of planets and their moons, in comets and in interstellar dust grains. Space is also filled with radiation of various kinds, including charged particles of the solar wind, more energetic charged particles from solar and galactic cosmic rays and electrons and ions trapped in the magnetic field of planets. It was in this context that the first experiments on the sputtering of water ice by MeV ions were undertaken [l]. Fig. 1 shows an early result: the energy dependence of the sputtering of ice from thin films at low temperatures under bombardment by protons [z]. The peak in the sputtering yield at about 100 keV, where the electronic stopping power of protons is a maximum in

* Fax: + 1 908 582 2300.

ice, reveals a dependence very different than that anticipated on the basis of sputtering produced by momentum transferring collisions between the protons and the atoms of the water that initiate collision cascades in the solid. Electronic excitation is producing the sputtering. Sputtering of ice by MeV ions has been examined for a number of different particles and energies, with the result shown in Fig. 2 [3]. With interesting systematic variations associated with the difference in sputtering with He ions at the same electronic stopping power but above and below the electronic stopping power maximum, the data shows an approximately quadratic dependence of sputtering yield on electronic stopping power over several orders of magnitude in the sputtering yield. It had been expected that at the low stopping power end of the experiments the yield would begin to vary linearly with stopping power but no linear regime has been found. For solid films of nitrogen [4] and oxygen [5] such a transition in the dependence of sputtering yield on electronic stopping power dependence from quadratic to linear does occur with decreasing stopping power, but not for ice. Fig. 3 illustrates the problem of mechanism schematically [4]. In part (a) corresponding to some intermediate stopping power, there is a track of excitations, whose spheres of influence overlap. In this regime it is relatively straightforward to account for a quadratic dependence because of the overlap in the time dependent expansion of the energy deposited in single excitations. However, at low stopping power, as illustrated in (b), the individual spheres become so widely separated that they act independently. The sputtering in this regime would be expected to be linear in the stopping power because the probability of an excitation occurring within a layer sufficiently close to the surface to produce sputtering is linear in stopping power.

0168-583X/96/$15.00 Copyright 0 1996Published by Elsevier Science B.V. All rights reserved PII SO168-583X(96)00119-X

2

W.L. Brown/Nucl.

Instr. and Meth. in Phys. Res. B 116 (1996) 1-12

H+ENERGY (kev) yield (molecules lost/incident

Fig. 1. Sputtering

ion) for Hz0 at

77 K as a function of proton energy.

The type of process that seems to be needed is illustrated in (c). The events that lead to electronically stimulated sputtering are the result of two independent excitations -

(4

(4

03

Fig. 3. Pictorial representation of (b) spherical spread of the energy per molecule following a mini-cascade induced by nonradiative repulsive recombination events at the site of three original molecules; (a) the overlap of events as in (b) when the spacing between the ionization events is small; (c) a possible 2-molecule electronic state involving the coupling of two independently ionized or excited molecules.

1000 -

single excitations do not do the job. Postulating the requirement of two independent excitations makes the quadratic dependence intrinsic with this model. Whether

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111’

1 100

0 1000

H20/cm2)

Fig. 2. Sputtering yield for Ha0 ice at 77 K as a function of (dE/dx),, the electronic energy loss, of hydrogen, helium, oxygen, carbon and flourhre ions.

0

0 ~‘J’I’~‘~‘*‘~ 1

2

3

4

5

6

7

Dose (1018 photons/cm*)

Fig. 4. Fhrence dependence of the photodesorption yield at 50 K of ice that is mainly amorphous (grown at 50 K) and polycrystallme (grown at 140 K).

W.L. Brown/Nucl.

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Instr. and Meth. in Phys. Res. B 116 (1996) 1-12

t

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6

Photon Flux (lOi4 photons cmy2d)

Fig. 5. Desorptionflux from an ice film at temperature T = 100 K plotted against Lyman-a flux on the sample for an accumulated dose of - 5 X 10” photons cm-*. The linear behaviour shows the absence of thermal effects. this is right is still not clear and if it is, the details of the excitation states involved have not been identified. The enigma of water ice has continued with recent

3

experiments studying the sputtering of water ice with Lyman alpha (10.2 eV) photons [6,7]. Fig. 4 shows the dose dependence of this sputtering at 50 K. The film is 5000 A thick, much thicker than the 450 A absorption depth of Lyman alpha photdns in ice. The points correspond to the sputtering measured for differential doses of between 1 and 4 X lOI photons/cm2. To come to a steady state sputtering yield, a dose of greater than 3 X lo’* photons/cm2 is required. The solid and open points show that the result is not sensitive to the temperature at which the film was grown. This rise to a steady state sputtering yield over a large dose range indicates that it is necessary to create an active species (with one photon) in order for a second photon to be able to produce ejection of material from the film surface. This has a strikingly similar ring to the two-excitation state postulated for the ion bombardment quadratic dependence on stopping power discussed above. After saturation, the sputtering (desorption) is proportional to the added dose, as shown in Fig. 5 (in this case at 100 K). The development of a steady state is itself temperature dependent, as is illustrated by the curves of Fig. 6 for films grown and radiated at different temperatures [6]. Not only does the saturation occur sooner at higher temperatures, but the steady state sputtering yield increases with increasing temperature. The authors suggest that a precursor state (probably radicals) is needed, and the diffusivity of such radicals to reach and interact with a second excitation is higher at higher temperatures. Fig. 7 shows the temperature dependence explicitly and comp&es it with temperature dependences reported for electron and ion bombardment [6]. The similarities (and differences) are intriguing. It is evident that the case of water ice still

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Dose (10’s photons cmQ)

Fig. 6. Photodesorption yield of water ice as a function of Lyman+ photon dose, for different temperatures T. The growth and irradiation temperatures were the same. Photon fluxes during measurements were in the range (0.5-5) X lOI photons cm-* s-‘.

0

0

20

40

60

80

100

120

140

0

Temperature (K)

Fig. 7. Photodesorption yield of water ice as a function of temperature for Lyman-a photons. Also shown are desorption yields for fast protons and electrons (from Ref. [7]>.

W.L. Brown/Nucl.

4

Instr. and Meth. in Phys. Res. B 116 (1996) 1-12

remains an enigma as a special case of sputtering induced by electronic excitation.

n

-1

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3. Hydrocarbon

films

The radiation effects in insulating films formed by condensation of hydrocarbon molecules on a cold surface provide other examples important for their astrophysical implications because hydrocarbon molecules are present in the space environment, often incorporated with water or ammonia in planetary solids. The formation of new molecules from radiation of simple mixed condensed gas solids of different composition and with different radiation doses and even different ionization densities in particle tracks is a very rich field of study. (Workers in this area are always looking for something to crawl out of their vacuum chamber after a bombardment, justifying their optimism that they are on the trail of the origin of life.) However, this discussion will be limited to some of the simplest cases, of films formed from single molecular species of methane or benzene. (There was no report of a creature crawling out of the chambers in any of these experiments.) First, the case of methane bombarded with MeV light ions. Fig. 8 shows signals at several different masses as measured with a quadrupole mass spectrometer during 2 MeV He+ ion bombardment of doubly isotopically labeled (with deuterium and i3C) methane films 1500, 3000 and 4500 A thick at 27 K @I. Note first the signal from D,. It starts out small, rises to a rather sharp peak at a fluence (dose) of 0.5 X 1015 ions/cm2, falls to a plateau and then

27K

FLIJENCE

(2.0

MeV

He) ~10’~ Ct’fi2

Fig. 8. D,, CD, and C,D, emission from CD,, as a function of 2 MeV He+ ion fluence at 27 K for three different film thicknesses. The D, release is proportional to the molecular thickness of the initial film. The results for CD, and C,D, are the same for the three thicknesses.

I

2

3

FLUENCE (lO16 1.5 MeV H+kn+)

Fig. 9. The quadrupole mass spectrometer signal for D, molecules as a function of fluence of 1.5 MeV Hf ions incident on a 2.8X 10” molecules/cm’ film. Note the sharp onset of major emission at - 9 X lOI ions/cm*.

tails off at fluences about twice that of the peak. The shape of the curves for the three thicknesses of film is the same. The magnitude is almost exactly proportional to the initial film thickness (note the vertical log scale). This implies that the whole thickness of the film is responsible for the release of D, produced from the decomposition of methane. The fact that there is a threshold in fluence before the D, is released efficiently suggests that the film must undergo sufficient decomposition to develop a percolation path for D, through the film to the surface before it is released. In contrast to the D, release, the release of the parent molecule and of the ethylene product molecule as shown in the figure is independent of the film thickness. Their release is associated with the surface and deep layers in the film do not contribute. This is not at all surprising considering the temperature of the experiment and the limited diffusivity these large molecules would be expected to have. Fig. 9 returns to consideration of the D, release peak, in this case on a linear vertical scale and for 1.5 MeV H+ ions, rather than 2.0 MeV He” [9]. The fluence of the maximum is at a value 20 times higher than in Fig. 8. The stopping power of 1.5 MeV protons is about ten times lower than for 2 MeV He. If a particular energy deposition were required for the formation of a percolation path and D, release, then the threshold fluence for release with the protons should have been only 10 times larger than for the helium ions. This point has been examined for a variety of stopping powers with results displayed in Fig. 10 [91. The ordinate in this case is the energy deposited per molecule of i3CDq at the threshold of release. The abscissa is the stopping power of H and He ions used (marked also with their energy and species). If the process depended only on the amount of energy deposited, a constant ordinate would have been obtained. The fact that the value drops with higher stopping power ions indicates that the process of D, formation is more efficient when the ionization density

W.L. Brown /Nucl.

Instr. and Meth. in Phys. Res. B 116 (1996) l-12

5

r 60-c

1000

Fig, 10. The D, threshold as a function of the electronic stopping power, S,, of H+ and He+ ions of different energies. D, is the energy deposited per molecule at the fluence threshold &,: D, = Ql&

along an ion track is higher. The probability of decomposition to form new species is enhanced in competition with the process of reformation of the original molecule when two (or more?) excitations occur close together in space. Non-linear effects are extremely common in radiation effects in organic solids. These are technologically important in radiation exposure of resists in ion beam lithography. They are also very important in radiation damage to human tissue. Experiments of the type discussed above have one serious limitation: they do not provide direct information about the property of the film as bombardment proceeds since they only examine the species that are being ejected. Infrared absorption and Raman scattering enable identification of new molecular species in the film. In situ (and also ex situ) measurements have been made on a number of hydrocarbon films by Strazzulla and Baratta [lo], but only their case of benzene will be discussed here. Fig. 11 shows Raman spectra of benzene before and after irradiation to a deposited energy of 30 eV/C atom. The radiation has been done with 3 keV He+ which has very low film penetration. In order to form a thick enough film to provide adequate Raman signal, benzene was continuously deposited as it was bombarded, the rate of deposition being adjusted (for a given ion beam flux) to provide the desired dose to the film. This method does not allow rebombardment of a film to a higher energy per atom; a new film must be prepared. The Raman spectrum of Fig. 11 shows the very clear development of an amorphous carbon peak at about 1600 cm- ‘. When the film is heated to room

1500

2000

2500

Raman Shift

(cm-l)

3000

3500

Fig. 11. Raman spectra of frozen benzene: as deposited (77 K), after radiation with 3 keV He ions during film deposition (77 K) and of the organic residue left over after ion irradiation and warm up (300 Kl.

temperature any volatile species will leave the film, but there is an organic residue left behind which has the same Raman signature. The tendency of organic species to decompose, releasing hydrogen and becoming more and more carbon-rich, was also evident in the case of methane discussed above. In all such cases there is a brown or black deposit (depending on the initial film thickness and the bombardment dose) that remains at room temperature. Infra-red spectra for the system shown with Raman scattering in Fig. 11 are displayed in Fig. 12. In this case, the spectra are for three different films, deposited with

CsHs Deposited + 3keV He+ (77K)

24eV/CH-mol

Organic Residue (3OOK) After 16eV/Cl+mol

I I I, 4000

I I 81 I, 3000

1 I I I, 2000

I I I 1000

Wavenumbers (cm-‘)

Fig. 12. IR spectra of virgin benzene and of the organic residue left over after irradiation with 3 keV ions at three different doses are shown.

6

W.L. Brown/Nucl.

Ins@. and Meth. in Phys. Res. B 116 (1996) 1-12

Y

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Low Power RF Plasma

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‘;’

-Si-Si-Si-Si,

-

g$$ 0” d 0

,

(IR)

20

Cli,

dH3

Rigid Crosslinked Amorphous Methylsilicon Hydride Polymer

Fig. 14. Deposition of plasma polymerized methylsilane (PPMS) from methylsilane gas in a low density plasma.

,

’ t “I

Methytsilane Gas

I ‘: I

I:

Room Temp.

m* ’ n s 40

Ion Dose (&.X-atom)

Fig. 13. Ratios of peak absorbance for different IR bands and the H/C ratio from elastic recoil detection analysis (ERDA) as a function of the radiation dose.

energy per CH molecular group. In these spectra the authors identify three different IR bands that develop between 2800 and 3200 cm-’ and change both their relative and their absolute strengths with increasing bombardment dose. These features can be associated with different types of C-H bonding. In the lowest spectrum in the figure, the organic residue at room temperature is shown. It has fewer features than the corresponding spectrum taken at 77 K. As anticipated from the bonding deduced from the 77 K features, much of the material is still volatile after an energy deposition of 16 eV/molecule. Strazzulla and Baratta [ll] have further characterized the residues of their films with forward recoil scattering to obtain the hydrogen to carbon ratio. Fig. 13 is a summary of the IR and FRS results as benzene films progressively lose hydrogen and the character of the C-H bonding changes. The progression is broken into three regions in the figure: a damaged molecular solid, a polymer like material and finally IPHAC (ion produced hydrogenated amorphous carbon). The details of the chemistry taking place during the modification of hydrocarbon films by ion bombardment are complex. However, the changes in chemical structure are being ever more completely identified and the common trends such as those of Fig. 13 more confidently established. different

4. Lithography

attention because without them patterning the fine geometric structures demanded by VLSI would be impossible. This section deals with radiation effects in a new kind of resist that holds promise as a material useful for patterning structures of 0.25 Frn or less using 248 and 193 nm photons. The new material is plasma polymerized methylsilane [ 111. It is formed in a low power plasma from methylsilane gas and has the structure shown in Fig. 14. It is an amorphous organosilicon hydride network. As deposited it has a very high absorption coefficient in the UV (at either 248 or 193 nm), but in the presence of oxygen it photobleaches [12]. Fig. 15 illustrates the spectral absorption characteristics of a very thin layer before and after increasing fluences of 193 nm radiation. The dramatic drop in absorption with exposure makes it possible to expose the full thickness of a resist layer 0.25 pm thick or even thicker, a result very difficult to achieve with hydrocarbon based resists. The changes in the i&a-red absorption spectrum during UV exposure in air are shown in Fig. 16 [12], together with the indication of the chemical change that is taking place. UV exposure breaks Si-Si bonds and allows insertion of 0 which replaces them with SiO bonds. The radiation effect of the 248 or 193 nm photons is concentrated in this particular bond-breaking process. As the figure shows, with increasing exposure, the Si-0-Si feature in the IR spectrum increases, as does also an Si-O-H

in the deep UV

Radiation effects in polymer or polymerizable materials have been important for many reasons, among them the ability to change the mechanical properties of polymers by cross linking, for example in the plastic sheath on electrical cables after the sheath has been applied by extrusion. In the silicon integrated circuit industry, radiation effects in lithographic resist materials have received enormous

300 Wavelength (nm)

Fig. 15. UV absorption spectrum of PPMS as prepared and as bleached by increasing exposure dose of 193 urn radiation in air.

WL. Brown/Nucl.

Instr. andMeth.

7

in Phys. Res. B 116 (1996) 1-12

CHsSiH, RF-plasma Deposition 743

743

-Si-Si-

PPMS (n z 1.7)

f

I:

7

-Si-Si-&I3

dH,

I

UV Exposure In Air

CH,

CH,

0-Si-0-Si 1, H -

PPMS (n E 1.5) 2000

0 c,

3000

2000

1000

Wavenumbers

H

O-+-O-+

Cii,

dH3

Fig. 16. IR absorption spectrum of PPMS as prepared and as exposed for different times to 193 nm radiation in air.

feature, and the Si-H feature decreases. At this point in the process the film still contains all of its original C and most of its original H. Nevertheless the Si atoms are sufficiently thoroughly bonded to 0 so that the material does not etch in a chlorine plasma. As a result, an exposure pattern is left as a negative if the PPMS film is developed in a chlorine plasma after UV exposure in air [ill. Although the primary process in either 248 or 193 nm UV exposure is the same - breaking Si-Si bonds - the result is not quite the same for the two wavelengths. Breaking an isolated Si-Si bond requires a photon with a

wavelength of 215 nm or less; breaking a bond in an Si-Si-Si structure requires 242 nm, but breaking a bond in an Si-Si-Si-Si structure requires only 265 nm. Thus, in a 193 nm exposure it is possible to insert 0 into all

-O_~i_O_~i__O_H

_&lAi-

A ;ouvh_O_$i-O_&-

ti&!_

H

Cli,AH3

Cl-1,

dH,

PPMSO 47% Si by Weight

PPMS 64% Si by Weight

Downstream Oxygen Plasma Strip or UV

I

Ozone Process

High Temp. A

b

_O-$i-O-$i-O-

p Calsolidation

A

B

C

1

D

Fig. 17. The complete sequence of processing steps of PPMS from DUV exposure in air, to replacement of remaining CH, groups

Fig. 18. The chemical (a) and physical (b)+(c) properties of PPMS: A (as deposited in low density plasma); B (after inserting 0 by deep UV exposure in air); C (after treatment in oxygen plasma to add more 0, strip the carbon and produce fully oxidized

with 0 in an oxygen

Si); D (after 4WC

SiO,

Low Density Silica

plasma, to densification

by annealing

iu 0,.

anneal to densify the oxide).

8

W.L. Brown/Nucl.

Insrr. and Meth. in Pkys. Res. B 116 (1996) 1-12

Si-Si bonds, but at 248 nm these dimers, and also trimers survive. This does not seem to be a critical difference for use in lithography. The selectivity in the chemical processes that is available by using UV photons of different energy cannot be anticipated if the material were exposed to energetic ions or electrons. These particles introduce a wide variety of excitations without the sharp cut-off that photons provide. The processing of the film exposed to UV in air can be carried further. Fig. 17 shows two additional steps [1 11. The exposed film is immersed in an oxygen plasma which removes the C and some of the H and converts the film to relatively low density silica. This film can finally be consolidated at high temperature to form a high density silica. This SiO, may be of high enough quality to be used directly in IC devices, but it certainly is a very high quality resist for defining the patterns required by IC processing. It is interesting to follow the physical and chemical characteristics of PMMS films from deposition to final densification. Fig. 18a traces the chemical composition and Figs. 18b and 18~ the physical properties of films of three different initial thickness [ 111. The huge uptake of 0 in the photo-oxidation step is evident in all parts of the figure. The extraction of C and the addition of 0 to bring the composition to nearly SiO, occurs in the oxygen plasma “strip” with a reduction in index of refraction to nearly the value of thermally grown SiO,. At the completion of processing, the film has decreased in thickness by only 15% from its initial value as PPMS. Radiation effects in resists are the primary processes being sought in deep UV lithography. They are not, however, the only place that radiation effects arise in the optical lithography process. The optical system required for producing an image with well defined pattern features of 0.25 pm or less is extremely demanding. The limitations of diffraction require that the pattern be produced with deep UV photons. The desire for high throughput in lithographic exposure in step-and-repeat operations with a rectangular exposure field size of 10 to 20 mm on a side repeated over a 150 or 200 mm diameter wafer drives the desire for high light intensity. The optical material of choice for fabrication of lens elements is silica because of

=6 .ZZ

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s

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m4z d z=:: a 0

_ 200 193

250

300

350

Wavelength(nm)

Fig. 19. Absorption spectrum of high OH fused silica induced by pulsed irradiation at 193 mn.

0

0

1

2

3

4

5

6 7~10~

Number of Pulses

Fig. 20. Effect of irradiation history on the 215 nm absorption coefficient and the 633 nm stress birefringence. Three regimes are noted: (a) a linear increase.at high (50 mJ/cm’/pulse) fluence up to - lo6 pulses; (b) a sublinear increase at the same fluence for the next - 1.5 X lo6 pulses, and (c) partial bleaching but no change in birefringence upon reducing the fluence to 12 mJ/cm*/pulse.

its acceptable transparency at wavelengths as short as 193 nm. Upfortunately, the combination of short wavelength and high light intensity results in radiation damage to the lens elements [13] and any degradation in the optical properties of the material is serious for an optical system that is so near its ultimate limit. Two types of UV induced radiation effects in amorphous, silica have been identified [13]. The first is the introduction of absorption due to defect color center formation. Fig. 19 shows the W absorption spectrum in commercial fused silica following exposure to repeated pulses of 193 nm radiation from an ArF excimer laser. Two absorption bands are observed. The stronger of these, at 215 nm, has been connected with the E’ defect, a positively charged oxygen vacancy. The second effect of UV radiation is observed as the development of birefringence in the silica. This is due to mechanical stress resulting from compaction of the silica. Compaction produces stress and hence birefringence when there are spatial gradients in the compaction arising from variations in optical intensity over an optical element. These two optical effects are correlated, but are not simply two manifestations of the same radiation induced process. This is evident from Fig. 20 in which absorption (at 215 nm) and biiefringence (observed at 633 nm) are shown as a function of the number of 193 nm pulses for 50 mJ/cm* and subsequently 12 mJ/cm2 [13]. The two effects rise very nearly together and even tend toward saturation together. However, when the laser energy density is changed, the absorption decreases (bleaches) toward a new steady state value, but the birefiingence does not. While the absorption has been associated with the E’ color center, the nature of the matrix relaxation that leads to

W.L. Brown/Nucl. Instr. and Meth. in Phys. Res. B 116 (1996) 1-12 I

a, 0.88 5. -1 5 0.86 -. C 9 0.84 _. e i5 0.62 .

I

9 ,

I

f 5 MinutePause Between Pulses

0.84 I 0

’

’ 1OD

’ 200

Numbers of

’

’ 300

’ ’ 400 k

193 nm Pulses

Fig. 21. Transmission through 1 cm thick Suprasil 2 versus 77

mJ/cm*, 193 nm pulse count compaction is still uncertain. Photons of 193 nm (6.4 eV) cannot ionize amorphous silica which has a ba_nd gap of more than 8 eV. Two photons can. A quadratic dependence of E’ center growth on photon fluence has been observed supporting the conclusions that a two-photon absorption process is responsible for production of these defects. Interestingly, the development of birefringence is nearly quadratic with laser fluence. However, birefringence is quadratically dependent on strain, so it seems that the compaction process leading to birefringence varies linearly (not quadratically) with laser fluence [ 141. The development of absorption in amorphous silica varies widely with the particular grade of silica examined. Pigs. 21 and 22 illustrate two examples. The ahsorption in these cases was measured as the transmission of incident 193 pulses [14]_ At 193 nm the absorption coefficient of the 215 nm E’ center is about 60% of its maximum value [13]. Note that the pulse count scales of Figs. 21 and 22 are different by more than factor of 10 and that the transmission scales cover a span that is different by a factor of 2. In the case of Suprasil 2, there is an approximately linear decrease in transmission with pulse count, but the transmission through SV2Gl silica drops very quickly and then actually begins to increase again after only a few thousand pulses. A pause after about 20 K

0

10

20

30k

Numbers of 193 nm PUlSeS

Fig. 22. Transmission through 1 cm thick SV2Gl silica versus 80 mJ/cm’, 193 nm pulse count.

pulses shows a remarkably fast recovery in the transmission, which quickly returns to continue the slow upward trend after only about a thousand more pulses. Differences in formation rate and recovery of E’ centers are associated with differences in OH content in the silica, but other factors are also involved. Understanding these differences is a serious challenge to materials scientists.

5. Optical fiber communications

Radiation induced changes in the optical properties of glass (silica) are highly undesirable when near perfection of the grass is essential in lenses for diffraction limited projectivn lithvgraphy as discussed in the previous section. In contrast, in optical communications, radiation induced changes in the optical properties of fibers are technologically valuable. Optically induced changes in index of refraction enable fabrication of unique optical components. Fig. 23 illustrates a typical case [15]. A UV laser beam is split and the two halves are recombined in an interference pattern that exposes an optical fiber from the side. The cylindrical lenses increase the light intensity in this pattern by focusing it to a line along the fiber. The UV fluence levels may be as large as 1 J/cm2. The high intensity

uv Laser Beam

Wavelength, pm

pig. 23, Enterferurneter used

to

fabricate t&r Bragg gratings by exposure from the side and the transmission spectrum of a fiber &awing the

development of a bansmission notch filter as a resuh of increasing numbers of 244 nm laser pulses.

W.L. Brown/Nucl.

10

Instr. and Meth. in Phys. Res. B 116 (1996) l-12

regions of the interference pattern produce changes in the index of refraction of the fiber core. With this scheme the period of the interference grating can be selected by choice of the convergence angle theta of the interfering beams: A = A/(2 sin 0). The transmission characteristics of the fiber are altered by different numbers of 244 nm UV laser pulses as shown in the figure [15]. With the value of theta chosen, the periodic index changes produced a phase matched distributed reflector that acts as a notch transmission filter centered at about 1.3 pm. The depth and breadth of the notch increases as the index changes in the grating increase with larger numbers of pulses. For exposures of the type shown, the fiber must be very stably positioned with respect to the interference pattern over times that may be as long as several minutes. A single 20 ns excimer laser pulse is sufficient in some cases to produce a grating. This completely removes the requirement of mechanical stability and allows grating formation even directly on a fiber draw tower [16]. The index of refraction changes produced by optical radiation effects are typically small, 10m5 to 10w4. They have been produced in fibers doped with germanium, europium or cerium but the germanium doping is particularly important because fiber cores of this material are extensively used in communication applications. Gratings have been written with photons of many different wavelengths, from the visible (where the process is two-photon) to the UV. A primary role of the dopant seems to be in

Incident

Silica Glass Phase Grating (Zero Order , Suppressed)

Optical Fiber

Diffracted Beams

+ 1st Order

- 1st Order /

i -

Zero Order (4% of throughput) Fiber Core Fringe Pattern Pitch = l/2 Grating Pitch

Fig. 24. An alternative method of writing Bragg gratings in the core of a photosensitive fiber using a phase mask with suppressed zero order.

-a -5

-10

$j a

5 t $

-15

.S 0 Q B s2

-20,-

-261

-

, 800

I

I

1000

1

I

1200

14Ol

Wavelength (nm)

Fig. 25. Wavelength dependence of the photoerasure (defined as the percentage of UV-induced shift erased by light). Note, no photoerasure was measured at 1320 nm after more than 1 h of exposure; the bar plotted represents maximum measurement uncertainty.

providing absorption bands which the writing laser can access and into which it can deposit energy. In germanium doped silica fibers a strong absorption band at 240 nm offers the most attractive writing regime. The change in index of refraction that is important is, of course, not at the writing wavelength, but at the wavelength where the grating is to be used, typically in the IR. The index change is an increase, as would be expected from a compaction process. Another method for externally writing gratings is shown in Fig. 24 [17]. Rather than using interference, as in Fig. 23, it depends on diffraction from a phase grating that has been fabricated with the desired periodicity in a silica glass plate. This phase grating is designed so that the intensity of the zeroth order diffraction is made small. The desired grating is written into the optical fiber by the first order diffraction beams as shown, with the phase grating in contact or near contact with the fiber. This approach reduces coherence requirements for the writing laser and also allows more complex tailoring of the written grating by design of the phase grating which has been lithographically produced. Gratings can be erased, presumably as a result of localized relaxation of the compacted matrix by more gentle optical absorption than is used in writing. The

W.L. Brown/N&.

Instr. and Meth. in Phys. Res. B I16 (1996) 1-12

output kg

k Input

11

1.55 Wavelength, pm

Input ---

/...,

R = 99.5%

output

‘.A. +

R = 99.5%

1.61 MHz

J

-10

-5

0

5

10

15

Detuning From Center Frequency, MHz

Fig. 26. Fiber optic bandpass filters using intra-core Bragg gratings. Upper filter is a Michelson type. and lower filter is a Fabry-Perot type.

extent of the erasure is a strong function of the wavelength of the erasing light [ 1 I]. Using lasers of different wavelength and adjusting the power in each case to 10 mW in an exposure of 5 min, the result in Fig. 25 was obtained [ 1 I]. No measurable erasure was observed at 1320 nm, even after exposure for 1 h. Fig. 26 illustrates two of the types of devices that can be made by combining Bragg gratings [15]. A grating by itself is a notch filter, as shown in Fig. 23. However, but combining two of them in a fiber coupler, the result is a bandpass filter. Part (a) of the figure shows the broadband spectrum of the input light (from an LED) and the output light that appears in a band just about 1.55 pm. Part (b) of the figure shows the result of two Bragg gratings spaced 10 cm apart in a single fiber. They act as a Fabry-Perot etalon and provide a very narrow pass band, only 1.6 MHz in width.

very low energy charged particles and low intensity, low energy photons. Radiation effects in insulators are often considered to be deleterious, but it is clear that they also provide material changes that are unique and advantageous. With the richness of the phenomena that characterize the mechanisms at work and the diversity of materials in which they are important, this area of research seems destined for a long and fruitful future.

References

[ll W.L. Brown, L.J. Lanzerotti, J.M. Poate and W.M. Augustyniak, Phys. Rev. L&t. 40 (1978) 1027.

El W.L. Brown, W.M. Augustyniak, E. Brody, B. Cooper, L.J. Lanzerotti, A. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321.

6. Perspective Whether the insulator is a mixed ice layer condensed over eons on the surface of an interstellar silicate grain from atoms and molecules in space or a carefully tailored metallic oxide crystal epitaxially prepared in hours in UHV or a radially graded optical fiber drawn from a hot preform at meters per second, radiation effects in these materials are pervasive. The big band gap of the insulator provides an increment of energy large enough to displace atoms in the bulk or to eject them from the surface. Excitation across the band gap can be produced by all but

[31 W.L. Brown, L.J. Lanzerotti

and R.E. Johnson, Science 218 (1982) 525. [41 W.L. Brown, L.J. Lanzerotti, K.J. Marcantonio, R.E. Johnson and C.T. Reimaan, Nucl. Instr. and Meth. B 14 (1986) 392. El K.M. Gibbs, W.L. Brown and R.E. Johnson, Phys. Rev. B 38 (1988) 11001. [61 MS. Westley, R.A. Baragiola, R.E. Johnson and G.A. Baratta, Nature 373 (1995) 405. [71 M.S. Westley, R.A. Baragiola, R.E. Johnson and G.A. Baratta, Planet, Space Sci. 409 (1995) 7946. [81 W.L. Brown, Nucl. Instr. Meth. B 32 (1988) 1. [91 W.L. Brown, G. Foci, L.J. Lanzerotti, J.E. Bower and R.E. Johnson, Nucl. Instr. and Meth. B 19/20 (1987) 899.

12

W.L. Brown/Nucl.

Ins@. and Merh. in Phys. Rex B 116 (1996) l-12

[IO] G. Strazzulla and G.A. Baratta, Astron. Astrophys. 266 (1992) 434. [l l] T.W. Weidman, 0. Jouber, A.M. Joshi, J.T-C Lee, D. Bouliu, E.A. Chaudross, R. Cirelli, F.P. Klemens, H.L. Maynard and V.M. Donnelly, J. Photopolym. Sci. Techuol. 8 (1995) 4. [12] 0. Jouber, T.W. Weidmau, R. Cirelli, S. Stein, J.T.C. Lee and S. Vaidya, J. Vat. Sci. Techuol. B 12 (1994) 3909. 1133M. Rothschild, D.J. Ehrlich and D.C. Shaver, Appl. Phys. Lett. 55 (1989) 13.

1141 R. Schenker, L. Eichner, H. Vaidya, S. Vaidya, P. Schermerhorn, D. Fladd and W.G. Oldham, SPIE Proc. 2428 (1995). [ISI W.W. Morey, G.A. Ball and G. Mel& Opt. Photon. News (1994) 8. [16] C.G. Askins et al., Opt. Lett. 17 (1992) 833. [17] K.O. Hill, B. Malo, F. Bilodeau and D.C. Johnson, Ann. Rev. Mater. Sci. 23 (1993) 125.