- Email: [email protected]
Glass for high performance optics and laser technology
Journal of Non-Crystalline Solids 129 (1991) 19-30 North-Holland
19
Glass for high performance optics and laser technology Doris Ehrt and Wolfgang Seeber Otto Schott Institute, Department of Chemistry, Friedrich Schiller University, .lena, Germany
Fluoride phosphate glasses having special optical properties are attractive candidates for applications in high performance optics and laser technology.Their potentially good UV-transmittance, luminescenceand solarizationproperties depend on the melting conditions and trace impurities from raw materials and crucible materials. Glasses melted under reducing conditions using normal quality batch materials yield glasses with higher UV-transmittance near 250 nm but with stronger luminescence and solarizationeffects than glasses melted under normal conditions. Fluoride phosphate glass with low impurity content can be UV-transmitting optical materials that are complementaryto silica glass and fluoride crystals.
I. Introduction
The so-called fluorophosphate optical glasses have been known since the 1960s and have been commercially melted at Schott [1]. They possess anomalous partial dispersions, making them desirable for optical lens designs that reduce the secondary spectrum. In the past decade, a large variety of fluoride phosphate glasses have been investigated. Glasses based on fluorides generally have low refractive indices and low dispersions together with low non-linear refractive indices and a negative temperature dependence of the refractive index. Another favorable property is their potentially good transparency from the ultraviolet to the infrared region of the optical spectrum. Their special optical properties make them attractive candidates for applications in high performance optics and laser technology [2]. Detailed structure-property relations of fluoride phosphate glasses were investigated in the 1980s at Jena [3-8]. The structure model can be described as AI(F, O)6-octahedral chains bonding by mono- and diphosphate groups and cations. The strong ionic bonding character of fluoroaluminates and fluorides results in a low melt viscosity, which is comparable with the viscosity of water at room temperature, and a great tendency to devitrify. Phosphates possess a more covalent bonding character. The melt viscosity and
glass-forming ability of fluoroaluminate glasses increase with increasing phosphate content. The structure and properties of these glasses depend mainly on the relation between fluorides and phosphates. Recently, the demand for UV-transmitting materials has increased for lens systems in microlithography equipment, for windows, lens blanks and substrate materials for excimer laser system optics, for fiber optics, for space applications, and for solid-state laser applications. Fluoride single crystals and vitreous silica are well known traditional materials for ultraviolet optics. Crystal sizes are limited and glass is better for fabricating high-accuracy optical surfaces. Moreover, for lens systems, a variety of glasses with different refractive indices and dispersions is required. Fluoride phosphate glasses with a low content of phosphate are attractive candidates as UV-transmitting materials. Their values for the UV resonance wavelengths calculated using a two-term Sellmeier dispersion formula are comparable with those of silica, CaF2, LiF and BeF2 glass [4,8,9]; however, the UV transmission of glass is frequently limited by trace impurities introduced by raw materials and the possible contamination from the melting technique and method of processing used. Other importance properties of optical materials are the resistance to radiation damage and luminescence characteristics. Moreover, the output
0022-3093/91/$03.50 © 1991 - ElsevierSciencePublishers B.V. (North-Holland)
20
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
power and efficiency that can be obtained from a solid-state laser system depend fundamentally upon the loss at the lasing wavelength within the active medium [10,11]. All of these properties are influenced by glass structure and impurity effects. In this paper we describe the properties of a variety of fluoride phosphate glasses and their dependence on trace impurities.
2. Experimental procedure Different fluoride phosphate glass compositions containing between 3 and 20 mol% M(PO3) 2 were melted in 50 to 1000 g batches in a special platinum crucible or in a carbon crucible using a glove box. The raw materials used were metaphosphates (Sr(PO3)2, Ba(PO3)2, La(PO3)3) and fluorides (A1F3, MgF 2, CaF 2, N H 4 F . HF) of normal optical quality. In some cases raw materials of special high purity (ultrapure) were used (Fe: 1-5 ppm, transition metals < 0.5 ppm). Trace amounts of iron in the raw materials and glasses were determined by means of the photometric phenanthroline method after sample decomposition by heating with sulfuric acid (fluorides, glasses), by fusion with potassium disulfate (aluminium fluoride), and with hydroxide (polyphosphates) [12]. Optical emission spectral analysis of the raw materials and glasses showed that other trace elements ( P b < 0 . 5 ppm, C u < 0 . 5 ppm, C 0 < 0 . 2 ppm, Ni < 0.2 ppm, Mn < 0.5 ppm) were negligible with regard to their influence on glass transmittance. The large melts were stirred during refining using temperatures between 1050 and 650 o C. The resulting glass castings were annealed at temperatures between 430 and 500 ° C and subsequently cooled at a rate of 10 o C / h . The castings were selectively cut, ground, and polished into different sample shapes for various measurements. The optical absorption spectra were obtained using SPECORDs, U V - V I S and N I R and special equipment for the VUV region. The luminescence emission spectra were measured using excitation wavelengths of 360 nm and 270 nm (sample thick-
ness: 11 mm) and detected by a photomultiplier in the region of 360 to 850 nm. Refractive index values were obtained on a Pulfrich refractometer. The density of the glass was determined using Archimedes' principle and measuring the buoyancy of a polished glass sample in CC14. Solarization tests were performed on polished glass samples (20 mm x 20 nun) having a thickness of 2 mm. For radiation a Kr F excimer laser was used ( k = 2 4 8 nm). Transmission spectra measurements were taken after various doses. The energy densities per pulse were in the range of 50-200 m J / c m 2. The accumulated incident energy densities were kept between 80 and 400 J / c m 2, i.e., the number of superimposed pulse varied between 1600 and 8000. In some cases X-rays (Cu K~, k = 1.541 .~, 30 kV, 3 mA, distance = 45 cm) were used for irradiation. To determine the thermal stability of the color centers formed during irradiation, thermal tests were carried out. After irradiation, glass samples were heat treated at temperatures of 350°C, cooled at a rate of 20 o C / h , and the transmission was measured.
3. Results and discussion 3.1. Transmission in the UV and VUV-spectral region Fluoride phosphate glasses are potentially good UV-transmitting materials. The intrinsic absorption in the UV region is due to electron transitions. The calculated and the measured absorption bands in the ultraviolet for fluoride phosphate glasses occur at energies higher than those due to bridging oxygen in oxide glasses [4,5,8,13,14]. The resonance wavelength of the UV transition calculated by a dispersion formula increases with the increasing phosphate content of the glass [5,8]. The measured UV transmittance of glass is strongly affected by extrinsic absorption due to trace impurities, mainly transition metals. Charge transfer bands having a molar extinction coefficient c > 103 are preferred over electron transition bands with e = 10 -2 to 102. The U V - V I S transmitting characteristics of a fluoride phosphate laser
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology 250
100, -.
.
.
.
21
350 Xlnm
300
$20 s
s
;--; ;;
T/%
S"
/
% /I ///¢/ //
,/
d
=0.2cm
//
50
J
iiiii /
-
-
$20 F e 2 O p p m /
.....
S10 Fe 30ppm
.....
58 •
/
Fe3Oppm ~ -
i
/ /
/
. . . . .
J
-
i
/
/ / /
/
//i//
j
:;:2:1- ................. 50
£6
42
3~
34
3'o
~/iO00cm-1
Fig. 1. UV-transmission spectra of three different fluoride phosphate glasses melted in platinum crucibles with raw materials of normal optical quality and ultrapure quality (glass with index s).
glass LG-810 were investigated b y C o o k and M a d e r [15] as a means of diagnosing transition metal c o n t a m i n a t i o n and in previous work on glass OK1 b y U r u s o v s k a y a et al. [16]. Because iron is one of the undesirable transition metal impurities that is very easily introduced into glasses through raw materials and fabrication processing, we studied the effect of iron in some detail.
Figure 1 shows a c o m p a r i s o n of the UV-transmission spectra of three different fluoride phosphate glasses (table 1 lists average values of selected properties) melted in p l a t i n u m crucibles with two different purities of the raw materials. In one case, raw materials of n o r m a l optical quality were used and the analyzed Fe content of these glasses was 2 0 - 3 0 ppm. In the other case, raw
Table 1 Selected properties of glasses Glass type
Composition (mol%)
Refractive index n 1.45159
Abbe const. ~e 90.6
Tg ( o C)
Density (g/cm3)
L5
6 phosphates 94 fluorides
440
3.47
$8
8 phosphates 92 fluorides
1.44482
92.0
440
3.40
$10
10 phosphates 90 fluorides
1.46604
86.8
445
3.44
$20
20 phosphates 80 fluorides
1.50361
80.1
450
3.54
22
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
E 200 [---
3oo
250
bm]
1,2I\ \ \ /
0.8
f. "-. ""
d= 0,1cm ~
~" " \ \.\
\ \ ...
o2
\
0 I
50000
,
L
l
40 000
I
I
I
I
~
I
.~ Ecru+] -
30000
Fig. 2. Optical density spectra of a fluoride phosphate glass, melted in a corundum crucible (K), melted in a platinum crucible (Pt) and melted in a corundum crucible doped with 100 ppm iron (Fe).
m a t e r i a l s of special high p u r i t y (glass with i n d e x s ) were used a n d the a n a l y z e d F e c o n t e n t of the glass was b e t w e e n 4 a n d 7 p p m . T h e results d e m o n s t r a t e t h a t large differences in U V t r a n s m i s s i o n d e p e n d m a i n l y o n the differences of the F e content which cause two b r o a d b a n d s c e n t e r e d a r o u n d 250 n m a n d 200 nm. T h e b a n d a r o u n d 250 n m is a t t r i b u t e d to F e 3÷ a n d the b a n d n e a r 200 n m to F e 2+. T h e s p e c t r u m of glass c o n t a i n i n g Pt i o n s is similar to that of F e - d o p e d glass. T w o b r o a d b a n d s n e a r 250 n m a n d 200 n m are also a t t r i b u t e d to Pt ions. F i g u r e 2 shows the o p t i c a l d e n s i t y s p e c t r a of a fluoride p h o s p h a t e glass m e l t e d in a c o r u n d u m crucible (K), in a p l a t i n u m c r u c i b l e (Pt) a n d in a c o r u n d u m crucible d o p e d with 100 p p m i r o n (Fe). It is k n o w n that fluoride p h o s p h a t e glasses m e l t e d in c a r b o n crucibles in a n inert a t m o s p h e r e h a v e m u c h higher U V t r a n s m i s s i o n that p l a t i n u m m e l t e d m a t e r i a l [5,8,12,13,15,17,18]. T h e p r e p a r a t i o n of f l u o r o p h o s p h a t e glasses in c a r b o n crucibles was r e p o r t e d b y Soviet scientists m a n y years ago [19,20]. T h e increase of the U V t r a n s m i s s i o n was a t t r i b u t e d to a r e d u c t i o n o f F e 3+ to F e 2+ d u e to c a r b o n c r u c i b l e - g l a s s i n t e r a c t i o n
100 -
220
240
260
Mnm
-
SIOs/C Fe6ppm T/%
"
Fe 6ppm
/
1
/
.- . !
/
/
/
i s./
i/
50-
,''
$10/PI
/ 6'8
t
/'
/
/
/
d =0,2cm
/ S10/RC Fe 27ppm
/
6%
6'0
36 100b/em-~
Fig. 3. UV-transmission spectra of the glass type $10 with high iron content (27 ppm Fe) melted in a platinum crucible (S10/Pt) and remelted in a carbon crucible (S10/RC) and with low iron content (Fe: 6 ppm) melted in a platinum crucible (S10s/Pt) and melted in a carbon crucible (S10s/C).
D. Ehrt, W. Seeber / Glass for
highperformance optics and laser technology
(see fig. 3). Figure 3 presents a comparison of the UV-transmission spectra of the glass composition $10 with high iron content (Fe: 27 ppm) melted in a platinum crucible (S10/Pt) and remelted in a carbon crucible (S10/RC), and with low iron content (Fe: 6 ppm) melted in a platinum crucible (S10s/Pt) and in a carbon crucible (S10s/C). The effect is large if the iron content is high, i.e. 27 ppm; the glass S 1 0 / R C remelted in a carbon crucible has a much higher-UV-transmission above 230 nm and a transmission below 230 nm lower
250
100
than the platinum melted glass S10/Pt. In the glass S10s containing only 6 ppm Fe, the crucible effect is smaller. Instead of using a carbon crucible as reducing agent, it is also possible to dope the raw materials with reducing agents such as N H 4 F - H F or bromides [21]. Figures 4(a) and 4(b) show the transmission spectra of a fluoride phosphate glass doped with 0.5% Fe203 melted in a platinum crucible, glass no. 6 under normal conditions (air atmosphere), and no. 4 with the addition of 6%
300
&O0
o, o,o
T/°/
:
// t/
5C
d=O,O/,cm
i
500
~lnm
.................
: .....
///
--4
/
//~ \
i/
23
/I
/
/,'~ d=O,lcm
c:O, lcm
T;_ so
a
4~
42
3e
34
30
1
2B
"
1,5
z'2
"
2i
lOO
la
~hoood,~ 1
X/,um 3i
6
T/%
#/
50"
%
o,so/o
/
/
3 b
12
1"0
a
B
";mo06c ~1
Fig. 4. (a, b) Transmission spectra of a fluoride phosphate glass highly doped with 0.5% Fe203 melted in a platinum crucible: curve 6, under normal conditions; curve 4, under reducing conditions, caused by adding 6% NH4F. HF as the reducing agent.
D. Ehrt, IV.. Seeber / Glass for high performance optics and laser technology
24
16.0
100
180
2,00
220
2zt,O , 260 ,
---. . . . .
TI%
, ;Wnm,
-,7 ...........
-;;::=-~-'
/
/
"
5O
/
l
i
,~"
/ ii
l ii I J
66
I
I I
i' i !
#
!i
/
/ ,/
/
I
/
I
\
/
o:o,O
.... ss~ Fe 6ppm
.... s8
Fe29ppm
- - - SIO/C Fe27ppm (d=O,O5cm)
,,',.," j / 62
5t,
58
50
46
42
38
~/1000cml
Fig. 5. Vacuum ultraviolet transmission spectra of two different types of fluoride phosphate glasses melted in platinum crucibles (S8s and $8) and in a carbon crucible (S10/C) in comparison with silica glass SQ1.
N H 4 F - H F as reducing agent (air atmosphere). Colorless glasses were obtained in both cases. Glass no. 6, however, has an UV-edge near 380 nm and only very weak bands in the I R range which can be attributed to Fe 2+. That means that most of the iron exists in the Fe 3÷ state. The UV-edge of glass no. 4 is shifted into the shorter wavelength region and shows a weak band centered 100 160
,
180
,
around 260 nm due to Fe 3+, a strong band near 220 nm, and the tailing of a strong band below 220 nm. In the I R region the typical (d --->d) due to Fe 2÷ centered around 1 and 2 ~m are observed. M6ssbauer studies have shown that glass no. 6 contains only 12% Fe 2÷ and 88% Fe 3+ locating in three different sites; in glass no. 4 more than 97% 200
220
240
Fe2+
Fe3+
r/O/o
..........
so
I,'"
i," il
' /
62
_._~y
."
,""
I/ " /,, ,,
26,0 , ;',/rim,
,." sTos
/ _~l~o~~ , ~ /
d:O,2cm
Fe?ppm /
........
/!
5B
St,
50
#+6
t+2
~2100()c t~l
Fig. 6. Vacuum ultraviolet transmission spectra of silica glass glass SQI, two fluoride phosphate glasses with 8 (S8s) and 20 (S20s) mol% phosphate melted in platinum crucibles and a pure phosphate glass (Sr(PO3)2) melted in a silica crucible, containing nearly the same impurity content (Fe: 6-7 ppm).
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
of Fe 3+ was reduced to Fe 2+ located in two different sites [8]. Vacuum ultraviolet transmission measurements (fig. 5) of the glass S 1 0 / C with 27 p p m Fe remelted in a carbon crucible show a sharp absorption band near 210 nm and an absorption band near 170 n m overlapping the V U V / c u t . We are tempted to deduce from our Mi3ssbauer studies that both absorption bands can be attributed to Fe 2+. Although the position and shape of this band near 210 n m is typical for a S ~ P transition and could thus be caused by p3+ [23], the intensity of the band depends on the Fe 2+ content (compared with luminescence spectra) and is stable under heat treatment up to 400 ° C. A fluoride phosphate glass with similar high iron content $8 (Fe: 29 ppm) melted traditionally in a platinum crucible shows a broad band due to Fe 3+ near 250 nm and a broad band due Fe 2÷ between 200 and 160 nm. The same glass composition with lower iron content S8s (Fe: 6 p p m ) melted in a platinum crucible shows only weak bands for Fe 2÷ and Fe 3÷. The VUV-cut near 160 nm is comparable with that of vitreous silica SQ1. Figure 6 compares the V U V transmission spectra of vitreous silica SQ1 two fluoride phosphate glasses with 8 (S8s) and 20 (S20s) mol% phosphate melted in platinum crucibles, and a pure phosphate glass (Sr(PO3)2) melted in a silica crucible containing nearly the same impurity content (Fe: 6 - 7 ppm). Two significant results are: (1) the UV-cutoff is shifted to longer wavelength with
25
increasing phosphate content, which provides the trend of calculations for the resonance wavelength using a Sellmeier dispersion formula [5,8]; and (2) the Fe2+/Fe3+-relation is shifted to Fe 2+ with increasing phosphate content.
3.2. Luminescence of fluoride phosphate glasses Luminescence spectra reveal that minority species are efficient emitters under special excitation [24]. An important requirement for UV-transmitting optical material is low background luminescence [25,26]. The luminescence spectra of these fluoride phosphate glasses excited at 360 nm show only a weak broad b a n d around 800 nm which is typical for Fe 3+. These glasses generally have low luminescence intensity in comparison with pure oxide glasses (i.e. B K 7 standard glass = 1). With an excitation wavelength of 270 nm a second luminescence band near 400 cm (fig. 7) was obtained which has a low intensity for most glasses melted in platinum crucibles under normal conditions. Evidence of platinum in glasses from luminescence spectra was not possible. The b a n d around 800 nm increases w i t h increasing iron content. The 800 n m band is completely absent when melted under reducing conditions in a carbon crucible (fig. 8) and the band centered at 400 n m increases with increasing iron content. The luminescence spectra of one glass composition L5 melted in a platinum crucible with increas-
J 0,1
a I $8 /29ppmFe b) S8s/ 6ppmFe
Pt-C 0
0,06
0,02
400
500
600
700
800 A/nm
Fig. 7. Luminescenceemission spectra ()~exc.= 270 nm, 30 o C) of glass type $8 (Fe: 29 ppm) and S8s (Fe: 6 ppm) traditionally melted in platinum crucibles.
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
26
Q
a) $10
c-c
22 ppm Fe
b) $10 s
6 pprnFe
\\\\\\ ~
I/"
400
500
600
8b0 Xlnm
700
Fig. 8. Luminescence emission spectra (Xexc.= 270 nm, 3 0 ° C ) of glass type S10 (Fe: 27 ppm and S10s (Fe: 6 ppm) melted under reducing conditions in carbon crucibles.
nm. A p3+ luminescence near 400 nm is a possibility, but no dependence on the iron content in the p p m range would be expected.
ing reduced conditions (adding 2 to 4 mol% bromides) are shown in fig. 9. The total iron content of the glass was constant at 30 p p m Fe. The luminescence intensity of the band at 400 n m increases with increasing reduction; however the weak 800 n m band decreases slightly. The total luminescence intensity of glass melted under reducing conditions is much greater than those of normal conditions. The strong luminescence band centered at 400 nm is assumed to be due to Fe+2+ , although the luminescence of Fe 2÷ in glasses is unknown. The position and shape of this band are typical of S ~ P-luminescence in oxide glasses [24,25]. However, in fluoride phosphate glasses, the Pb 2+ luminescence is located below 360 n m and the C e 3 + luminescence is centered around 310
2.0 ¸
3
c /-~ \
b.
3.3. Effects of radiation of the transmission of fluoride phosphate glasses Another important property of optical material is its resistance to radiation damage. Ionizing radiation produces free holes and electrons in glass which become trapped forming defect centers. These defect centers cause a decrease in optical transmission in the ultraviolet and visible range of the spectrum. The decreased optical transmission is commonly referred to as solarization [25,27]. Excimer lasers are considered to be applicable
0,1 a) L 5 / O B r b) L 5 / 2Br
\
Pt-C
Cl
"] //
~1 LSJ~B~
x\
, 1,0
// // // // /
400
500
600
\\
//
700
800 X/nm
700
b /
/
c
\
\
\
800 X/nm
Fig. 9. Luminescence emission spectra (Xexc.= 270 nm, 30 o C) of glass type L5 (Fe: 30 ppm) melted in a platinum crucible with an increasing reduction effect. Curves: (a) without adding bromide; (b) adding 2 mol% bromide; (c) adding 4 mol% bromide.
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology 100200
250
To/o
300
400
27
500 ^/nm
//'
.'~
C l / / / /"/"/ / I'" "
.
l
/./"
~
//
re:z/pp
C
50
t. 0
30
"~/1000cm-I
20
Fig. 10. Transmission spectra of glass type $10 with 27 p p m Fe (sample thickness 2 mm) traditionally melted in a platinum crucible before and after exposure to radiation from an Kr F excimer laser, from X-ray Cu and after reheating for recovery. Curves: (a) before irradiation; (b) after 80 J / c m 2 (400 pulses at 200 m J / c m 2) at 248 nm; (c) after 3 h X-ray (Cu K~); (d) after reheating at temperature of 350 o C.
for deep UV lithography because short-wavelength illumination can provide submicron resolution [22]. One of the interesting wavelengths for illumination is 248 nm from a KrF excimer laser. Solarization effects of glass type S10 depend on iron content, melting conditions, radiation doses,
200 100
'
'
250 . . . .
and heat treatment and are shown in figs. 10-13. By comparison, no solarization effect was observed on the tested vitreous silica sample SQ1. Figure 10 shows an example of the transmission spectra of glass type S10 with high iron content (27 ppm) traditionally melted in a platinum cruci-
400
300
500 Mnm
T/%
SIO/RC
50-
', /.lil
J
50
d:O,2cm
Fe:27ppm
40
30
'~/lO00cm -1
20
Fig. 11. Transmission spectra of glass type S10 with 27 ppm Fe remelted under reducing conditions in a carbon crucible (sample thickness 2 mm). Curves: (a) before irradiation; (b) after 80 J / c m 2 (400 pulses at 200 m J / c m 2) at 248 nm; (c) after 160 J / c m 2 (800 pulses at 200 m J / c m 2) at 248 nm; (d) after 3 h X-ray (Cu K , ) ; (e) after reheating at a temperature of 350 o C.
28
D. Ehrt, IV.. Seeber / Glass for high performance optics and laser technology
200 100
250
/ /c
300
././"
SlOs / Pt Fe:6ppm
./
50
/
d/
so
400
500 klnrr
d:O,2cm
.J
40
30
~/~Ooocm4
20
Fig. 12. Transmission spectra of glass type S10s with 6 ppm Fe traditionally melted in a platinum crucible (sample thickness 2 mm). Curves: (a) before irradiation; (b) after 160 J/cm 2 (800 pulses at 200 mJ/cm2) at 248 nm; (c) after 400 J/cm 2 (2000 pulses at 200 mJ/cm2) at 248 nm; (d) after 3 h X-ray (Cu K~); (e) after reheating at a temperature of 350 o C.
ble (a) before a n d (b) after exposure to a n accum u l a t e d dose of 80 J / c m 2 from a n excimer laser o p e r a t i n g at 248 n m . T h e energy density per pulse was high, a p p r o x i m a t e l y 200 m J / c m 2 per pulse with a pulse length of a b o u t 20 ns, a n d 400 pulses were accumulated. O n l y m i n o r solarization effects are observed. A f t e r X-ray r a d i a t i o n (curve c: 3 h C u K s ) the glass shows strong solarization effects
250
100200
a n d a b r o w n color. T h e color centers p r o d u c e d are stable at r o o m temperature. A nearly c o m p l e t e recovery from solarization was observed for wavelengths > 360 n m after heat t r e a t m e n t at 3 5 0 ° C (curve d). Glass type S10 with a high iron c o n t e n t of 27 p p m melted u n d e r r e d u c i n g c o n d i t i o n s i n a c a r b o n crucible (fig. 11) has a better U V - t r a n s m i t tance near 250 n m before i r r a d i a t i o n (curve a)
300
1,00
500 klnm
T/% J /
C
SIOslC Fe:6ppm
d=O,2cm
30
~'11000cr n-1
50 ¸
50
~0
20
Fig. 13. Transmission spectra of glass type Sl0s with 6 ppm Fe melted under reducing conditions in a carbon crucible (sample thickness 2 ram). Curves: (a) before irradiation; (b) after 80 J/cm 2 (400 pulses at 200 mJ/cm2) at 248 nm; (c) after 400 J/cm 2 (2000 pulses at 200 mJ/cm2) at 248 nm.
D. Ehrt, IV. Seeber / Glass for high performance optics and laser technology
since most of the iron exists in the Fe z+ state, but stronger solarization effects were induced by excimer laser radiation than the same glass melted in a platinum crucible. The color centers produced by 160 J / c m 2 of the excimer laser (curve c) are comparable with those produced by X-ray radiation (curve d). A very interesting effect was observed after reheating at a temperature of 3 5 0 ° C (curve e). The transmittance near 250 nm is lower than before irradiation. This is attributed to a partial re-oxidation of Fe 2+ to Fe 3÷. Examples of the solarization of glass type S10s with low iron content (6 ppm) traditionally melted in a platinum crucible and melted under reducing conditions in a carbon crucible are shown in figs. 12 and 13. Little susceptibility to solarization was observed at low irradiation density. The solarization effects are due to two-photon absorption [18]. By comparison, the radiation intensity present at a lens in a wafer stepper is much less. The energy density in stepper objectives is estimated to be less than 10 m J / c m 2, and a typical photoresist is exposed to 100 pulses with an energy density of approximately 1 m J / c m 2 per pulse, accumulated to a total dose of 100 m J / c m 2 [18]. The formation of color centers by excimer laser radiation is likely to be the result of photoionizable multivalent metal ions present as trace impurities, mainly iron, in the glass. Fluoride phosphate glasses with a low content of impurities could be a suitable candidate for lens systems in wafer steppers.
4. Conclusions
Fluoride phosphate glasses possess special optical properties making them attractive candidates for applications in high performance optics and laser technology. They are potentially good UVtransmitting materials since their electron transition is comparable to that of BeF2 glass and fluoride crystals. Their actual UV-transmittance is strongly affected by extrinsic absorption due to trace impurities, mainly iron and other transition metals. Glasses melted under reducing conditions (a carbon crucible in a glove box or a platinum
29
crucible and adding reducing agents) using normal quality batch materials yield glasses with higher UV-transmittance near 250 nm due to the reduction of Fe 3+ to Fe 2÷. For maximum improvements in UV-transmission, it is important to use ultrapure raw materials and carbon crucibles. Traditionally, platinum-crucible-melted fluoride phosphate glasses possess only small amounts of luminescence dominated by a weak Fe 3+ luminescence centered around 800 nm. A strong luminescence near 400 nm is observed due to reducing melting conditions. The formation of color centers by excimer laser and X-ray radiation depend on the trace impurities, mainly of the iron content in the glass. Batch materials normally used to produce optical quality glass when melted under reducing melting conditions yield glasses with higher UV-transmittance near 250 nm; however, they show a stronger susceptibility to solarization than glasses melted under normal conditions. Fluoride phosphate glasses with a low content of impurities melted in carbon crucibles can be UV-transmitting optical materials that are complementary to silica glass and fluoride crystals. The authors with to thank R. Atzrodt, M. Carl, T. Kittel, J. Kleinschmidt, T. Kloss, W. Mikkeleit and R. Ranch for their assistance in experiments and helpful discussions.
References [1] W. Jahn, Glastechn. Ber. 34 (1961) 107. [2] J.T. Wenzel et al., SPIE 204 (1979) 59. [3] D. Ehrt and W. Vogel, 13 Friihjahrsschule Optik, Beitrage zur Optik und Quantenelektronik 6 (1981) 98; Feinger]tetechnik 31 (1982) 147. [4] D. Ehrt, R. Atzrodt and W. Vogel, in: Proc. 2nd Int. Otto Schott Colloquium, July 1982, Jena, Wiss. Z. FriedrichSchiller Univ. Jena, Math.-Naturwiss. Reihe 32 (1983) 509. [5] D. Ehrt, thesis B, Friedrich-Schiller-Universitiit Jena (1984). [6] D. Ehrt, C. J~iger and W. Vogel, in: Proc. 3rd Int. Otto Schott Colloquium, July 1986, Jena, Wiss, Z. FriedrichSchiller Univ. Jena, Math.-Naturwiss. Reihe 36 (1987) 867. [7] M. Dubiel and D. Ehrt, Phys. Status Solidi (a)100 (1987) 415.
30
D. Ehrt, IV. Seeber / Glass for high performance optics and laser technology
[8] D. Ehrt, W. Mikkeleit, W. Burckhardt and H. Mehner, in: Proc. 2nd Int. Ernst-Abbe-Conference (EAC), FriedrichSchiller-Universit~it, Jena, 1989. [9] R.T. Williams, D.J. Nagel, P.H. Klein and H.J. Weber, J. Appl. Phys. 52 (1981) 6279. [10] D.L. Sapak, J.M. Ward and J.E. Marion, SPIE, 970 (1988) 107. [11] S.E. Stokowski and D. Krashkerich, Mater. Res. Soc. Symp. Proc. 61 (1986) 273. [12] W. Mikkeleit, L. Priplata and G. Heidemann, 'Iron trace determination in fluoride phosphate systems', poster 4, 4th Int. Otto Schott Colloquium, Jena, July 1990. [13] M.J. Liepmann, A.J. Marker III and J.M. Melvin, SPIE 1128 (1989) preprint. [14] T. Izumitani and S. Hirota, J. Non-Cryst. Solids 73 (1985) 255. [15] L.M. Cook and K.-H. Mader, J. Am. Ceram. Soc. 65 (1982) 597. [16] L.N. Urusovskaya, A.N. Svenigorodskaya and P.N. Sakirova, Z. Prikl. Spectros. 6 (1980) 1076.
[17] L.M. Cook, M.J. Liepmann and A.J. Marker III, Mater. Sci. Forum 19-20 (1987) 305. [18] M.J. Liepmann, A.J. Marker III and U. Sowada, SPIE 970 (1988) 170. [19] L.A. Golubcov et al., Z. Prikl. Khim. 1 (1971) 1980. [20] E. Garibin, V.D. Chalilev, K.S. Evdropjev and B.S. Doladugina, Soy. J. Opt. Techn. 5 (1977) 286. [21] W. Seeber and D. Ehrt, Patent WP CO3C 392-6, 28.12.87. [22] D.A. Markle, SPIE 774 (1987) 108. [23] S.G. Lunter and J.K. Fedorov, Fiz. Khim. Stekla 14 (1988) 72. [24] W.B. White and D.S. Knight, Mater. Res. Soc. Symp. Proc. 61 (1986) 273. [25] T. Kloss and C. Beeker, Beitr~ige Opt. Quantenelektr. 21 (1989) 131. [26] G. Schmidt, K. Motzke, J. Steinert and D. Ellguth, Beitr~ige Opt. Quantenelektr. 21 (1989) 223. [27] J.J. Setta, R.J. Scheller and A.J. Marker III, SPIE 970 (1988) 179.
19
Glass for high performance optics and laser technology Doris Ehrt and Wolfgang Seeber Otto Schott Institute, Department of Chemistry, Friedrich Schiller University, .lena, Germany
Fluoride phosphate glasses having special optical properties are attractive candidates for applications in high performance optics and laser technology.Their potentially good UV-transmittance, luminescenceand solarizationproperties depend on the melting conditions and trace impurities from raw materials and crucible materials. Glasses melted under reducing conditions using normal quality batch materials yield glasses with higher UV-transmittance near 250 nm but with stronger luminescence and solarizationeffects than glasses melted under normal conditions. Fluoride phosphate glass with low impurity content can be UV-transmitting optical materials that are complementaryto silica glass and fluoride crystals.
I. Introduction
The so-called fluorophosphate optical glasses have been known since the 1960s and have been commercially melted at Schott [1]. They possess anomalous partial dispersions, making them desirable for optical lens designs that reduce the secondary spectrum. In the past decade, a large variety of fluoride phosphate glasses have been investigated. Glasses based on fluorides generally have low refractive indices and low dispersions together with low non-linear refractive indices and a negative temperature dependence of the refractive index. Another favorable property is their potentially good transparency from the ultraviolet to the infrared region of the optical spectrum. Their special optical properties make them attractive candidates for applications in high performance optics and laser technology [2]. Detailed structure-property relations of fluoride phosphate glasses were investigated in the 1980s at Jena [3-8]. The structure model can be described as AI(F, O)6-octahedral chains bonding by mono- and diphosphate groups and cations. The strong ionic bonding character of fluoroaluminates and fluorides results in a low melt viscosity, which is comparable with the viscosity of water at room temperature, and a great tendency to devitrify. Phosphates possess a more covalent bonding character. The melt viscosity and
glass-forming ability of fluoroaluminate glasses increase with increasing phosphate content. The structure and properties of these glasses depend mainly on the relation between fluorides and phosphates. Recently, the demand for UV-transmitting materials has increased for lens systems in microlithography equipment, for windows, lens blanks and substrate materials for excimer laser system optics, for fiber optics, for space applications, and for solid-state laser applications. Fluoride single crystals and vitreous silica are well known traditional materials for ultraviolet optics. Crystal sizes are limited and glass is better for fabricating high-accuracy optical surfaces. Moreover, for lens systems, a variety of glasses with different refractive indices and dispersions is required. Fluoride phosphate glasses with a low content of phosphate are attractive candidates as UV-transmitting materials. Their values for the UV resonance wavelengths calculated using a two-term Sellmeier dispersion formula are comparable with those of silica, CaF2, LiF and BeF2 glass [4,8,9]; however, the UV transmission of glass is frequently limited by trace impurities introduced by raw materials and the possible contamination from the melting technique and method of processing used. Other importance properties of optical materials are the resistance to radiation damage and luminescence characteristics. Moreover, the output
0022-3093/91/$03.50 © 1991 - ElsevierSciencePublishers B.V. (North-Holland)
20
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
power and efficiency that can be obtained from a solid-state laser system depend fundamentally upon the loss at the lasing wavelength within the active medium [10,11]. All of these properties are influenced by glass structure and impurity effects. In this paper we describe the properties of a variety of fluoride phosphate glasses and their dependence on trace impurities.
2. Experimental procedure Different fluoride phosphate glass compositions containing between 3 and 20 mol% M(PO3) 2 were melted in 50 to 1000 g batches in a special platinum crucible or in a carbon crucible using a glove box. The raw materials used were metaphosphates (Sr(PO3)2, Ba(PO3)2, La(PO3)3) and fluorides (A1F3, MgF 2, CaF 2, N H 4 F . HF) of normal optical quality. In some cases raw materials of special high purity (ultrapure) were used (Fe: 1-5 ppm, transition metals < 0.5 ppm). Trace amounts of iron in the raw materials and glasses were determined by means of the photometric phenanthroline method after sample decomposition by heating with sulfuric acid (fluorides, glasses), by fusion with potassium disulfate (aluminium fluoride), and with hydroxide (polyphosphates) [12]. Optical emission spectral analysis of the raw materials and glasses showed that other trace elements ( P b < 0 . 5 ppm, C u < 0 . 5 ppm, C 0 < 0 . 2 ppm, Ni < 0.2 ppm, Mn < 0.5 ppm) were negligible with regard to their influence on glass transmittance. The large melts were stirred during refining using temperatures between 1050 and 650 o C. The resulting glass castings were annealed at temperatures between 430 and 500 ° C and subsequently cooled at a rate of 10 o C / h . The castings were selectively cut, ground, and polished into different sample shapes for various measurements. The optical absorption spectra were obtained using SPECORDs, U V - V I S and N I R and special equipment for the VUV region. The luminescence emission spectra were measured using excitation wavelengths of 360 nm and 270 nm (sample thick-
ness: 11 mm) and detected by a photomultiplier in the region of 360 to 850 nm. Refractive index values were obtained on a Pulfrich refractometer. The density of the glass was determined using Archimedes' principle and measuring the buoyancy of a polished glass sample in CC14. Solarization tests were performed on polished glass samples (20 mm x 20 nun) having a thickness of 2 mm. For radiation a Kr F excimer laser was used ( k = 2 4 8 nm). Transmission spectra measurements were taken after various doses. The energy densities per pulse were in the range of 50-200 m J / c m 2. The accumulated incident energy densities were kept between 80 and 400 J / c m 2, i.e., the number of superimposed pulse varied between 1600 and 8000. In some cases X-rays (Cu K~, k = 1.541 .~, 30 kV, 3 mA, distance = 45 cm) were used for irradiation. To determine the thermal stability of the color centers formed during irradiation, thermal tests were carried out. After irradiation, glass samples were heat treated at temperatures of 350°C, cooled at a rate of 20 o C / h , and the transmission was measured.
3. Results and discussion 3.1. Transmission in the UV and VUV-spectral region Fluoride phosphate glasses are potentially good UV-transmitting materials. The intrinsic absorption in the UV region is due to electron transitions. The calculated and the measured absorption bands in the ultraviolet for fluoride phosphate glasses occur at energies higher than those due to bridging oxygen in oxide glasses [4,5,8,13,14]. The resonance wavelength of the UV transition calculated by a dispersion formula increases with the increasing phosphate content of the glass [5,8]. The measured UV transmittance of glass is strongly affected by extrinsic absorption due to trace impurities, mainly transition metals. Charge transfer bands having a molar extinction coefficient c > 103 are preferred over electron transition bands with e = 10 -2 to 102. The U V - V I S transmitting characteristics of a fluoride phosphate laser
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology 250
100, -.
.
.
.
21
350 Xlnm
300
$20 s
s
;--; ;;
T/%
S"
/
% /I ///¢/ //
,/
d
=0.2cm
//
50
J
iiiii /
-
-
$20 F e 2 O p p m /
.....
S10 Fe 30ppm
.....
58 •
/
Fe3Oppm ~ -
i
/ /
/
. . . . .
J
-
i
/
/ / /
/
//i//
j
:;:2:1- ................. 50
£6
42
3~
34
3'o
~/iO00cm-1
Fig. 1. UV-transmission spectra of three different fluoride phosphate glasses melted in platinum crucibles with raw materials of normal optical quality and ultrapure quality (glass with index s).
glass LG-810 were investigated b y C o o k and M a d e r [15] as a means of diagnosing transition metal c o n t a m i n a t i o n and in previous work on glass OK1 b y U r u s o v s k a y a et al. [16]. Because iron is one of the undesirable transition metal impurities that is very easily introduced into glasses through raw materials and fabrication processing, we studied the effect of iron in some detail.
Figure 1 shows a c o m p a r i s o n of the UV-transmission spectra of three different fluoride phosphate glasses (table 1 lists average values of selected properties) melted in p l a t i n u m crucibles with two different purities of the raw materials. In one case, raw materials of n o r m a l optical quality were used and the analyzed Fe content of these glasses was 2 0 - 3 0 ppm. In the other case, raw
Table 1 Selected properties of glasses Glass type
Composition (mol%)
Refractive index n 1.45159
Abbe const. ~e 90.6
Tg ( o C)
Density (g/cm3)
L5
6 phosphates 94 fluorides
440
3.47
$8
8 phosphates 92 fluorides
1.44482
92.0
440
3.40
$10
10 phosphates 90 fluorides
1.46604
86.8
445
3.44
$20
20 phosphates 80 fluorides
1.50361
80.1
450
3.54
22
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
E 200 [---
3oo
250
bm]
1,2I\ \ \ /
0.8
f. "-. ""
d= 0,1cm ~
~" " \ \.\
\ \ ...
o2
\
0 I
50000
,
L
l
40 000
I
I
I
I
~
I
.~ Ecru+] -
30000
Fig. 2. Optical density spectra of a fluoride phosphate glass, melted in a corundum crucible (K), melted in a platinum crucible (Pt) and melted in a corundum crucible doped with 100 ppm iron (Fe).
m a t e r i a l s of special high p u r i t y (glass with i n d e x s ) were used a n d the a n a l y z e d F e c o n t e n t of the glass was b e t w e e n 4 a n d 7 p p m . T h e results d e m o n s t r a t e t h a t large differences in U V t r a n s m i s s i o n d e p e n d m a i n l y o n the differences of the F e content which cause two b r o a d b a n d s c e n t e r e d a r o u n d 250 n m a n d 200 nm. T h e b a n d a r o u n d 250 n m is a t t r i b u t e d to F e 3÷ a n d the b a n d n e a r 200 n m to F e 2+. T h e s p e c t r u m of glass c o n t a i n i n g Pt i o n s is similar to that of F e - d o p e d glass. T w o b r o a d b a n d s n e a r 250 n m a n d 200 n m are also a t t r i b u t e d to Pt ions. F i g u r e 2 shows the o p t i c a l d e n s i t y s p e c t r a of a fluoride p h o s p h a t e glass m e l t e d in a c o r u n d u m crucible (K), in a p l a t i n u m c r u c i b l e (Pt) a n d in a c o r u n d u m crucible d o p e d with 100 p p m i r o n (Fe). It is k n o w n that fluoride p h o s p h a t e glasses m e l t e d in c a r b o n crucibles in a n inert a t m o s p h e r e h a v e m u c h higher U V t r a n s m i s s i o n that p l a t i n u m m e l t e d m a t e r i a l [5,8,12,13,15,17,18]. T h e p r e p a r a t i o n of f l u o r o p h o s p h a t e glasses in c a r b o n crucibles was r e p o r t e d b y Soviet scientists m a n y years ago [19,20]. T h e increase of the U V t r a n s m i s s i o n was a t t r i b u t e d to a r e d u c t i o n o f F e 3+ to F e 2+ d u e to c a r b o n c r u c i b l e - g l a s s i n t e r a c t i o n
100 -
220
240
260
Mnm
-
SIOs/C Fe6ppm T/%
"
Fe 6ppm
/
1
/
.- . !
/
/
/
i s./
i/
50-
,''
$10/PI
/ 6'8
t
/'
/
/
/
d =0,2cm
/ S10/RC Fe 27ppm
/
6%
6'0
36 100b/em-~
Fig. 3. UV-transmission spectra of the glass type $10 with high iron content (27 ppm Fe) melted in a platinum crucible (S10/Pt) and remelted in a carbon crucible (S10/RC) and with low iron content (Fe: 6 ppm) melted in a platinum crucible (S10s/Pt) and melted in a carbon crucible (S10s/C).
D. Ehrt, W. Seeber / Glass for
highperformance optics and laser technology
(see fig. 3). Figure 3 presents a comparison of the UV-transmission spectra of the glass composition $10 with high iron content (Fe: 27 ppm) melted in a platinum crucible (S10/Pt) and remelted in a carbon crucible (S10/RC), and with low iron content (Fe: 6 ppm) melted in a platinum crucible (S10s/Pt) and in a carbon crucible (S10s/C). The effect is large if the iron content is high, i.e. 27 ppm; the glass S 1 0 / R C remelted in a carbon crucible has a much higher-UV-transmission above 230 nm and a transmission below 230 nm lower
250
100
than the platinum melted glass S10/Pt. In the glass S10s containing only 6 ppm Fe, the crucible effect is smaller. Instead of using a carbon crucible as reducing agent, it is also possible to dope the raw materials with reducing agents such as N H 4 F - H F or bromides [21]. Figures 4(a) and 4(b) show the transmission spectra of a fluoride phosphate glass doped with 0.5% Fe203 melted in a platinum crucible, glass no. 6 under normal conditions (air atmosphere), and no. 4 with the addition of 6%
300
&O0
o, o,o
T/°/
:
// t/
5C
d=O,O/,cm
i
500
~lnm
.................
: .....
///
--4
/
//~ \
i/
23
/I
/
/,'~ d=O,lcm
c:O, lcm
T;_ so
a
4~
42
3e
34
30
1
2B
"
1,5
z'2
"
2i
lOO
la
~hoood,~ 1
X/,um 3i
6
T/%
#/
50"
%
o,so/o
/
/
3 b
12
1"0
a
B
";mo06c ~1
Fig. 4. (a, b) Transmission spectra of a fluoride phosphate glass highly doped with 0.5% Fe203 melted in a platinum crucible: curve 6, under normal conditions; curve 4, under reducing conditions, caused by adding 6% NH4F. HF as the reducing agent.
D. Ehrt, IV.. Seeber / Glass for high performance optics and laser technology
24
16.0
100
180
2,00
220
2zt,O , 260 ,
---. . . . .
TI%
, ;Wnm,
-,7 ...........
-;;::=-~-'
/
/
"
5O
/
l
i
,~"
/ ii
l ii I J
66
I
I I
i' i !
#
!i
/
/ ,/
/
I
/
I
\
/
o:o,O
.... ss~ Fe 6ppm
.... s8
Fe29ppm
- - - SIO/C Fe27ppm (d=O,O5cm)
,,',.," j / 62
5t,
58
50
46
42
38
~/1000cml
Fig. 5. Vacuum ultraviolet transmission spectra of two different types of fluoride phosphate glasses melted in platinum crucibles (S8s and $8) and in a carbon crucible (S10/C) in comparison with silica glass SQ1.
N H 4 F - H F as reducing agent (air atmosphere). Colorless glasses were obtained in both cases. Glass no. 6, however, has an UV-edge near 380 nm and only very weak bands in the I R range which can be attributed to Fe 2+. That means that most of the iron exists in the Fe 3÷ state. The UV-edge of glass no. 4 is shifted into the shorter wavelength region and shows a weak band centered 100 160
,
180
,
around 260 nm due to Fe 3+, a strong band near 220 nm, and the tailing of a strong band below 220 nm. In the I R region the typical (d --->d) due to Fe 2÷ centered around 1 and 2 ~m are observed. M6ssbauer studies have shown that glass no. 6 contains only 12% Fe 2÷ and 88% Fe 3+ locating in three different sites; in glass no. 4 more than 97% 200
220
240
Fe2+
Fe3+
r/O/o
..........
so
I,'"
i," il
' /
62
_._~y
."
,""
I/ " /,, ,,
26,0 , ;',/rim,
,." sTos
/ _~l~o~~ , ~ /
d:O,2cm
Fe?ppm /
........
/!
5B
St,
50
#+6
t+2
~2100()c t~l
Fig. 6. Vacuum ultraviolet transmission spectra of silica glass glass SQI, two fluoride phosphate glasses with 8 (S8s) and 20 (S20s) mol% phosphate melted in platinum crucibles and a pure phosphate glass (Sr(PO3)2) melted in a silica crucible, containing nearly the same impurity content (Fe: 6-7 ppm).
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
of Fe 3+ was reduced to Fe 2+ located in two different sites [8]. Vacuum ultraviolet transmission measurements (fig. 5) of the glass S 1 0 / C with 27 p p m Fe remelted in a carbon crucible show a sharp absorption band near 210 nm and an absorption band near 170 n m overlapping the V U V / c u t . We are tempted to deduce from our Mi3ssbauer studies that both absorption bands can be attributed to Fe 2+. Although the position and shape of this band near 210 n m is typical for a S ~ P transition and could thus be caused by p3+ [23], the intensity of the band depends on the Fe 2+ content (compared with luminescence spectra) and is stable under heat treatment up to 400 ° C. A fluoride phosphate glass with similar high iron content $8 (Fe: 29 ppm) melted traditionally in a platinum crucible shows a broad band due to Fe 3+ near 250 nm and a broad band due Fe 2÷ between 200 and 160 nm. The same glass composition with lower iron content S8s (Fe: 6 p p m ) melted in a platinum crucible shows only weak bands for Fe 2÷ and Fe 3÷. The VUV-cut near 160 nm is comparable with that of vitreous silica SQ1. Figure 6 compares the V U V transmission spectra of vitreous silica SQ1 two fluoride phosphate glasses with 8 (S8s) and 20 (S20s) mol% phosphate melted in platinum crucibles, and a pure phosphate glass (Sr(PO3)2) melted in a silica crucible containing nearly the same impurity content (Fe: 6 - 7 ppm). Two significant results are: (1) the UV-cutoff is shifted to longer wavelength with
25
increasing phosphate content, which provides the trend of calculations for the resonance wavelength using a Sellmeier dispersion formula [5,8]; and (2) the Fe2+/Fe3+-relation is shifted to Fe 2+ with increasing phosphate content.
3.2. Luminescence of fluoride phosphate glasses Luminescence spectra reveal that minority species are efficient emitters under special excitation [24]. An important requirement for UV-transmitting optical material is low background luminescence [25,26]. The luminescence spectra of these fluoride phosphate glasses excited at 360 nm show only a weak broad b a n d around 800 nm which is typical for Fe 3+. These glasses generally have low luminescence intensity in comparison with pure oxide glasses (i.e. B K 7 standard glass = 1). With an excitation wavelength of 270 nm a second luminescence band near 400 cm (fig. 7) was obtained which has a low intensity for most glasses melted in platinum crucibles under normal conditions. Evidence of platinum in glasses from luminescence spectra was not possible. The b a n d around 800 nm increases w i t h increasing iron content. The 800 n m band is completely absent when melted under reducing conditions in a carbon crucible (fig. 8) and the band centered at 400 n m increases with increasing iron content. The luminescence spectra of one glass composition L5 melted in a platinum crucible with increas-
J 0,1
a I $8 /29ppmFe b) S8s/ 6ppmFe
Pt-C 0
0,06
0,02
400
500
600
700
800 A/nm
Fig. 7. Luminescenceemission spectra ()~exc.= 270 nm, 30 o C) of glass type $8 (Fe: 29 ppm) and S8s (Fe: 6 ppm) traditionally melted in platinum crucibles.
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology
26
Q
a) $10
c-c
22 ppm Fe
b) $10 s
6 pprnFe
\\\\\\ ~
I/"
400
500
600
8b0 Xlnm
700
Fig. 8. Luminescence emission spectra (Xexc.= 270 nm, 3 0 ° C ) of glass type S10 (Fe: 27 ppm and S10s (Fe: 6 ppm) melted under reducing conditions in carbon crucibles.
nm. A p3+ luminescence near 400 nm is a possibility, but no dependence on the iron content in the p p m range would be expected.
ing reduced conditions (adding 2 to 4 mol% bromides) are shown in fig. 9. The total iron content of the glass was constant at 30 p p m Fe. The luminescence intensity of the band at 400 n m increases with increasing reduction; however the weak 800 n m band decreases slightly. The total luminescence intensity of glass melted under reducing conditions is much greater than those of normal conditions. The strong luminescence band centered at 400 nm is assumed to be due to Fe+2+ , although the luminescence of Fe 2÷ in glasses is unknown. The position and shape of this band are typical of S ~ P-luminescence in oxide glasses [24,25]. However, in fluoride phosphate glasses, the Pb 2+ luminescence is located below 360 n m and the C e 3 + luminescence is centered around 310
2.0 ¸
3
c /-~ \
b.
3.3. Effects of radiation of the transmission of fluoride phosphate glasses Another important property of optical material is its resistance to radiation damage. Ionizing radiation produces free holes and electrons in glass which become trapped forming defect centers. These defect centers cause a decrease in optical transmission in the ultraviolet and visible range of the spectrum. The decreased optical transmission is commonly referred to as solarization [25,27]. Excimer lasers are considered to be applicable
0,1 a) L 5 / O B r b) L 5 / 2Br
\
Pt-C
Cl
"] //
~1 LSJ~B~
x\
, 1,0
// // // // /
400
500
600
\\
//
700
800 X/nm
700
b /
/
c
\
\
\
800 X/nm
Fig. 9. Luminescence emission spectra (Xexc.= 270 nm, 30 o C) of glass type L5 (Fe: 30 ppm) melted in a platinum crucible with an increasing reduction effect. Curves: (a) without adding bromide; (b) adding 2 mol% bromide; (c) adding 4 mol% bromide.
D. Ehrt, W. Seeber / Glass for high performance optics and laser technology 100200
250
To/o
300
400
27
500 ^/nm
//'
.'~
C l / / / /"/"/ / I'" "
.
l
/./"
~
//
re:z/pp
C
50
t. 0
30
"~/1000cm-I
20
Fig. 10. Transmission spectra of glass type $10 with 27 p p m Fe (sample thickness 2 mm) traditionally melted in a platinum crucible before and after exposure to radiation from an Kr F excimer laser, from X-ray Cu and after reheating for recovery. Curves: (a) before irradiation; (b) after 80 J / c m 2 (400 pulses at 200 m J / c m 2) at 248 nm; (c) after 3 h X-ray (Cu K~); (d) after reheating at temperature of 350 o C.
for deep UV lithography because short-wavelength illumination can provide submicron resolution [22]. One of the interesting wavelengths for illumination is 248 nm from a KrF excimer laser. Solarization effects of glass type S10 depend on iron content, melting conditions, radiation doses,
200 100
'
'
250 . . . .
and heat treatment and are shown in figs. 10-13. By comparison, no solarization effect was observed on the tested vitreous silica sample SQ1. Figure 10 shows an example of the transmission spectra of glass type S10 with high iron content (27 ppm) traditionally melted in a platinum cruci-
400
300
500 Mnm
T/%
SIO/RC
50-
', /.lil
J
50
d:O,2cm
Fe:27ppm
40
30
'~/lO00cm -1
20
Fig. 11. Transmission spectra of glass type S10 with 27 ppm Fe remelted under reducing conditions in a carbon crucible (sample thickness 2 mm). Curves: (a) before irradiation; (b) after 80 J / c m 2 (400 pulses at 200 m J / c m 2) at 248 nm; (c) after 160 J / c m 2 (800 pulses at 200 m J / c m 2) at 248 nm; (d) after 3 h X-ray (Cu K , ) ; (e) after reheating at a temperature of 350 o C.
28
D. Ehrt, IV.. Seeber / Glass for high performance optics and laser technology
200 100
250
/ /c
300
././"
SlOs / Pt Fe:6ppm
./
50
/
d/
so
400
500 klnrr
d:O,2cm
.J
40
30
~/~Ooocm4
20
Fig. 12. Transmission spectra of glass type S10s with 6 ppm Fe traditionally melted in a platinum crucible (sample thickness 2 mm). Curves: (a) before irradiation; (b) after 160 J/cm 2 (800 pulses at 200 mJ/cm2) at 248 nm; (c) after 400 J/cm 2 (2000 pulses at 200 mJ/cm2) at 248 nm; (d) after 3 h X-ray (Cu K~); (e) after reheating at a temperature of 350 o C.
ble (a) before a n d (b) after exposure to a n accum u l a t e d dose of 80 J / c m 2 from a n excimer laser o p e r a t i n g at 248 n m . T h e energy density per pulse was high, a p p r o x i m a t e l y 200 m J / c m 2 per pulse with a pulse length of a b o u t 20 ns, a n d 400 pulses were accumulated. O n l y m i n o r solarization effects are observed. A f t e r X-ray r a d i a t i o n (curve c: 3 h C u K s ) the glass shows strong solarization effects
250
100200
a n d a b r o w n color. T h e color centers p r o d u c e d are stable at r o o m temperature. A nearly c o m p l e t e recovery from solarization was observed for wavelengths > 360 n m after heat t r e a t m e n t at 3 5 0 ° C (curve d). Glass type S10 with a high iron c o n t e n t of 27 p p m melted u n d e r r e d u c i n g c o n d i t i o n s i n a c a r b o n crucible (fig. 11) has a better U V - t r a n s m i t tance near 250 n m before i r r a d i a t i o n (curve a)
300
1,00
500 klnm
T/% J /
C
SIOslC Fe:6ppm
d=O,2cm
30
~'11000cr n-1
50 ¸
50
~0
20
Fig. 13. Transmission spectra of glass type Sl0s with 6 ppm Fe melted under reducing conditions in a carbon crucible (sample thickness 2 ram). Curves: (a) before irradiation; (b) after 80 J/cm 2 (400 pulses at 200 mJ/cm2) at 248 nm; (c) after 400 J/cm 2 (2000 pulses at 200 mJ/cm2) at 248 nm.
D. Ehrt, IV. Seeber / Glass for high performance optics and laser technology
since most of the iron exists in the Fe z+ state, but stronger solarization effects were induced by excimer laser radiation than the same glass melted in a platinum crucible. The color centers produced by 160 J / c m 2 of the excimer laser (curve c) are comparable with those produced by X-ray radiation (curve d). A very interesting effect was observed after reheating at a temperature of 3 5 0 ° C (curve e). The transmittance near 250 nm is lower than before irradiation. This is attributed to a partial re-oxidation of Fe 2+ to Fe 3÷. Examples of the solarization of glass type S10s with low iron content (6 ppm) traditionally melted in a platinum crucible and melted under reducing conditions in a carbon crucible are shown in figs. 12 and 13. Little susceptibility to solarization was observed at low irradiation density. The solarization effects are due to two-photon absorption [18]. By comparison, the radiation intensity present at a lens in a wafer stepper is much less. The energy density in stepper objectives is estimated to be less than 10 m J / c m 2, and a typical photoresist is exposed to 100 pulses with an energy density of approximately 1 m J / c m 2 per pulse, accumulated to a total dose of 100 m J / c m 2 [18]. The formation of color centers by excimer laser radiation is likely to be the result of photoionizable multivalent metal ions present as trace impurities, mainly iron, in the glass. Fluoride phosphate glasses with a low content of impurities could be a suitable candidate for lens systems in wafer steppers.
4. Conclusions
Fluoride phosphate glasses possess special optical properties making them attractive candidates for applications in high performance optics and laser technology. They are potentially good UVtransmitting materials since their electron transition is comparable to that of BeF2 glass and fluoride crystals. Their actual UV-transmittance is strongly affected by extrinsic absorption due to trace impurities, mainly iron and other transition metals. Glasses melted under reducing conditions (a carbon crucible in a glove box or a platinum
29
crucible and adding reducing agents) using normal quality batch materials yield glasses with higher UV-transmittance near 250 nm due to the reduction of Fe 3+ to Fe 2÷. For maximum improvements in UV-transmission, it is important to use ultrapure raw materials and carbon crucibles. Traditionally, platinum-crucible-melted fluoride phosphate glasses possess only small amounts of luminescence dominated by a weak Fe 3+ luminescence centered around 800 nm. A strong luminescence near 400 nm is observed due to reducing melting conditions. The formation of color centers by excimer laser and X-ray radiation depend on the trace impurities, mainly of the iron content in the glass. Batch materials normally used to produce optical quality glass when melted under reducing melting conditions yield glasses with higher UV-transmittance near 250 nm; however, they show a stronger susceptibility to solarization than glasses melted under normal conditions. Fluoride phosphate glasses with a low content of impurities melted in carbon crucibles can be UV-transmitting optical materials that are complementary to silica glass and fluoride crystals. The authors with to thank R. Atzrodt, M. Carl, T. Kittel, J. Kleinschmidt, T. Kloss, W. Mikkeleit and R. Ranch for their assistance in experiments and helpful discussions.
References [1] W. Jahn, Glastechn. Ber. 34 (1961) 107. [2] J.T. Wenzel et al., SPIE 204 (1979) 59. [3] D. Ehrt and W. Vogel, 13 Friihjahrsschule Optik, Beitrage zur Optik und Quantenelektronik 6 (1981) 98; Feinger]tetechnik 31 (1982) 147. [4] D. Ehrt, R. Atzrodt and W. Vogel, in: Proc. 2nd Int. Otto Schott Colloquium, July 1982, Jena, Wiss. Z. FriedrichSchiller Univ. Jena, Math.-Naturwiss. Reihe 32 (1983) 509. [5] D. Ehrt, thesis B, Friedrich-Schiller-Universitiit Jena (1984). [6] D. Ehrt, C. J~iger and W. Vogel, in: Proc. 3rd Int. Otto Schott Colloquium, July 1986, Jena, Wiss, Z. FriedrichSchiller Univ. Jena, Math.-Naturwiss. Reihe 36 (1987) 867. [7] M. Dubiel and D. Ehrt, Phys. Status Solidi (a)100 (1987) 415.
30
D. Ehrt, IV. Seeber / Glass for high performance optics and laser technology
[8] D. Ehrt, W. Mikkeleit, W. Burckhardt and H. Mehner, in: Proc. 2nd Int. Ernst-Abbe-Conference (EAC), FriedrichSchiller-Universit~it, Jena, 1989. [9] R.T. Williams, D.J. Nagel, P.H. Klein and H.J. Weber, J. Appl. Phys. 52 (1981) 6279. [10] D.L. Sapak, J.M. Ward and J.E. Marion, SPIE, 970 (1988) 107. [11] S.E. Stokowski and D. Krashkerich, Mater. Res. Soc. Symp. Proc. 61 (1986) 273. [12] W. Mikkeleit, L. Priplata and G. Heidemann, 'Iron trace determination in fluoride phosphate systems', poster 4, 4th Int. Otto Schott Colloquium, Jena, July 1990. [13] M.J. Liepmann, A.J. Marker III and J.M. Melvin, SPIE 1128 (1989) preprint. [14] T. Izumitani and S. Hirota, J. Non-Cryst. Solids 73 (1985) 255. [15] L.M. Cook and K.-H. Mader, J. Am. Ceram. Soc. 65 (1982) 597. [16] L.N. Urusovskaya, A.N. Svenigorodskaya and P.N. Sakirova, Z. Prikl. Spectros. 6 (1980) 1076.
[17] L.M. Cook, M.J. Liepmann and A.J. Marker III, Mater. Sci. Forum 19-20 (1987) 305. [18] M.J. Liepmann, A.J. Marker III and U. Sowada, SPIE 970 (1988) 170. [19] L.A. Golubcov et al., Z. Prikl. Khim. 1 (1971) 1980. [20] E. Garibin, V.D. Chalilev, K.S. Evdropjev and B.S. Doladugina, Soy. J. Opt. Techn. 5 (1977) 286. [21] W. Seeber and D. Ehrt, Patent WP CO3C 392-6, 28.12.87. [22] D.A. Markle, SPIE 774 (1987) 108. [23] S.G. Lunter and J.K. Fedorov, Fiz. Khim. Stekla 14 (1988) 72. [24] W.B. White and D.S. Knight, Mater. Res. Soc. Symp. Proc. 61 (1986) 273. [25] T. Kloss and C. Beeker, Beitr~ige Opt. Quantenelektr. 21 (1989) 131. [26] G. Schmidt, K. Motzke, J. Steinert and D. Ellguth, Beitr~ige Opt. Quantenelektr. 21 (1989) 223. [27] J.J. Setta, R.J. Scheller and A.J. Marker III, SPIE 970 (1988) 179.