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Study of HfO2 films prepared by ion-assisted deposition using a gridless end-hall ion source
Thin Solid Films 350 (1999) 203±208
Study of HfO2 ®lms prepared by ion-assisted deposition using a gridless end-hall ion source M. Gilo a,*, N. Croitoru b a
b
ELOP Electrooptics Industries Ltd. P.O. Box 1165, Rehovot 76100, Israel Faculty of Engineering, Department of Physical Electronics, Tel-Aviv University, Tel-Aviv, Israel Received 12 May 1998; received in revised form 15 February 1999; accepted 9 March 1999
Abstract HfO2 thin ®lms were deposited using e-beam gun evaporation with ion assisted deposition (IAD) of low energy oxygen ions (40±100 eV) from an end-Hall ion source. A comparison was made using Hf and HfO2 starting materials. The index of refraction was measured as a function of the ion source voltage and compared to results without IAD. Application to high power laser mirrors was veri®ed by measurements of laser damage thresholds at 1.06 mm. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ion assisted deposition; Coatings, HfO2, Atomic force microscopy
1. Introduction HfO2 is a very commonly used material for optical coatings. The large transparent range from the UV to the IR (0.22±12 mm) and ease of evaporation adds to its popularity. The index of refraction is relatively high for materials in the visible. Hafnia also exhibits high laser damage thresholds [1], therefore it is a high index material of choice in multilayer coating designs, with SiO2 as the low index material. These designs include AR coatings, mirrors and polarizers for laser inter-cavity applications. Hafnia has been coated using conventional e-beam gun evaporation from either oxide or metal state. There are reports of using sputtering [2,3] and ion assisted deposition (IAD) [4,5] methods to coat with this material. In our work we examine the effect of IAD on e-beam gun evaporated Hafnia layers, using a gridless end-hall ion source. 2. Experimental set-up All samples were prepared in a Balzers BAK710 bell-jar coating system. The samples were placed on a rotary cage. The vacuum pressure of 10 26 mbar was achieved using a diffusion pump coupled through a liquid-nitrogen trap. The rate of evaporation was controlled by a quartz crystal monitor, whereas the layer re¯ectivity was recorded by an optical * Corresponding author. Tel.: 1972-8-938-6668; fax: 1972-8-9386597.
monitor in the visible. The substrates were heated to 2508C. A Commonwealth (Commonwealth Scienti®c Corp., Alexandria, Virginia) Mark II end-Hall ion source, with oxygen gas ¯ow, supplied the ion current. This source has two main advantages: 1. A wide angle ion beam ( ^ 308) that is suitable for industrial high volume applications. 2. Low ion energy with high current, resulting in index increase in oxides and layer stabilization, without the surface damage associated with high energy ions. The operation of this ion source is described by Kaufman et al. [6]. The voltage and current setting of the ion source are maintained constant by controlling the oxygen ¯ow by an electronic feedback circuit. The ion energy at the substrates is about 60% of the energy generated by the anode voltage. The material was evaporated by an e-gun, with deposition rate of 0.1 nm/s. The distance between the ion source and the substrates was 30 cm. The ion gun voltage was varied from 100±170 V (ion energies of 60±100 eV), while the current was maintained at 1 A. The resulting current density was about 1 mA/cm 2 on the substrates, which was neutralized to prevent charge build-up. Spectral transmittance and re¯ectance were measured using a Perkin±Elmer Lambda 900 spectrophotometer. The roughness of the surface was measured using an atomic force microscope (AFM) (NanoScope II AFM/STM, Digital Instruments, Santa Barbara, CA, tested under ambient conditions with a 140 mm scanner). The damage thresholds at 1.06 mm were measured
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00226-6
204
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
Fig. 1. Spectral transmittance of HfO2 layer with an optical thickness of nine quarterwaves at a wavelength of 480 nm on BK7 glass.
using an in-house laser system, and the composition of the coatings was analyzed using auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS), which will be described later.
3. Results and discussion 3.1. Index of refraction variations Thin HfO2 ®lms with an optical thickness of nine quarterwaves at a wavelength of 480 nm (within 3%) were deposited on transparent BK7 glass disks using ion-assistance, with different voltages of the ion gun. The index of refraction was evaluated from the transmittance measurements of the glass disks using the turning point method [7]. The spectral transmittance of a ®lm deposited using ion-
assistance was compared to a ®lm deposited on a heated (2508C) substrate without operating the ion gun. The transmittance spectrum of the two ®lms is shown in Fig. 1. Since the halfwave turning points were very close to transmittance of uncoated BK7 glass, zero absorption was assumed. The indices of refraction using different anode voltage of the ion gun for unheated substrates, with HfO2 starting material are shown in Fig. 2. These results are compared to ®lms that were obtained from Hf in metal form, that were evaporated and oxidized by bleeding of oxygen gas during the evaporation, and shown in Fig. 3. In Fig. 4 the effect of heating the substrates prior to the evaporation is shown for Hf and HfO2 starting material, with constant ion gun voltage. It can be seen that as the ion gun voltage increases, the index of refraction increases. When the voltage is above 120 V, the indices rise above the value obtained by heating the substrate without IAD. Films deposited on a heated
Fig. 2. Index of Hafnia obtained from HfO2 starting material, with different anode voltage of the ion gun. Substrates were unheated, except where noted.
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
205
Fig. 3. Index of Hafnia obtained from Hf starting material with different anode voltage of the ion gun. Substrates heated to 2508C.
substrate with IAD showed higher indices than ®lms deposited on unheated substrates with the same IAD conditions. The index obtained from Hf in metal form is slightly higher than values obtained from oxide form, with the same temperature and ion gun voltage. Tikhonravov et al. [8] have recently shown that small refractive index inhomogeneities alter the transmittance curves of ideal homogeneous layers, therefore introducing inaccuracies to the turning point index measurements. This and repeated measurements of HfO2 deposited layers lead us to evaluate the index accuracy obtained with this method to be 0.02. With this accuracy it is still possible to see the effect of IAD on the layer's index of refraction. McNally et. al. [9] used a Kaufman ion source with 300 eV O21 ions and current density of 28 mA/cm 2 to study the
properties of HfO2. They have found similar results for the increase of index of refraction, from 2.00 to 2.11, measured at 350 nm. Their data is comparable to the results we obtained at 450±480 nm (Figs. 2±4). 3.2. Laser damage threshold measurements The set-up for laser damage threshold measurements is shown in Fig. 5. The system is based on a pulsed YAG laser that is focused on the test piece, with a pulse rate of 10 pps. The beam shape was monitored by a Big Sky Laser Diagnostic system. The beam intensity was controlled by the angle between two linear polarizers. We used two test procedures to evaluate the thresholds:
Fig. 4. Comparison of indices obtained with and without heating the substrates, for Hf and HfO2 starting material.
206
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
Fig. 5. Laser damage testing set-up.
3.2.1. N £ 1 In this procedure the beam intensity is raised gradually (with a rate of about 500 MW/cm 2 per min) until the surface is damaged, while all the pulses are positioned on the same site. The threshold value determined is 250 MW/cm 2 lower than the intensity value of the observed damage. This is to compensate the relatively few pulses at each intensity, and to comply with the 1 £ 1 procedure. 3.2.2. 1 £ 1 In this procedure the beam intensity is set and then tested
on one site. Afterwards the intensity is raised by 250 MW/ cm 2, and tested on a different site on the glass. This procedure continues until damage is observed. The threshold value determined is 250 MW/cm 2 lower than the value of the intensity of the observed damage. Of these two procedures, the N £ 1 procedure is expected to achieve higher thresholds due to an annealing effect that raises the damage resistance of the site. Test pieces with different coating conditions and starting materials were tested with both procedures. Each test site was exposed to at least 200 pulses at 10 Hz repetition rate.
Fig. 6. Laser damage threshold results using two testing procedures. The points plotted are 250 MW/cm 2 below the damaging intensity. Test procedures are described in the text.
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
207
cially with the 1 £ 1 procedure. Coatings with HfO2 starting material, compared to metallic starting material, showed similar thresholds when coated without IAD, and higher thresholds when performed with IAD. The threshold values that were found are very high (above 500 MW/cm 2 with IAD and above 1.5 GW/cm 2 without IAD) compared to other coating materials with similar index of refraction. For comparison, we tested TiO2±SiO2 mirrors and the threshold was about 300 MW/ cm 2. The high threshold values of the HfO2 samples contributed to our assumption of very low absorption. 3.3. Moisture effects Fig. 7. The dependence of the spectral shifts on the ion gun voltage of heated substrates evaporated with HfO2 from Hf metal.
The damage threshold values are summarized in Fig. 6. The results of the N £ 1 testing procedure are higher than those of the 1 £ 1 procedure, by about 500 MW/cm 2, at all the conditions, as expected. Comparing the deposition parameters, the coating performed with IAD shows a decrease in resistance to laser damage, as the voltage increases, espe-
The exposure of the layers to the atmosphere humidity results in partial replacement of the air voids in the layers with water molecules. The spectral shift due to humidity adsorption (`dry to wet' spectral shift), in layers grown with IAD, was compared to layers prepared without IAD. One spectral measurement was performed within 10 min after opening the chamber, and another measurement after 6 months of exposure to room temperature and humidity. The dependence of the spectral shifts on the ion gun voltage of heated substrates evaporated with HfO2 from Hf metal is shown in Fig. 7. The resulting spectral shifts show that the employment of ion-assistance lowers greatly the spectral shifts. The difference between 120 and 150 V is less significant. These low spectral shifts are attributed to the reduction of the microscopic voids and the densi®cation of the layers by the IAD process. This phenomena has been observed in other materials using IAD [10] and ion plating [11,12] of low energy ions. 3.4. Surface roughness
Fig. 8. (a): AFM picture of HfO2 deposited without IAD. Average roughness: 2.27 nm RMS. (b): AFM picture of HfO2 deposited with IAD. Ion gun voltage: 170 V. Average roughness: 4.51 nm RMS.
Atomic force microscopy (AFM) was used to study the surface topography of the thin ®lms. The microstructure of thermally and sputtered deposited layers tend to be columnar resulting in microscopically rough surfaces. Insulating surfaces (such as Hafnia) can be dif®cult to image using Scanning Electron Microscopy due to the charging of the surface. These thin ®lm surfaces are appropriate for AFM imaging because the surfaces are hard and the surface structure sizes (surface features of 1±500 nm) are in the range of AFM observations. This technique results in quantitative pro®le characterization of the roughness. We compared Hafnia layers deposited on heated substrates with and without IAD. The AFM scanned squares of 5 £ 5 mm. The roughness of uncoated substrates was measured to be 1.10 nm. The results of layers deposited with and without IAD are shown in Fig. 8. The RMS roughness was averaged over a horizontal line with no special features. The deposition with IAD (170 V, in Fig. 8b) shows that the IAD process increased the average roughness of the surface from about 2.3 to 4.5 nm. Since the surface roughness and light scattering are
208
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
Table 1 Atomic concentration (%) of the elements present in the AES measurement Sample
Hf
O
C
With IAD Without IAD
37.4 34.6
54.1 61.3
8.4 4.2
related, it is interesting to note that MaNally et. al. [9] have observed only a slight decrease in the scattering due to deposition with IAD, using their ion gun. The different energy and current regimes may account for the difference. 3.5. Surface analysis Hafnia coated glass samples deposited with and without IAD were analyzed using auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) in order to see differences in the surface composition and contamination. AES measurements have been performed with a PHI model 590A scanning auger microscope using a primary beam of 5 KeV and 1 mA electrons. The analysis area was 100 £ 100 mm 2. The sputtering was done by Ar 1 ions with sputter rate of 0.5 nm/min. The rate was measured on a 100 nm Ta2O5 standard. The AES spectra demonstrated the presence only of hafnium, oxygen and carbon in carbide form. Table 1 summarizes the elemental analysis of average results of three different areas after sputtering off 50 nm. No standard has been used for quantitative analysis, only relative results are given of comparison between the samples. Therefore, the stoichiometric information cannot be evaluated. SIMS measurements have been performed with a Cameca IMS 4f instrument using a primary beam of 8 KeV O21 with an analyzed area of 60 mm diameter. The measurement has shown the presence of several additional metals in both samples (Al, Si, Ti, Cr, Fe and Cu). The HfO2 sample without IAD showed also the presence of indium, while the HfO2 sample with IAD had traces of tungsten. The impurity levels of both samples are of the same order of magnitude, which is believed to be less than 100 ppm. Due to the absence of appropriate standards and lack of known tabulated relative sensitivity factors, there was no attempt to calculate the concentrations of these impurities. The main impurity, carbon, is usually observed in coatings prepared under vacuum condition and can be attributed to the diffusion pump. While there is a distinguished difference of concentration in the two samples, this difference had not noticeably affected the absorption of the ®lm. The sources of the other impurities can be numerous. Indium can be related to a former coating in the same chamber, and tungsten introduced from the ion gun's ®lament. Since the concentrations of these impurities are very low (not including carbon), it is presumed they do not affect the laser thresholds.
Additional depth pro®le measurements may show if there are differences in the composition of the two samples. This will help us understand the growth process and the effect of IAD on the HfO2 layer. Unfortunately, it is beyond the scope of this study. 4. Summary HfO2 thin ®lms were deposited using e-beam gun evaporation with IAD of low energy oxygen ions from an end-Hall ion source. The IAD increased the index of refraction of HfO2 layers evaporated either from metallic or oxide form, with the increase of the anode voltage. The indices were slightly higher when the evaporated material was Hf in metal form compared to oxide form. Laser damage thresholds tests at 1.06 mm gave similar results for both starting materials when evaporated without IAD. When evaporated with IAD there was a marked decrease in the thresholds, which was more pronounced for Hf in metal form. AFM measurements showed that the IAD process increased the roughness of the coating. AES surface analysis measurements showed a higher carbon impurity in the HfO2 layers evaporated with IAD. These two ®ndings can be responsible for the lowering of the damage thresholds. The increase in roughness may result in loosely attached molecules that can be initial damage sites. Likewise, the impurities in the layer can be initial damage sites due to residual absorption. The tests we performed are not conclusive as to the prime reason of the lowering of the damage thresholds. The overall threshold values that were found were high compared to other coating materials with similar index of refraction. Therefore, from our experience using the Mark II ion gun, it is recommended to evaporate Hafnia without IAD for high intensity laser applications. References [1] S.R. Foltyn, B.E. Newnam, SPIE 288 (1981) 21. [2] S.M. Edlou, A. Smajkiewicz, G.A. Al-Jumaily, Appl. Opt. 32 (1993) 5601. [3] C.T. Kuo, R. Kwor, K.M. Jones, Thin Solid Films 213 (1992) 257. [4] P.J. Martin, R.P. Netter®lld, Thin Solid Films 199 (1991) 351. [5] M.L. Fulton, SPIE 2253 (1994) 374. [6] H.R. Kaufman, R.S. Robinson, R.I. Seddon, J. Vac. Sci. Technol. A 5 (1987) 2081. [7] H.A. Macleod, Thin-Film Optical Filters, 2nd (1986) 369. [8] A.V. Tikhonravov, M.K. Trubetskov, B.T. Sullivan, J.A. Dobrowolski, Appl. Opt. 36 (1997) 7188. [9] J.J. McNally, G.A. Al-Jumaily, S.R. Wilson, J.R. McNeil, SPIE 540 (1985) 479. [10] M. Gilo, N. Croitoru, Thin Solid Films 283 (1996) 84. [11] A.J. Waldorf, J.A. Dobrowolski, B.T. Sullivan, L.M. Plante, Appl. Opt. 32 (1993) 5583. [12] A. Zoller, R. Gotzelmann, H. Hagedon, W. Klug, K. Matl, SPIE, Proc., 3133 (1997).
Study of HfO2 ®lms prepared by ion-assisted deposition using a gridless end-hall ion source M. Gilo a,*, N. Croitoru b a
b
ELOP Electrooptics Industries Ltd. P.O. Box 1165, Rehovot 76100, Israel Faculty of Engineering, Department of Physical Electronics, Tel-Aviv University, Tel-Aviv, Israel Received 12 May 1998; received in revised form 15 February 1999; accepted 9 March 1999
Abstract HfO2 thin ®lms were deposited using e-beam gun evaporation with ion assisted deposition (IAD) of low energy oxygen ions (40±100 eV) from an end-Hall ion source. A comparison was made using Hf and HfO2 starting materials. The index of refraction was measured as a function of the ion source voltage and compared to results without IAD. Application to high power laser mirrors was veri®ed by measurements of laser damage thresholds at 1.06 mm. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Ion assisted deposition; Coatings, HfO2, Atomic force microscopy
1. Introduction HfO2 is a very commonly used material for optical coatings. The large transparent range from the UV to the IR (0.22±12 mm) and ease of evaporation adds to its popularity. The index of refraction is relatively high for materials in the visible. Hafnia also exhibits high laser damage thresholds [1], therefore it is a high index material of choice in multilayer coating designs, with SiO2 as the low index material. These designs include AR coatings, mirrors and polarizers for laser inter-cavity applications. Hafnia has been coated using conventional e-beam gun evaporation from either oxide or metal state. There are reports of using sputtering [2,3] and ion assisted deposition (IAD) [4,5] methods to coat with this material. In our work we examine the effect of IAD on e-beam gun evaporated Hafnia layers, using a gridless end-hall ion source. 2. Experimental set-up All samples were prepared in a Balzers BAK710 bell-jar coating system. The samples were placed on a rotary cage. The vacuum pressure of 10 26 mbar was achieved using a diffusion pump coupled through a liquid-nitrogen trap. The rate of evaporation was controlled by a quartz crystal monitor, whereas the layer re¯ectivity was recorded by an optical * Corresponding author. Tel.: 1972-8-938-6668; fax: 1972-8-9386597.
monitor in the visible. The substrates were heated to 2508C. A Commonwealth (Commonwealth Scienti®c Corp., Alexandria, Virginia) Mark II end-Hall ion source, with oxygen gas ¯ow, supplied the ion current. This source has two main advantages: 1. A wide angle ion beam ( ^ 308) that is suitable for industrial high volume applications. 2. Low ion energy with high current, resulting in index increase in oxides and layer stabilization, without the surface damage associated with high energy ions. The operation of this ion source is described by Kaufman et al. [6]. The voltage and current setting of the ion source are maintained constant by controlling the oxygen ¯ow by an electronic feedback circuit. The ion energy at the substrates is about 60% of the energy generated by the anode voltage. The material was evaporated by an e-gun, with deposition rate of 0.1 nm/s. The distance between the ion source and the substrates was 30 cm. The ion gun voltage was varied from 100±170 V (ion energies of 60±100 eV), while the current was maintained at 1 A. The resulting current density was about 1 mA/cm 2 on the substrates, which was neutralized to prevent charge build-up. Spectral transmittance and re¯ectance were measured using a Perkin±Elmer Lambda 900 spectrophotometer. The roughness of the surface was measured using an atomic force microscope (AFM) (NanoScope II AFM/STM, Digital Instruments, Santa Barbara, CA, tested under ambient conditions with a 140 mm scanner). The damage thresholds at 1.06 mm were measured
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0040-609 0(99)00226-6
204
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
Fig. 1. Spectral transmittance of HfO2 layer with an optical thickness of nine quarterwaves at a wavelength of 480 nm on BK7 glass.
using an in-house laser system, and the composition of the coatings was analyzed using auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS), which will be described later.
3. Results and discussion 3.1. Index of refraction variations Thin HfO2 ®lms with an optical thickness of nine quarterwaves at a wavelength of 480 nm (within 3%) were deposited on transparent BK7 glass disks using ion-assistance, with different voltages of the ion gun. The index of refraction was evaluated from the transmittance measurements of the glass disks using the turning point method [7]. The spectral transmittance of a ®lm deposited using ion-
assistance was compared to a ®lm deposited on a heated (2508C) substrate without operating the ion gun. The transmittance spectrum of the two ®lms is shown in Fig. 1. Since the halfwave turning points were very close to transmittance of uncoated BK7 glass, zero absorption was assumed. The indices of refraction using different anode voltage of the ion gun for unheated substrates, with HfO2 starting material are shown in Fig. 2. These results are compared to ®lms that were obtained from Hf in metal form, that were evaporated and oxidized by bleeding of oxygen gas during the evaporation, and shown in Fig. 3. In Fig. 4 the effect of heating the substrates prior to the evaporation is shown for Hf and HfO2 starting material, with constant ion gun voltage. It can be seen that as the ion gun voltage increases, the index of refraction increases. When the voltage is above 120 V, the indices rise above the value obtained by heating the substrate without IAD. Films deposited on a heated
Fig. 2. Index of Hafnia obtained from HfO2 starting material, with different anode voltage of the ion gun. Substrates were unheated, except where noted.
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
205
Fig. 3. Index of Hafnia obtained from Hf starting material with different anode voltage of the ion gun. Substrates heated to 2508C.
substrate with IAD showed higher indices than ®lms deposited on unheated substrates with the same IAD conditions. The index obtained from Hf in metal form is slightly higher than values obtained from oxide form, with the same temperature and ion gun voltage. Tikhonravov et al. [8] have recently shown that small refractive index inhomogeneities alter the transmittance curves of ideal homogeneous layers, therefore introducing inaccuracies to the turning point index measurements. This and repeated measurements of HfO2 deposited layers lead us to evaluate the index accuracy obtained with this method to be 0.02. With this accuracy it is still possible to see the effect of IAD on the layer's index of refraction. McNally et. al. [9] used a Kaufman ion source with 300 eV O21 ions and current density of 28 mA/cm 2 to study the
properties of HfO2. They have found similar results for the increase of index of refraction, from 2.00 to 2.11, measured at 350 nm. Their data is comparable to the results we obtained at 450±480 nm (Figs. 2±4). 3.2. Laser damage threshold measurements The set-up for laser damage threshold measurements is shown in Fig. 5. The system is based on a pulsed YAG laser that is focused on the test piece, with a pulse rate of 10 pps. The beam shape was monitored by a Big Sky Laser Diagnostic system. The beam intensity was controlled by the angle between two linear polarizers. We used two test procedures to evaluate the thresholds:
Fig. 4. Comparison of indices obtained with and without heating the substrates, for Hf and HfO2 starting material.
206
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
Fig. 5. Laser damage testing set-up.
3.2.1. N £ 1 In this procedure the beam intensity is raised gradually (with a rate of about 500 MW/cm 2 per min) until the surface is damaged, while all the pulses are positioned on the same site. The threshold value determined is 250 MW/cm 2 lower than the intensity value of the observed damage. This is to compensate the relatively few pulses at each intensity, and to comply with the 1 £ 1 procedure. 3.2.2. 1 £ 1 In this procedure the beam intensity is set and then tested
on one site. Afterwards the intensity is raised by 250 MW/ cm 2, and tested on a different site on the glass. This procedure continues until damage is observed. The threshold value determined is 250 MW/cm 2 lower than the value of the intensity of the observed damage. Of these two procedures, the N £ 1 procedure is expected to achieve higher thresholds due to an annealing effect that raises the damage resistance of the site. Test pieces with different coating conditions and starting materials were tested with both procedures. Each test site was exposed to at least 200 pulses at 10 Hz repetition rate.
Fig. 6. Laser damage threshold results using two testing procedures. The points plotted are 250 MW/cm 2 below the damaging intensity. Test procedures are described in the text.
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
207
cially with the 1 £ 1 procedure. Coatings with HfO2 starting material, compared to metallic starting material, showed similar thresholds when coated without IAD, and higher thresholds when performed with IAD. The threshold values that were found are very high (above 500 MW/cm 2 with IAD and above 1.5 GW/cm 2 without IAD) compared to other coating materials with similar index of refraction. For comparison, we tested TiO2±SiO2 mirrors and the threshold was about 300 MW/ cm 2. The high threshold values of the HfO2 samples contributed to our assumption of very low absorption. 3.3. Moisture effects Fig. 7. The dependence of the spectral shifts on the ion gun voltage of heated substrates evaporated with HfO2 from Hf metal.
The damage threshold values are summarized in Fig. 6. The results of the N £ 1 testing procedure are higher than those of the 1 £ 1 procedure, by about 500 MW/cm 2, at all the conditions, as expected. Comparing the deposition parameters, the coating performed with IAD shows a decrease in resistance to laser damage, as the voltage increases, espe-
The exposure of the layers to the atmosphere humidity results in partial replacement of the air voids in the layers with water molecules. The spectral shift due to humidity adsorption (`dry to wet' spectral shift), in layers grown with IAD, was compared to layers prepared without IAD. One spectral measurement was performed within 10 min after opening the chamber, and another measurement after 6 months of exposure to room temperature and humidity. The dependence of the spectral shifts on the ion gun voltage of heated substrates evaporated with HfO2 from Hf metal is shown in Fig. 7. The resulting spectral shifts show that the employment of ion-assistance lowers greatly the spectral shifts. The difference between 120 and 150 V is less significant. These low spectral shifts are attributed to the reduction of the microscopic voids and the densi®cation of the layers by the IAD process. This phenomena has been observed in other materials using IAD [10] and ion plating [11,12] of low energy ions. 3.4. Surface roughness
Fig. 8. (a): AFM picture of HfO2 deposited without IAD. Average roughness: 2.27 nm RMS. (b): AFM picture of HfO2 deposited with IAD. Ion gun voltage: 170 V. Average roughness: 4.51 nm RMS.
Atomic force microscopy (AFM) was used to study the surface topography of the thin ®lms. The microstructure of thermally and sputtered deposited layers tend to be columnar resulting in microscopically rough surfaces. Insulating surfaces (such as Hafnia) can be dif®cult to image using Scanning Electron Microscopy due to the charging of the surface. These thin ®lm surfaces are appropriate for AFM imaging because the surfaces are hard and the surface structure sizes (surface features of 1±500 nm) are in the range of AFM observations. This technique results in quantitative pro®le characterization of the roughness. We compared Hafnia layers deposited on heated substrates with and without IAD. The AFM scanned squares of 5 £ 5 mm. The roughness of uncoated substrates was measured to be 1.10 nm. The results of layers deposited with and without IAD are shown in Fig. 8. The RMS roughness was averaged over a horizontal line with no special features. The deposition with IAD (170 V, in Fig. 8b) shows that the IAD process increased the average roughness of the surface from about 2.3 to 4.5 nm. Since the surface roughness and light scattering are
208
M. Gilo, N. Croitoru / Thin Solid Films 350 (1999) 203±208
Table 1 Atomic concentration (%) of the elements present in the AES measurement Sample
Hf
O
C
With IAD Without IAD
37.4 34.6
54.1 61.3
8.4 4.2
related, it is interesting to note that MaNally et. al. [9] have observed only a slight decrease in the scattering due to deposition with IAD, using their ion gun. The different energy and current regimes may account for the difference. 3.5. Surface analysis Hafnia coated glass samples deposited with and without IAD were analyzed using auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS) in order to see differences in the surface composition and contamination. AES measurements have been performed with a PHI model 590A scanning auger microscope using a primary beam of 5 KeV and 1 mA electrons. The analysis area was 100 £ 100 mm 2. The sputtering was done by Ar 1 ions with sputter rate of 0.5 nm/min. The rate was measured on a 100 nm Ta2O5 standard. The AES spectra demonstrated the presence only of hafnium, oxygen and carbon in carbide form. Table 1 summarizes the elemental analysis of average results of three different areas after sputtering off 50 nm. No standard has been used for quantitative analysis, only relative results are given of comparison between the samples. Therefore, the stoichiometric information cannot be evaluated. SIMS measurements have been performed with a Cameca IMS 4f instrument using a primary beam of 8 KeV O21 with an analyzed area of 60 mm diameter. The measurement has shown the presence of several additional metals in both samples (Al, Si, Ti, Cr, Fe and Cu). The HfO2 sample without IAD showed also the presence of indium, while the HfO2 sample with IAD had traces of tungsten. The impurity levels of both samples are of the same order of magnitude, which is believed to be less than 100 ppm. Due to the absence of appropriate standards and lack of known tabulated relative sensitivity factors, there was no attempt to calculate the concentrations of these impurities. The main impurity, carbon, is usually observed in coatings prepared under vacuum condition and can be attributed to the diffusion pump. While there is a distinguished difference of concentration in the two samples, this difference had not noticeably affected the absorption of the ®lm. The sources of the other impurities can be numerous. Indium can be related to a former coating in the same chamber, and tungsten introduced from the ion gun's ®lament. Since the concentrations of these impurities are very low (not including carbon), it is presumed they do not affect the laser thresholds.
Additional depth pro®le measurements may show if there are differences in the composition of the two samples. This will help us understand the growth process and the effect of IAD on the HfO2 layer. Unfortunately, it is beyond the scope of this study. 4. Summary HfO2 thin ®lms were deposited using e-beam gun evaporation with IAD of low energy oxygen ions from an end-Hall ion source. The IAD increased the index of refraction of HfO2 layers evaporated either from metallic or oxide form, with the increase of the anode voltage. The indices were slightly higher when the evaporated material was Hf in metal form compared to oxide form. Laser damage thresholds tests at 1.06 mm gave similar results for both starting materials when evaporated without IAD. When evaporated with IAD there was a marked decrease in the thresholds, which was more pronounced for Hf in metal form. AFM measurements showed that the IAD process increased the roughness of the coating. AES surface analysis measurements showed a higher carbon impurity in the HfO2 layers evaporated with IAD. These two ®ndings can be responsible for the lowering of the damage thresholds. The increase in roughness may result in loosely attached molecules that can be initial damage sites. Likewise, the impurities in the layer can be initial damage sites due to residual absorption. The tests we performed are not conclusive as to the prime reason of the lowering of the damage thresholds. The overall threshold values that were found were high compared to other coating materials with similar index of refraction. Therefore, from our experience using the Mark II ion gun, it is recommended to evaporate Hafnia without IAD for high intensity laser applications. References [1] S.R. Foltyn, B.E. Newnam, SPIE 288 (1981) 21. [2] S.M. Edlou, A. Smajkiewicz, G.A. Al-Jumaily, Appl. Opt. 32 (1993) 5601. [3] C.T. Kuo, R. Kwor, K.M. Jones, Thin Solid Films 213 (1992) 257. [4] P.J. Martin, R.P. Netter®lld, Thin Solid Films 199 (1991) 351. [5] M.L. Fulton, SPIE 2253 (1994) 374. [6] H.R. Kaufman, R.S. Robinson, R.I. Seddon, J. Vac. Sci. Technol. A 5 (1987) 2081. [7] H.A. Macleod, Thin-Film Optical Filters, 2nd (1986) 369. [8] A.V. Tikhonravov, M.K. Trubetskov, B.T. Sullivan, J.A. Dobrowolski, Appl. Opt. 36 (1997) 7188. [9] J.J. McNally, G.A. Al-Jumaily, S.R. Wilson, J.R. McNeil, SPIE 540 (1985) 479. [10] M. Gilo, N. Croitoru, Thin Solid Films 283 (1996) 84. [11] A.J. Waldorf, J.A. Dobrowolski, B.T. Sullivan, L.M. Plante, Appl. Opt. 32 (1993) 5583. [12] A. Zoller, R. Gotzelmann, H. Hagedon, W. Klug, K. Matl, SPIE, Proc., 3133 (1997).