6th NDT in Progress Quantitative Characterization of Surface and Subsurface. Topography of An Optical Nano-Film/Substrate Using Scanning

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1 6th NDT in Progress 2011 International Workshop of NDT Experts, Prague, Oct 2011 Quantitative Characterization of Surface and Subsurface Topography of An Optical Nano-Film/Substrate Using Scanning Probe Acoustic Microscopy Qian CHENG, WenGang YAO, MengLu QIAN Institute of Acoustics, Tongji University, Shanghai, China Phone/Fax: , Abstract Using Scanning Probe Acoustic Microscopy (SPAM), we characterize the surface and subsurface topography of a nano HfO 2 film on SiO 2 substrate quantitatively. Surface topography shows that the roughness of the film s surface is less than 3 nm, while the acoustic image of Sample Vibration Mode (SVM) shows that there are dozens of random lines at the film s subsurface, which might be scratches from the polishing procedure. We also find that the detection depth of SPAM is sensitive to the frequency. After laser irradiation, the depth of laser-damaged hole in the film seems close to its thickness. Residual stress within film due to heat absorption is observed using a higher frequency. The phase images of Probe-Vibration Mode (PVM) show that the hydrophilicity of the damaged area seems higher than the undamaged area. This leads to stronger water vapor absorption in the air. Keywords: Scanning probe acoustic microscopy, optical nano-film, subsurface topography, scratch, laser-damaged, residual stress, hydrophilicity 1. Introduction In the high power laser system, the optical films are generally the weakest but crucial component, and their quality limits the laser output energy. When there are impurities or defects within or underneath a film, it s absorbed laser energy might exceed the damage threshold of the film, resulting in a local damage on the film [1-3]. One important kind of defects is scratch on the substrate due to polishing. It is the key feature responsible for adhesive strength between film and substrate. Besides deformation, the residual thermal stress might occur, the locally porosity and porous structure might change in the damaged area of film, causing the change in film s hydrophilicity. It s a challenge to make a qualitative and quantitative definition of a film s subsurface damage. In this study, we investigate the structural and mechanical properties in and under a HfO 2 film on SiO 2 substrate using SPAM. SPAM is a new nano-scale detection tool under continuing development for materials characterization Both the surface

2 and subsurface images of a sample can be obtained from SPAM in situ simultaneously, the local elastic/electric/magnetic/thermal properties can also be obtained with different type of probes. There are three operation modes of SPAM: Sample Vibration Mode (SVM, i.e. contact mode) 4-6, Probe Vibration Mode (PVM, i.e. intermittent contact mode) 7-18, and resonant difference-frequency mode 16. In this paper, we set up a SPAM system by adding an acoustic module to a Dimension 3100 SPM system (Veeco Inc.) and operate in the first two modes. The thicknesses of HfO 2 film and SiO 2 substrate are ± 0.10 nm and 1.50 ± 0.01 mm respectively. The probe type is AppNano Sicon-200, the probe s resonant frequency is f 0 = 5-25 khz, the elastic constant is k = N/m, the relative humidity is %, and the temperature is Surface and subsurface topographies 2.1 Surface topography Topography shows that the HfO 2 film s surface is relatively smooth. The roughness is less than 2.61 ± 0.01 nm. 0 nm 500 nm

3 Fig.1 Topography of a nano HfO 2 film on SiO 2 substrate 2.2 Subsurface topography Acoustic phase image of Sample Vibration Mode (SVM) shows some random lines on the subsurface of the sample while surface topography has not. Reviewing the whole coating procedure, we believe these lines should come from substrate surface scratching during the polishing procedure. As shown in Fig. 2, the width of a scratch is measured as µm, which is consistent with nm radius of abrasive. The acoustic frequency of sample vibration is f SV = 45 khz. 0 30

4 Width = µm Section Fig.2 Subsurface topography with some random lines. The detection of the subsurface structural and mechanical properties of a material is an advantage of the SPAM. One feature of the SPAM is the detection depth is very sensitive to the frequency. The lower the frequency, the deeper the detection depth. For this sample and this probe (f 0 = khz, k = N/m), 45 khz is the best frequency to detect the depth of film as shown in Fig. 3.

5 0 30 a) 24.6 khz b) 35.2 khz

6 c) 35.8 khz d) 45.0 khz e) 88.2 khz

7 f) khz g) System frequency sweep Fig.3 Scratches in different acoustic phase images with different frequencies. The frequency (45 khz) of the biggest resonant peak is the best frequency to detect the depth of film 3. Laser-damaged nano-film To detect the damage threshold of the film, the sample was irradiated for 10 ns by an 1064 nm laser of 1/e 2 mm radius, 10 J/cm 2 power. During the high power laser's irradiation, HfO 2 film absorbs the heat, which leads to thermal and stress gradient and might even melt and be damaged locally when laser power exceeds the damage threshold. Except for damaged hole, the residual thermal stress might occur, the porosity and porous structure might change in the film damaged area leading to the change of film s hydrophilicity. 3.1 Depth of laser - damaged hole There are some laser-damaged holes appeared on the surface of the film. As shown in Fig. 4, the hole is irregular circular and the vertical depth of the hole was ± 0.10 nm, a little smaller than the thickness of the film ± 0.10 nm. We also find the width of scratches is somewhere between nm.

8 Depth = nm 0 nm 500 nm a) b) Width = µm 0 mv 160 mv c) d) Fig. 4 a) Topography of laser-damaged hole; b) Section of the hole along the white line in a) with nm depth close to the film thickness; c) Amplitude image of acoustic signal shows the scratches underneath the film; and d) Width of the scratches is around nm. 3.2 Residual stress The phase image of acoustic signal from another hole shows there is residual thermal stress around the laser-damaged hole. The average acoustic phase of this area is about 20 degrees higher than other area.

9 Fig. 5 residual thermal stress around damaged area 3.3 Hydrophilicity When the laser irradiation power exceeds the film s damage threshold, local porosity and porous structure might change in the film s damaged area which leads to the change of film s hydrophilicity. The sample in Fig. 6 was ongoing exposed to the air during the experiment. It is imaged with the PVM which is sensitive to water layer absorbed on the sample surface. The damping force of water layer will produce another vibration of the probe additional to the resonant vibration, which has greater effect on phase image of PVM than on height image. Fig. 6a) and b) were pictures of the sample when it is just taken out from the sealed bag. Fig. 6c) and d) were pictures 15 minutes afterwards, and Fig. 6e) and f) another 15 minutes later. Fig. 6a), c) and e) are height images, the others are phase images. Fig. 6g) is the section of 6f). The phase images show clearly that the film s damaged area and one side of splatter spot absorbed increasing amount of water from the air with the time exposed to the air while the undamaged area of film was hardly changed. The working frequency of this tip-sample system in PVM is f PV = 240 khz, while relative humidity is 45%. 0 nm 500 nm 0 20

10 a) b) c) d) e) f) g)

11 Fig. 6 The phase images of PVM shows that the absorption to water vapor in air in the damaged area of HfO 2 film was stronger than in the undamaged area. f PV = 240 khz. 4. Conclusion Quantitative and qualitative results of structural and mechanical properties of a nano HfO 2 film on SiO 2 substrate using SPAM are presented in this paper. Surface topographies show that the roughness of film surface is less than 3 nm, while a lot of scratches due to polishing are observed on the SiO 2 substrate s surface under the HfO 2 film with SVM. After high power laser irradiation, damaged holes with depth close to the thickness of film appear in the film. Residual stress due to heat absorbing is observed within film with SVM. The phase images in PVM show that the hydrophilicity of the damaged area on the HfO 2 film is higher than the undamaged area, which leads to stronger absorption of water vapor in the air. We also find that the detection depth of SVM is sensitive to the sample vibration frequency, which means appropriate frequency should be chosen for detecting scratches and residual stress underneath or in a nano film. In conclusion, SPAM can be used as a quantitative and qualitative evaluation tools for subsurface damage of optical film. It will help better fabrication and usage of optical film. Acknowledgements This work was supported by the National Natural Science Foundation of China (No ). References 1. A. Callegari, E. Cartier, M. Gribelyuk, et al., 'Physical and electrical characterization of hafnium oxide and hafnium silicate sputtered films', J. Appl. Phys., Vol. 90, No. 12, pp , J.D.T. Kruschwitz and W.T. Pawlewicz, et al., 'Optical and durability properties of infrared transmitting thin films', Appl. Opt., Vol. 36, No. 10, pp , M. Alvisi, F. De Tomasi, M.R. Perronea, et al., 'Laser damage dependence on structural and optical properties of ion-assisted HfO2 thin films', Thin Solid Films, Vol. 396, No. 1-2, pp , H.R. Zeng, H.F. Yu, L.N. Zhang, et al., 'Local elastic response of individual grains in lead free Nb doped Bi4Ti3O12 piezoelectric ceramics', physica status solidi (a), Vol. 202, pp. R41 R43, Feb 2005.

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