IMAGING HARD X- Ray Optics Fabrication and Analysis Vinita Navalkar JRF, DAA CONTENTS  Introduction to Hard X-ray Optics  Principles of X-ray Multilayers  Installation of Sputtering System  Mirror fabrication process        Cutting of glass sheets Cleaning of glass sheets Slumping of glass sheets AFM studies SEM studies Sputtering process XRR studies HARD X-RAY OPTICS  Focusing optics have not been used at energies above 10 keV.  The critical angle ‘c’ for the total external reflection is inversely proportional to the photon energy ‘E’. So as the ‘E’ increases, the ‘c’ decreases further, thus reducing the overall field of view of the telescope. This in turn changes the ratio of the mirror diameter to focal length of telescope.  The solution to the problems, is the development of super mirrors (mirrors that can reflect hard X-rays) using multilayer coatings.  Reflectance at angles greater than the critical graze angle can be achieved by using depth graded multilayer coatings on the mirror surfaces.  The stack acts as a periodic lattice and the Bragg condition will create constructive interference according to mλ=2dsinθ PRINCIPLES OF X-RAY MULTILAYERS  A continuous reflectance could be obtained by having d-spacing given by di =dc / i 0.25  The increase in the number of layers increases reflectivity due to a more continuous distribution of Bragg peaks, up to a point where the absorption becomes dominant and outweighs the benefit of adding layers.  The probability of reflection at the interface is proportional to the density contrast between the two materials and hence multilayers are composed of a high density and low density material. MIRROR FABRICATION PROCESS CUTTING OF GLASS SHEETS Cutting of Glass:  The glass used as a substrate is a 0.2 mm and 0.3mm thin glass sheet available in 440x360mm dimensions. These sheets are needed to be cut into desired sizes.  This is done using a diamond cutter on hard clean bench with laminar air flow.  A diamond cutter is used to cut all the sheets MIRROR FABRICATION PROCESS CLEANING OF GLASS SHEETS Cleaning of glass sheets: The procedure used for cleaning involves: 1. Rinsing the glass by non-alkaline soap water thoroughly using a soft flint free flannel cloth. 2. Cleaning with distilled de-ionized water multiple times to remove all the trace of soap water on it. 3. Washing it with Acetone. Acetone being a good solvent for water, it absorbs all the moisture from surface of the glass. 4. Drying it by blowing hot nitrogen gas at around 70o C. It vaporizes the remains of acetone from the surface. MIRROR FABRICATION PROCESS SLUMPING OF GLASS SHEETS Slumping of glass sheets:  The glass sheets are molded into cylindrical shell using a quartz cylinder molds. For molding two programmable furnaces are being used.  The furnaces can be programmed to do multiple cycles of raising the temperature at a specified rate and then holding at that temperature for a specified time within the temperature range. MIRROR FABRICATION PROCESS SLUMPING OF GLASS SHEETS MIRROR FABRICATION PROCESS SLUMPING OF GLASS SHEETS MIRROR FABRICATION PROCESS SLUMPING OF GLASS SHEETS T2 T1 R1 H1 H2 R2 R1 10o C/min R2 5o C/min T1 529o C T2 670o C H1 10mins H2 5 mins MIRROR FABRICATION PROCESS ATOMIC FORCE MICROSCCOPE (AFM) STUDIES AFM studies:  Thermally formed glasses as well as plain glass before forming were scanned for surface quality using Atomic Force Microscopy (AFM).  Gwiddion and SPIP were used for analysis.  The average roughness of slumped glass was around 7 pm. MIRROR FABRICATION PROCESS SCANNING ELECTRON MICROSCCOPE (SEM) STUDIES SEM studies:  Cut glass pieces were scanned under SEM before subjecting it to cleaning procedure to check the cleanliness of the pieces.  SEM analysis also helped in developing a good cleaning procedure.  Tungsten coated glass pieces were also scanned under SEM to check for any non-uniformity during the sputtered deposition.  These images helped in developing the cleaning procedure, as they showed the stains of propanol/acetone left on the glass.  They also showed any minor bump on the surface of the glass after slumping. MIRROR FABRICATION PROCESS SCANNING ELECTRON MICROSCCOPE (SEM) STUDIES Cut glass piece scanned before cleaning Cleaned glass showing propanol stains MIRROR FABRICATION PROCESS SCANNING ELECTRON MICROSCCOPE (SEM) STUDIES Glass piece slumped with silicon cloth. Tungsten coated glass pieces of same sample MIRROR FABRICATION PROCESS SPUTTERING PROCESS Sputtering Process:  In hard X-ray telescope each mirror will be coated with multiple layers of Silicon and Tungsten by RF magnetron sputter deposition.  RF magnetron sputtering is used for silicon being a semiconductor and magnetron system helps in confining the plasma to area where magnetic field is strong. Nearness of plasma to target causes faster deposition rates, greater Argon ion replenishment and less substrate damage.  To get desired deposition, four major parameters are required to be fixed.  Applied RF power  Deposition pressure of Argon (Ar) gas inside the sputtering chamber  Rotation speed of the substrate holding table  The distance of the substrate from the target. INSTALLATION OF SPUTTERING SYSTEM  In hard X-ray telescope each mirror will be coated with multiple layers of Silicon and Tungsten by sputter deposition.  This RF magnetron sputtering system deposits alternate layers of tungsten and silicon.  RF magnetron sputtering is used for silicon being a semiconductor and magnetron system helps in confining the plasma to area where magnetic field is strong. Nearness of plasma to target causes faster deposition rates, greater Argon ion replenishment and less substrate damage. MIRROR FABRICATION PROCESS SPUTTERING PROCESS MIRROR FABRICATION PROCESS SPUTTERING PROCESS Sputtering system software and control unit MIRROR FABRICATION PROCESS XRR STUDIES XRR Studies:  XRR facility in AB-88 was used to test the coated samples for the deposited layer thickness and also the roughness at interfacial level.  ‘PANalytical X’Pert Epitaxy’ software was used to perform the XRR analysis.  XRR analysis were also done at RRCAT, Indore.  Samples with two bilayer coating on plain as well as curved glass, were sent to RRCAT in March 2014, to perform XRR analysis on it. The results were analyzed by me with the help of Dr. Sanjay Rai. MIRROR FABRICATION PROCESS XRD STUDIES XRD Analysis of W-Si 2 bilayers on Silicon wafer using TIFR XRD facility MULTILAYER FABRICATION PROCESS XRD STUDIES XRD Analysis of W-Si 1 bilayers on Silicon wafer using TIFR XRD facility ACKNOWLEDGEMENTS I would like to thank my supervisor, Prof. K. P. Singh, for all the guidance and support. I thank Dr. Sudip Bhattacharya for his guidance and support in data analysis of SXT. I thank Mr. Harshit Shah, Mr. Vishwas Risbud, Mr. Vilas Mhatre, Mr. Jayprakash Koyande, Mrs. Nilima Kamble, Mr. B.G. Bagade, for your valuable help and support in the laboratory work.