Synchrotron Radiation Mirrors (Introduction)

Synchrotron radiation is a high-performance instrument for many kinds of science and industry now. The increasing interest in this light source opens new possibilities for fundamental and applied researches. Synchrotron Radiation (SR) tools replace researchers into atomic scale world due to extremely small wavelengths of SR and ultra high vacuum chambers. Thus exceedingly stringent specifications are placed on the used optical component. The high heat conductivity and low thermal expansion are necessary for SR substrates (lower deformation) as well as good optical machinability and long-term stability.

The quality of grazing incident optics is accepted to characterize by Surface Figure Error user friendly. This term describes the maximum (PV) or average (RMS) deviation of the actual form from the ideal surface. Since the quality of the focus with grazing incident optics is primarily determinated by the slope distribution on the surface, it is more common to use the RMS Slope Error as a specification for global form accuracy. Typical value is from 0.05 microRad/rms (for flat surface) till 0.1 microRad/rms (aspherical surface). For visual Interferometric inspection is used the shape accuracy as part of test wavelength(pv or rms), e.g. « λ/130(rms)@633nm » - at the test wavelength λ=632.8nm. Interferometric measured technique is best suited for plane and spherical surfaces. Aspherical surfaces can be tested if a component-specific null lens is used. This lens compensates the wavefront aberrations resulting from the aspheric deformation. Micro-Roughness is measured by Microinterferometer or 3D Optical Profiler with resolution of better than 0.1nm, e.g."NewView 5000" Zygo Co.

Typical surface geometry of synchrotoron mirrors
  • Flat => ( best slope error is reached);
  • Sphere, Cylinder => ( very good slope error);
  • Toroids, Elliptic/parabolic cylinder, Elliptical toroid => (good slope error);
  • Ellipsoid (rotary), Paraboloid, Hyperboloid; FreeFORM Surface  => (good slope error).
Typical mirrors substrate materials
For low SR flux:
  • Zerodur®, Astrositall®( Sitall CO-115M)
  • Fused Silica
  • ULE™
  • Glasses (Pyrex, BK7, …)
For high SR flux:
  • Silicon (single crystal)
  • Silicon Carbide (CVD)
  • Cu with electroless Ni layer
  • Al with electroless Ni layer
Manufacturing Techniques
There are two techniques for SR Mirrors: Direct Manufacturing and Replication by negative masterform.
The direct manufacturing process generally includes the following steps:
  • Grinding approaches for pre-manufacturing of substrates and optical surface geometry;
  • Etching for reducing stress and subsurface damages;
  • Lapping for performing good thermal contact at the side faces and for optimizing the optical
    surface for subsequent steps;
  • Polishing for correcting and smoothing the surface by several steps.
To achieve the desired quality, a very close interaction between metrology and polishing is necessary.
Depending on the type of mirror geometry and on the required accuracy, the fine correction of residual errors has to be performed by:
  • conventional polishing; for Plane & Spherical mirrors, rms-Roughness:<2 nm; ~4 Å (Magnetorheological)
  • computer controlled fine-correction polishing -tool for high-end figuring of aspherical(<0.1 microRad)
  • Ion Beam Figuring - powerful tool for high-end figuring of optical surfaces with any form (<0.1 microRad)
  • Metal Mirrors can also be performed by Diamond Turning methods and Replication Technique.
C o a t i n g s
    Commonly used coating materials: Au, Pt, Rh, Ni, Pd, Al, Si, Ru, SiO2, Al/MgF2 etc. In some cases (e.g. Ru) a thin Cr binding layer (~0.4 nm) is necessary for reducing stress and also for keeping the microroughness performance. Practically each of coater-producers have “know-how” for Art coatings of Extra UV HR Mirrors. ZILTA offers the “Special EUV HR” (=>Xuv) for lambda < 50nmtoo. Nominal Reflection for different metallic coating at AOI =75 degree for VUV mirrors (Theoretically, nonpolarized ):
Platinum Gold
Standard EUV (Au_40nm/ Cr_binder)
Nickel
  R=60% (55-58%)@200nm -65nm   R=60% (55-58%)@200nm -65nm  R=64% (68-60%)@200nm -120nm
  R=66% (60-69%)@ 65nm -27nm   R=62% (55-65%)@ 65nm -25nm   R=58% (60-56%)@120nm - 40nm
  R=57% (60-55%)@ 27nm -22nm   R=65% (70-60%)@ 41nm - 30nm
  R=62% (60-65%)@ 22nm -12nm   R=64% (61-70%)@ 25nm -15nm   R=45% (60-30%)@ 30nm - 20nm
  R=52% (55-50%)@ 12nm -10nm   R=65% (71-70%)@ 15nm - 9nm   R=35% (30-40%)@ 20nm - 16nm
Mirror Reflectivity is calculated using the Fresnel equations for a semi-infinite medium on line
Synchrotron Optical Elements from ZILTA
  • PLANE MIRRORS used for deflection/offset and filtering(see on next picture);
  • SYNCHROTRON MIRRORS for focusing (any form of Surface);
  • Optical elements based on refraction:X-ray refractive lens(under construction)

Reflectivity at 1 Angstrom and Energy Bandpath at the critical angle for Au, Be and C coatings acting as a high energy cut-off.
Synchrotron Mirrors from ZILTA

 

Substrate material: Sitall CO-155M (Zerodur); Fused Silica; Si; SiC CVD etc.
HR COATING (with Ion Beam Assistance): Standard EUV (Cr_binder +Au_40nm): HR @ 25÷ 200 nm at AOI
Special EUV (Cr_binder +Au +Ingredients): =>XUV HR @ 2÷70 nm at AOI

 

Ultimate Specification Standard Specification Control method and Notes
Dimensions Up to 1200 mm Up to 500 mm Customer’s drawing
Clear aperture,
active size (CA)
Up to 100 % 90 %
Tolerances for
Radius of Curvature
Down to 0.01 % 0.5 %
Unique Diamond Samples specially for custom/drawing. Calibrated at Institute of Standard 
Micro-Roughness
[nm], RMS
0.4 ~2
Micromap measurements 0.05nm 3D
Optical Profiler "NewView 5000" Zygo Co
Slope error,
[microRad], RMS
0.1 – longitudinal
0.5 - transverse
5– longitudinal
15 - transverse
Please see on page 9 of TSAM
”Aspherical concave surface control”
Lateral shift Interferometer: Individual
3D-map topography of surface figure
Shape accuracy
for λ test =632nm
λ / 130 λ / 25
Sitall CO-115M (Astrositall®)

SITALL СО115М is a crystalline glass ceramic material with ultra low Coefficient of thermal expansion. SITALL СО115М is an ideal material to be used for manufacturing of astronomical telescopes mirrors and other optical parts that require linear dimensions and surface figure stability at significant temperature changes. Thermal stability of SITALL material enables the fabrication of high precision mirrors within quicker time of treatment process since testing measurements are able to be conducted not waiting until thermal equilibrium is achieved.
Astrositall blanks with various forms of lightweightening

THERMAL PROPERTIES
mean linear coefficient of thermal expansion (CTE)
within temperature range -60 to 60 oС
10-7 oC-1 0±0.6
CTE homogeneity 10-7 oC-1 < 0,2
specific heat capacity kJ kg-1 oC-1 0.22
heat conductivity kilocalorie h-1 m-1 0C-1 1.71
thermal resistance 0C 550
MECHANICAL PROPERTIES
density g cm-3 2.46
micro hardness 107Pa 786
lapping relative hardness
(with reference to glass, K-8 type)
- 1.78
modulus of elasticity (Young’s) GPa 92
bending strength 107Pa 16.6
Poisson’s coefficient - 0.28
OPTICAL PROPERTIES
refractive index ne - 1.538
refractive index nd - 1.536
average variance nF - nC - 0.0102
stress birefringence nm/cm up to 15
ZILTA: Mirror Manufacturing Draft Quality Plan

Metallic coating is deposited under cryogenic vacuum and the thickness is monitored by a controller. Complete mirror processing is done in clean room, mirror is handled only with gloves in order to void carbon contamination.
Test documentation
A complete report including all data of performed measurements as described before is established for each optical piece. Test documents are delivered together with optical pieces.
Packaging
Each optic will be packaged in its own protective container. The container is the membrane box, and is designed to prevent dust, contamination and contact of any part of clear aperture. The packaging of each optic will clearly identify its serial number.
Packing and Delivery
Packing for shipment will insure that each optic is insulated from severe shock and rough handling. Each optic will be delivered with its own documents: optical test report and conformance certificates.

Optical elements based on refraction: X-ray refractive lenses (under construction)
 
 
 

Z I L T A Co. unlimited © 2017, Lithuania European Union, Fax: +370 7000 4444, E-mail: zilta[eta]lasertechn.com

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