METROLOGY SERVICES
Inprentus has 10+ years of expertise in assessing key properties to benchmark a grating’s performance. With deep subject matter expertise stemming from first principles of optics, Inprentus can provide you with world class characterization of your optic. Please find below the techniques Inprentus offers as a metrology service. We love new challenges - if you have a characterization requirement that is outside of these techniques, please contact us!
Fizeau Interferometry
Inprentus has pioneered the use of Fizeau interferometry to produce an accurate wavefront displacement map that can be used to calculate the intrinsic resolving power for diffraction gratings. This technique is faster and more accurate than a long trace profilometer and has been used to measure the errors in diffraction gratings with resolving powers exceeding 1 x 105. This technique is optimized for constant line density gratings on planar surfaces, but large radius of curvature or a gentle variable line spacing grating may also be measured without requiring stitching.
Atomic Force Microscopy (AFM)
AFMs can provide fine detail on individual groove shapes and dimensions, as well as provide quantitative measurements on the high frequency roughness of a surface. While the technique can be generalized to looking at many surface properties, Inprentus has years of experience imaging thousands of gratings using in-house Nanosurf AFMs. Our metrology service leverages our experience in imaging a variety of gratings and coatings. Inprentus can provide imaging services for any grating, including blazed or binary structures, masters or replicas. Our imaging has been used on gratings of the following characteristics:
Pitch: 150 nm to 20 µm
Groove depth: < 10 nm to 4 µm
Angles*: 0.1° to > 70°
In addition to the raw scanning data, Inprentus will process the scans using our internal angle fitting software to provide statistics on the dimensions and angles for all grooves measured.
*Angles measured are limited by the AFM scanning probe tip and typically cannot be relied upon above 70°. Please see our FIB/SEM service for measuring grooves with steeper angles.
Focused Ion Beam (FIB) / Scanning Electron Microscopy (SEM)
SEM uses a focused electron beam to image nanoscale features with sub-nanometer resolution. While the SEM can be used to inspect a large variety of features, Inprentus mainly focuses on imaging diffraction grating groove shapes. Groove pitch and depth dimensions are often at or below the diffraction limit, making them impossible to resolve using traditional optical metrology techniques. Groove shapes may also contain vertical sidewalls or undercut features that are difficult to access using contact or probe-based methods like AFM. A combination of FIB and SEM produces cross-sectional images of diffraction gratings allowing accurate measurement of the groove height profile. Inprentus can provide FIB/SEM imaging services for gratings having blazed, binary, and even undercut structures. Along with the raw image data, height profile images using our internal angle fitting software to provide statistics on the dimensions and angles can also be provided.
Equipment Description: Helios NanoLab 600i (SEM/FIB DualBeam)
• SEM Resolution: 0.9 nm at 15 kV
• FIB (Ga+) Resolution 4.0 nm at 1.1 pA, 30 kV
• Maximum ion beam current 65 nA (rapid ion milling)
• Platinum deposition GIS (protective layer)
• Maximum substrate size 150 mm
Two circle diffractometer for Optical Blaze Angle Measurement
The defining feature of a blazed grating is the concentration of diffracted light into small patches of angle space. To measure the angular distribution of diffracted light, Inprentus uses a custom diffractometer built on a two-circle goniometer. The light source is a polarized He-Ne gas laser operating at 543 nm. Note that the source energy does not, in general, need to be at-wavelength.
The wave vectors of incoming and outgoing light can be selected independently, and the instrument can be configured to allow for pseudo-backscattering geometries. For a given surface profile (blaze), the angular distribution of 543-nm light can be rigorously calculated for each incident wave vector and compared with experiment to determine blaze fidelity. This technique thus gives the blaze and anti-blaze angle measurements at the grating level as opposed to localized measurements using techniques such as AFM or FIB/SEM.
Two circle diffractometer for Stray light Measurement
The defining feature of a blazed grating is the concentration of diffracted light into small patches of angle space. To measure the angular distribution of diffracted light, Inprentus uses a custom diffractometer built on a two-circle goniometer. The light source is a polarized He-Ne gas laser operating at 543 nm.
Random errors in the positions of the constituent grooves, and possibly in the shapes of the grooves, produce a dim, uniform scattering background in angle space. Inprentus can configure the diffractometer to measure a normalized (with respect to blazing) straylight measurement where the diffuse background light is collected at varying wavevectors. This measurement is available in two modes:
· Angle integrated mode
· Angle resolved mode
248 nm Reflectometer for Efficiency Measurement
Inprentus has commissioned a 248.6 nm reflectometer to measure efficiency of gratings used for lithography systems and DUV spectrometers. This set up consists of a NeCu pulsed laser and can make measurements in both S and P polarization. The reflectometer uses a beam splitter and two detectors to account for the shot to shot noise of the pulsed laser supply. Using a calibrated mirror as a reference, Inprentus provides absolute efficiency measurements as a service. Currently the reflectometer measurement is configured for UV wavelength but in principle can be extended to other wavelengths.
In addition to efficiency measurements, Inprentus simulation team has the ability to compare the measurement to that of an ideal grating of perfect grooves of specified angles and pitch. We can also compare it to simulations of sampled, real groove shapes within the grating.