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MRS-5 Magnification Reference Standard
1,500x - 1,000,000x
Pattern Design, Traceability, Recertification Program,
Operating Parameters and Magnification Requirements

Pattern Design

The MRS-5 is fabricated by using the highest accuracy electron direct write semiconductor manufacturing equipment available today. The pattern is built on a silicon wafer with a 400nm SiO2 layer and then 65nm of tungsten on top. Imaging contrast in both secondary and backscattered electron mode is very high. The overall size is ~/=2mm x 2mm x 0.5mm thick. For applications requiring an electrically conductive sample (SEM at ~/=1.5 keV and higher), the MRS-5 is coated with a proprietary carbonaceous material which allows for image observation at any accelerating voltage. A distinct advantage of this coating is that electron beam tracks from are removable by plasma etching. Applying a fresh coating restores the MRS-5 to like new condition. This is all done in the manufacturing Lab.

The geometric design of the MRS-5 has three different types of patterns.

Scanning Microscopies and Profilometry: STM, AFM, stylus and optical etc. The pattern height is 0.1µm.
Groups of nested squares spanning several orders of magnitude with pitches of 80nm, 100nm, 200nm, 500nm, 1/2µm, 1µm, and 2µm. to allow for more testing the 80, 100 and 200nm patterns are repeated four times.
Newly incorporated into these standards is an extension of the 1951 USAF 3-bar targets. These finer patterns have pitches ranging from 80nm to 3µm in 15 steps. They will find good use measuring the resolution of state-of-the-art optical microscopes (UV, confocal, laser scanning, etc.
The 1/2µm square test pattern will help analyze your images for all types of dimensional distortions, vibrations and magnetic fields. This pattern contains 1/2µm squares with a 1µm pitch over a 20µm X and Y field.

Pitches are measures and certified (the distance between repeating parallel lines using center-to-center or edge–to-edge spacing). These are is the only type of measurement that can be used to relate measurements from different microscopy techniques (see “Submicrometer Linewidth Metrology in Optical Microscopy”, Nyysonen & Larrabee, Journal of the Research of the National Bureau of Standards, Vol. 92, No. 3, 1987). Linewidth measurements (the measurement of a single line or space width) can only be related if the same type of illumination is used as for the calibrating instrument since edge effects lead to uncertainty in the edge locations. Using pitch measurements, errors from edge-to-edge locations cancel as long as like positions are measured.

Square boxes and the 1µm pitch square test patterns are used for measuring magnification simultaneously in the X and Y directions. This gives a measure of image skew, barreling, pincushion and other non-linearity’s which can have various origins, such as from stray magnetic fields. With the MRS-5, a large range of pitches are provided, to closely match the needs of your instrumentation.

The largest pattern has an overall dimension of 40µm square. It contains lines and spaces that are nominally 1µm wide. This can be used to check magnifications around 1,500x. The smaller patterns will allow calibrations up to 1,000,000x. The “ruler” has an overall length of 6mm in the X and Y directions. The 1µm increments have graphics every 10µm and emphasized lines and graphics every 50µm and 100µm.

The 3-bar targets were included as a response to many requests for a standard capable of measuring resolving power for patterns smaller than the 1951 USAF targets. They are often found covering a range of 0.25 to 228 cycles/mm. The standard target element consists of two patterns (two sets of lines) at right angles to each other. Each consists of three lines separated by spaces of equal width. The bar length to width ratio is 5X. The patterns change size exponentially in groups and elements. The range in line length for the original target was from 10mm to 0.08769mm. Others have expanded the range towards finer patterns. The range is now extended to a line length of 0.00004mm! With these 15 patterns measurement of modulation transfer functions is made much simpler.

Standard Resolving Power Test Target Element.
From MIL-STD-150A: "The patterns of lines are parallel lines 2.5 x millimeters long and 0.5 x millimeters wide with space 0.5 x millimeters wide between the parallel lines, where x equals the number of lines per millimeter."

The patterns can also be used for determining imaging and chemical spatial resolution and chemical mapping.

The patterns can also be used for resolution and chemical mapping.

Pattern	Pitch Spacings
Nested boxes	2µm	1µm	500nm	200nm 4 each	100nm 4 each	80nm 4 each
3 bar targets	3µm	2µm	1µm	1.5µm	1µm	900nm
3 bar targets continued	800nm	700nm	600nm	500nm	400nm	300nm
3 bar targets continued	200nm	100nm	80nm

AFM image taken with sharpened tip (NPL) - 80nm pitch pattern

geller mrs-5 magnification reference standard

Traceability

Scanning electron microscope,
type II secondary electrons

Scanning electron microscope,
type I secondary electrons

Why should you consider a traceable standard? Beyond the requirements of national and international quality standards, purchasing a CRM (certified reference material) from a national laboratory or a traceable standard from a certifying body guarantees dimensions. Most commercially available standards have unknown accuracies. Measurements determined by optical methods measuring pattern frequency do not tell you about the individual variations in pitch measurements- and this is what you image. The MRS-5 is offered with or without traceability. The nontraceable standard differs only in documentation and cost. Traceability in the X and Y dimensions is established from a MRS-5 that has been measured by NPL (National Physics Laboratory), the NIST counterpart in the U.K. The nonaccumulative 2s pitch uncertainty is ±3nm. Through international agreements, our ACLASS accreditation provides equivalency with NAVLP and A2LA.

Through national laboratory mutual recognition agreements NPL measurements are equivalent to NIST. Traceable measurements are only provided for the 2, 1 and 0.1 µm pitch patterns. Details can be found on the provided measurement certificates.

Recertification Program

It is often asked why the MRS needs re-certification. Under ISO-17025 guidelines your quality department should determine the re-certification interval as they are most familiar with your company’s quality system requirements. Recertification is a common practice for devices such as gage blocks and electronic instruments. Over the years several standards have been found which could not be recertified due to physical damage and excessive contamination. In a few cases, electron beam damage and corrosion from storage in a contaminating environment have been seen. Re-certification insures your standard will perform its proper task and that you will be meeting your quality system directives. Recertification includes cleaning and application of a new coating, if needed.

Operating Parameters

Optical microscopes can use the MRS-5 in reflected light imaging modes.

Magnification Measurements

Optical Microscope magnifications can be measured directly on viewing CRTs, in reticles mounted within the ocular, or directly on photomicrographs. For instruments with verniers or electronic calipers distance measurements can be verified using a pitch pattern of appropriate size. Magnification is simply the image size divided by the object size (be careful to use the same units).

Scanning Electron Microscopes can use the secondary electron signal at an accelerating voltage of <=5keV preferably with type II secondary electrons to get more of a surface image. The low voltage requirement is due to the line height variations, as shown in the AFM image to the left, which reveal penetration differences. Backscattered electron imaging is not recommended for the same reason. For SEM applications we recommend the device with the electrically conductive coating for two reasons. First, it the tungsten film is electrically isolated from the silicon chip by a thin SiO2 layer and second, the coating is removable. When it is removed electron beam tracks from contamination go with it. A cleaning service is provided for a nominal fee. Cleaning is included at no charge with the suggested yearly recertification program. To avoid pattern damage use <1 nA electron beam current.

Scanning Probe Microscope operators must be aware of the fine dimension of the pitch patterns. The 80nm pitch has a nominal space width of 40nm. The cantilever tip must be smaller to define the pattern. As shown in the AFM image to the left the pattern lines have variable height.

International Calibration Standards

ASTM E766–2003: Standard Practice For Calibrating The Magnification Of A Scanning Electron Microscope
ISO 16700:1994 MICROBEAM ANALYSIS — Scanning Electron Microscopy — Guidelines For Calibrating Image Magnification
There are no international standards for SPM magnification calibration at this time

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