Anne Fallon

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Introduction

The application of measuring surface texture with a white light optical profiler has been well-known for many years. As the capabilities of optical manufacturing and precision machining increase, the production of ‘super smooth’ or ‘sub-angstrom’ surfaces has become more common, and quantification of these surfaces is critical for effective process control. The NewView™ 6000 series of optical profilers using Scanning White Light Interferometry (SWLI) with MetroPro™ software and patented FDA analysis enable rather straightforward quantification of surfaces with texture measured on the order of fractions of a nanometer. With good control over the measurement environment, proper selection of measurement parameters, and effective instrument calibration, quantification of surfaces with roughness measured in tens of picometers (1x10-12 m) is possible. With the NewView 6300’s best-in-class acquisition speed and resolution, areal measurement of supersmooth surface texture has never been so easy.

Understanding System Noise

The first step required in making quantitative measurements of super smooth surfaces is to understand that every measurement system has an inherent baseline system noise. This noise results from a number of factors including electronic noise, sensor noise, small irregularities in the reference surface, and small vibrations caused by changes in the measurement environment. For most samples, the measurement noise in the NewView can be essentially ignored, as the measurement value is much larger than the noise floor. However for very smooth samples, this is not the case. For these samples it is important to understand the noise sources and to control them as tightly as possible. Many sources of noise can be virtually eliminated or at least significantly reduced by both tightly controlling the measurement environment (acoustics, air currents, temperature, etc.) and also by performing a number of measurements and averaging them together into a single data file.

Environmental Controls 
The first task in setting up measurements for a super smoothpart is establishing control over the measurement environment. The ideal environment would be one which:

• is mechanically and acoustically quiet to minimize part vibrations;

• has tight temperature control to minimize sample and objective changes during the measurement period;
• has well controlled airflow to minimize air currents between the microscope and the part.

In order of importance, vibration, noise, and temperature rank at the top. When the objective working distance is small, airflow control may essentially be a non-issue after mechanics, acoustics, and temperature have been addressed. With long working distance objectives, however, air currents will be more critical to control.

Measuring the System Merit Function 
ZYGO has developed a process for quantifying the expected system noise as a function of measurement averages which we will call the System Merit Function (refer to the last page of this document for an illustration of this measurement method). The measurement process involves acquiring a number of measurements—typically 10 or more—with a given number of averages. Each of these data files are saved as D1, D2,…Di. These data are averaged together into one single file, Dse which represents the total system error during the measurement period. This Dse file is then subtracted from each of the component Di files to create an error map indicative of the expected system noise for a single measurement. The rms of the individual difference maps are recorded, and the mean and standard deviation are calculated for the series of differences. By adding twice the standard deviation of the series to the average rms of the series, the expected system noise for a given number of measurement averages can be estimated with good confidence. This process can be automated by using standard MetroPro and some very simple MetroScripting. An example application is available upon request from ZYGO.

Predicting System Noise 

Once the value of the System Merit Function for measurements with no averaging is known, the predicted system noise for a specific number of averages can be predicted using the formula.

where SN1 is the system merit value measured with no measurement averages and AVG is the desired number of averages. For larger numbers of averages taking a longer time period to measure (typically greater than 32 measurements) this prediction can only hold true in very well controlled environments. For critical applications, it is recommended to test the measured noise floor against the predicted value and ensure that the environment is controlled well enough – morewell-controlled environments will generally require fewer averages. In the event that the environment is not satisfactory, the line for the measured values in Figure 1 will typically turn upward again and diverge from the predicted values. If a noise floor associated with averages beyond the upturn point were desired, it would be necessary to further improve the environment.

Figure 1 –Excellent correlation is observed between predicted and actual system noise on the NewView 6300

Phase Res - Which Level?

Depending upon the smoothness of the surface to be measured, it may be necessary to increase the internal precision of the calculations made by the MetroPro software. Starting with version 8.1.1, MetroPro allows for three levels of precision with the Phase Res Measurement Control—Normal, High, and Super.Normal is the lowest resolution—useful primarily for large steps and rough surfaces. High is the standard setting for most typical measurement situations using scans up to 150µm and texture down to approximately 0.050 nm. The newest and highest precision setting, Super, enables measurement of very smooth surfaces using a large number of averages. Only very smooth surfaces will require the use of Super. This should be taken into consideration when determining baseline system noise.

What Noise Floor do I Need?

There is no one right way of selecting the number of averages (and by extension, the noise floor) for a particular application. For rougher surfaces, where the surface is measured in tens of nanometers or more, striving for a system noise on the order of 10x lower is often recommended. However for a surface which is on the order of 0.05 nm (0.5 Å) achieving a noise floor 10 times lower would theoretically require approximately 3000 averages! Rather than a hard and fast rule, a more empirical rule of thumb employed by ZYGO is that the lowest practical noise floor for an application is recommended, but that system noise should be at least 2 to 4 times smaller than the desired measurement surface features.

System Error Characterization

After determining the phase resolution and the number of averages required for the desired noise floor, it is recommended that the user perform a system error characterization. This process entails measuring a number of physical locations on an optical grade flat using the desired number of averages per site. 
Typically, at least 8 distinct sites with no overlapping regions are recommended for generation of a system error file. For specific information and procedures for creating a system error file, please refer to the NewView MetroPro Microscope Application Booklet, OMP-0360 or Section 8, MetroPro Reference Guide, OMP-0347. The error map created will then be subtracted from each of the surface measurements made on the actual sample.

Figure 2 - A graphical representation for the process of measuring the System Merit Function

Conclusion

Using the methods and procedures described here, ZYGO has demonstrated the capability of measuring surfaces smoother than 0.05 nm. Tightly controlling the measurement environment, selecting an appropriate internal precision, and choosing the number of phase averages based on the System Merit Value all combine to provide the highest quality surface texture measurements available from an optical profiler.

Every part’s surface is made up of texture and roughness which varies due to manufacturing techniques and the part structure itself. To understand a component’s surface and to control the manufacturing process to the degree required in today’s modern world, it is necessary to quantify the surface in both two and three dimensions.

Surface texture parameters can be grouped into these basic categories: Roughness, Waviness, Spacing, and Hybrid.

An Introduction to Zygo Metrology Solutions for improving product quality and reducing cost

The Zygo Metrology Group at Lambda Photometrics delivers precision industrial metrology solutions for QA and Statistical Process Control in manufacturing to help reduce the cost of defects and ensure your customers receive the best possible engineered components. Non-contact metrology from Zygo for manufacturing and R&D provides you with the tools to do the job quickly and effectively whilst providing the flexibility to adapt to your needs as your customers demands change. At Lambda Photometrics we recognise that our customers are faced with growing pressure to improve the surface form, finish and performance of components they manufacture and we have developed leading edge metrology solutions to help deliver such capabilities to you. Our high speed and precision metrology tools are capable of measuring: 3D surface profiles, roughness, texture and machining marks, step height, features, cone angles, wear volume, flatness, surface form, radius of curvature and many other parameters associated with precision engineered components, optics and MEMS (micro-electro-mechanical systems).

A wide range of industry sectors including automotive, medical, optics, IT, semiconductor, aerospace and precision engineering have benefited from working with Lambda Photometrics:

Improved component quality Reduced waste Reputation for performance and quality from 1000’s of installed units worldwide Established over 35 years with experience to match Manufacturing and R&D metrology solutions tailored to your needs Installation and support on site Training to ensure you get the most from your metrology tool Measurement service for smaller runs of components or R&D

If you have a pressing metrology issue and would like to know if we can help, try us out and send us a component sample to be measured, for more details see our sample measurement programme

What we measure

Precision metrology for a wide range of surfaces and in some cases films encountered in engineering and science. Our solutions allow surface profiles, form, flatness, waviness, texture, roughness, machining marks, material features and finish to be measured using high speed non-contact imaging technology. This delivers:

• Non-contact measurement of complete surfaces in 3D not just point samples or a line profile but height information across a whole 2D image so you do not miss a thing
• Powerful process software with easy to use interface allows you to extract familiar data such as 2D profiles at will and in any orientation
• Capture data at far higher speeds than can be achieved with contact methods such as CMM or stylus profilers. Up to a million data points can be captured in seconds
• To rapidly take the 3D data captured and allow comparison with conventional contact metrology
• Offer far higher precision particularly in the height dimension than can be achieved with conventional metrology tools, typically down to 0.1nm
• Allow complex image processing of data to perform sophisticated QA measurements or extract specific quantitative data from the measurement e.g wear volume
• Automate the process to reduce or eliminate operator intervention
• Provide rapid measurement of serial components for factory floor 100% inspection

Typical measurements:

• Surface form and profile
• Departure from spherical and planar form
• Flatness
• Radius of curvature measurement for concave and convex surfaces
• Volumetric wear
• Surface features and machining marks
• Roughness and a wide range of texture measurements for surfaces including Ra, Rpm, Rz (conforms to new ISO standards). For a full list of measurements download this document Texture
• Waviness and a wide range of parameters user selectable through filters
• Cone angle and recessed features
• Step height and multiple surfaces
• Thin and thick films

Traceability to certified standards

Our metrology tools provide unrivalled performance that is traceable to certified standards to ensure you achieve the highest accuracy, reproductibility and repeatability in your application. For more details download this document standards measurement.

Other typical applications include:

Orthopaedic implants

Measurement of radius of curvature Roughness and other texture parameters Machining and wear marks Wear volume

Contact Lenses

Surface form Radius of curvature Roughness

Diesel Injectors

Flatness Roughness Multiple surfaces and step heights Feature dimensions Cone angles Recessed features

MEMS - Micro Electro Mechanical Systems

Deflection & Dynamic measurement Roughness Flatness Step heights Multiple surfaces Film thickness

Data storage

Surface quality and defects Flatness Runout

The tools we use

The GPI, is a Fizeau type interferometer system coupled to an electronic camera and computer system for control and image processing.

• Non-contact high speed imaging metrology tool
• Surface form, flatness and measurement of spherical components (both convex and concave)
• Measures deviation from form of a complete surface area typically from several sq cm to hundreds of sq cm in seconds
• Measurement is guaranteed to better than λ/10 (63nm) across the whole viewing aperture of the instrument, the resolution of the instrument is far higher. Higher performance to better than λ/100 (6.3nm)has been demonstrated in some applications
• Radius of curvature of both convex and concave components to several microns
• Simple user interface for fast and easy measurement even by unskilled operators
• Can be automated and placed in protective housing for production and shop floor environments
• Built in environmental noise monitoring

The NewView, a scanning white light interferometer system incorporating a microscope objective lens, led light source, electronic camera and computer system for control and image processing

• Non-contact imaging metrology tool, captures up to a million 3D surface data points at high speed
• Measures roughness, texture, flatness, surface height, feature dimensions and thin films with unprecedented performance over conventional metrology tools
• Wide range of built in Roughness parameters from Ra to Rz and other texture parameters
• Typically measures sub sq mm to several sq cm
• Repeatable surface height measurement to 0.1nm
• Ability to measure film thickness, typically from 1 to 75 microns
• Wide range of accessories that also allow measurement of difficult recessed or concave components and features (using super long working distance objectives)
• Simple user interface embedded with powerful metrology applications for rapid and easy measurement
• Can be automated and placed in dedicated protective enclosure for application on factory and shop floor environments
• Built in environmental noise monitoring

How they work

There are two components to the metrology tools we provide, the hardware, comprising the instrument with PC and frame grabber and the software that controls and processes the data from the system. The combination of hardware and software provides fast and powerful metrology tools that deliver automated measurement solutions for QA and R&D in a fast and flexible way.

We use and supply two key instruments, the GPI and the NewView, for surface and profile measurement both driven by a common software package known as MetroPro. Both instruments are non-contact interferometric imagers that provide a 3D image of a surface to a very high degree of precision and with very high speed compared with conventional mechanical systems. Both provide a snap shot of the surface which when processed by the software allows you to extract a wide variety of parameters about a surface including making comparison with conventional measurement techniques. In the case of manufacturing this allows for automation and 100% inspection of components. For R&D the fast turnaround coupled to an easy to use interface makes productivity and metrology fast and accurate and with a precision far above mechanical methods this allows you to explore future capabilities and options.

The GPI interferometer comprises a laser light source, optics for focussing, expanding and collimating the beam and a camera for recording interference fringes. The GPI can be used for measuring flatness using a precision Transmission Flat or spherical surfaces using a Transmission Sphere, in this particular example we shall consider the measurement of a spherical surface. The Transmission Sphere used for this measurement is a focussing lens taking the collimated (flat wavefront) laser beam and focussing it into spherical waves that converge onto the part to be measured. The transmission sphere is a precisely formed system of optics that for most applications can be considered to generate a perfect spherical light wave converging on the part under measurement.

GPI Basic Operation

Some of the collimated light on striking the curved surface of the transmission sphere (a glass/air interface) is reflected back into the instrument to create a reference beam of light. In this example a spherical sample has been placed in proximity to the Transmission Sphere such that the spherical wavefronts impinging on the surface strike the surface at a normal and hence the light is reflected back along its original path and into the instrument. The reflected light from the part under measurement and the reference beam are combined to create an interference pattern that is detected by the camera.

During a measurement the Transmission Sphere is translated linearly along the optical axis with a piezo electric device to create a moving fringe pattern that is interpreted by the computer system to show the deviation of the part under test from an almost perfect sphere. The system displays these departures from spherical form using a colour coded image plot and also an oblique surface form plot, click here for examples of a spherical hip socket. The same principles can also be applied to the measurement of flat surfaces using a Transmission Flat as the reference.

The NewView scanning white light interferometer has similar elements and operation to the GPI. It employs optics in the form of a modified microscope objective, a light source, in this case an incoherent broadband LED light source is split at the objective so that some of the light passes to a reference mirror and some is focussed onto the surface of the sample under measurement. Light from the mirror and the sample surface are reflected back into the instrument and imaged onto a camera. If the distance from the light splitter to the mirror and from the splitter to the surface are equidistant so that there is no optical path difference (OPD) then the camera will observe an interference pattern. This occurs when the objective is held so that the focal plane of the objective lies in the same plane as the surface.

NewView Basic Operation

In order to perform a measurement of the surface observed by the field of view of the objective, the objective lens is translated vertically and linearly so that the focal plane moves through the entire height range of the surface being measured. As it does so the interference fringes will move and follow the height profile of the surface and this information is processed by the instrument to calculate the height profile to a very high precision. If we take the simple example of a spherical surface and the objective moving downwards then the interference fringes will appear as a small set of concentric circles emanating from the top of the sphere as the focal plane of the objective intersects it.

The concentric fringes will then grow larger as the focal plane moves and intersects the sphere lower down. The NewView is able to measure and view an image field dictated by the field of view of the objective lens, for example a 10x objective with a resolution of 1.18 microns is able to observe an area of 1.1mmx1.1mm. As with the GPI the MetroPro software processes the interference data to create a colour coded height profile of the surface under measurement, click her for an example, To measure larger areas there is a facility on the NewView to stitch image fields together. A motor driven stage allows the system to move the surface under inspection a step at a time and in raster fashion to allow relatively large planar surfaces to be measured.

AirMicro is the first wireless digital microscope made for the iPad, iPhone or iPod touch.

AirMicro is a portable microscope that wirelessly pairs with an Apple iOS device to view live images, freeze and capture images to the Photo album of the device.

Zeiss has released details of the first type of NEW super high performance lens family Otus. The lens will be available in February 2014. It will be a 1.4/55 for F-Mount. Other types will follow.

This lens family will be a change of paradigm for 43mm F-Mount.

The contrast at F-stop 1.4 is far better than every other standard lens with large aperture. This lens will give you finally the possibility run fast imaging with the highest contrast also to the last pixel.

Watch the video outlining the performance of the New Otus lens.

For further information and assistance, email [email protected] or telephone 01582 764334.

Baumer introduce S-Mount kits for the MX series:

For further information and assistance, email [email protected] or telephone 01582 764334.

Lambda Photometrics Ltd are pleased to announce that they have entered in to a non-exclusive Distribution Agreement with APEX Technologies of Marcoussis, France to sell their range of high performance Optical Spectrum Analysers, and Optical Linear Sampling Oscilloscope in the UK and Eire.

The APEX product line includes:

• Ultra High Resolution Optical Spectrum Analyser,
• Optical Complex Spectrum Analyser,
• Optical Linear Sampling Oscilloscope,
• Optical MultiTest Platform.

For further information and assistance, please contact Adrian Harrison on 01582 764334 or send an email to [email protected].

Lambda Photometrics Ltd is a leading UK Distributor for Laser and Light based products in areas including Optics, Electro-optic Testing, Spectroscopy, Machine Vision, Optical Metrology, Instrumentation, Microscopy and Pulsed Xenon Light Systems.

Baumer introduce S-Mount kits for the MX series:

Baumer expands its GigE camera series with 10 new IP protected models that have been developed for particularly demanding industrial environments. With the IP 65 and IP 67 protective housing, every sensitive camera component, all the way to the lens, is protected from dust, water spray and temporary immersion.

The cameras have an x-coded M12 connection industrial design 12 to 24 volt or Power over Ethernet (PoE) supply. The GigE Vision compliant cameras feature CCD and CMOS sensors. High-performance SONY CCD sensors deliver up to 160 fps in VGA, 1.3 and 2 megapixel resolutions. Multi I/O 2  & 4 megapixel CMOS variants (CMOSIS CMV2000 and CMV4000) Frame rates of 55 respectively 29 fps. FPN (Fixed Pattern Noise) correction HDR (High Dynamic Range) Impressively homogenous images even in scenes with substantial brightness difference. Starter kits and power supply kits for the VisiLine® and VisiLine® IP series are available now.

For more information, please click here to email or contact Clive Phillips  on 01582 764334.

Lambda is a leading supplier of characterisation, measurement and analysis equipment, applied to signals from DC to Light. Our company provides hardware, software and integrated solutions throughout the UK & Ireland. Lambda’s scientists and engineers provide expert technical advice, working with you to improve; product discovery, product development and productivity. From fundamental materials research to volume manufacture, our consultative approach will ensure you receive the best advice and get the best solution for your needs. Lambda is a subsidiary of Polytec GmbH, a privately owned German technology company.