Zygo is a market leader when it comes to interferometric metrology instrumentation. With two key product lines in that category, Zygo offers an optical profiler and a laser interferometer as well as producing displacement measuring interferometers as a part of its PPS product line.
AZoM sat down with Matt Battistoni to learn more about Zygo, and discuss performance and the metrology of acquiring data on aspheric surfaces.
Could you please introduce yourself and tell us a little about what it is you do at Zygo?
My name is Matt Battistoni. I am a sales engineer working out of Zygo’s West Coast regional office in Santa Clara, California. Zygo is the market leader in manufacturing interferometric metrology instrumentation and is an optics manufacturer. Our facility in Richmond, California, is a world leader in producing high-quality aspheric surfaces.
Image Credit: ShutterStock/Pixel B
What is an aspheric surface and why is it useful?
An aspheric surface is a surface that departs its form, or its top surface departs from a spherical form.
We typically describe these types of surfaces using an aspheric lens equation or an aspheric equation. There are several variations, but the fundamental constants remain throughout many of those equations.
The aspheric coefficients, the conic constant base radius of curvature, and related information are required to establish a SAG table or a description of that design.
What are the typical manufacturing processes surrounding the production of aspheric surfaces?
The three main manufacturing methods are aspheric polishing, diamond turning, and injection molding or stamping, and they can be used with glass and polymer surfaces.
During aspheric polishing, a glass substrate is subjected to rough-cutting using a CNC grinder. A small contact polisher is applied, typically computer-controlled, along with an abrasive slurry to polish the surface.
A very high-quality surface finish is achieved as a result, but the process is extremely time-consuming and can involve many iterations to obtain the desired form.
MRF and IBF allow users to reach optics with an extremely tight and controlled surface wavefront specification and a fairly smooth surface roughness. Diamond turning is similar; it involves the mechanical removal of material from the surface of a substrate - the difference is that it is completed using a rotational spindle.
A diamond-tipped cutting tool removes the material as it translates across the substrate. A very high level of surface finish can be obtained with this technique faster than traditional polishing methods, making it a good fit for higher-volume applications.
Finally, there is molding which is useful for situations with a much higher volume. Molding is also suitable when there is a higher demand for the number of optics that need to be reduced, but maybe not at the extreme levels of quality that, say, a diamond turning or aspheric polishing would require.
This manufacturing method involves taking a semi-liquid/semi-solid material and either stamping or injecting it into a pressurized dye or a mold to take the form of that dye or a mold.
Once the material completely fills the mold, it is either cooled and hardened into place or cured. Some materials can be cured using a UV light to take the same shape as that mold.
How do you test these aspheric surfaces for practical purposes?
First, it is worth considering what solutions are available for measuring these surfaces, as their metrology is a more challenging than your typical plano or spherical optics.
Zygo's Mx software package runs our instrumentation and then analyzes data post-acquisition. The software includes types of aspheric file descriptions that can be used to describe surfaces. This acts as a reference point so that you can go back and look at those or configure your design later so that it can be imported into Mx.
Why is this useful and what is the benefit of using aspheric lenses?
A five-optic system is used instead of a three-optic system, which saves the user on the weight of the system as well as its system cost.
The two common aberrations that can plague an optical system are sphere collaborations and astigmatism. Another benefit could be the improved aberration reduction of an aspheric lens or that an aspheric lens can provide.
Why are aspheres difficult to measure?
A spherical transmission element is installed into your Zygo interferometer, creating a spherical wavefront that can be caught on a resulting spherical surface.
The optic’s position can be adjusted in free space to where the wavefront matches the prescription of the spherical surface.
In the case of aspheres, there is a departure from a spherical wavefront, resulting in the slope capture limit of the instrument being quickly approached.
Mild aspheres are occasionally acceptable but are typically outside of the slope capture limit. The wavefront must be adjusted or its position manipulated in a way that enables measurement of the entire asphere.
Why is interferometry necessary?
When dealing with a very high precision level or softer materials like polymers involved in molding techniques, dragging a stylus across the sample’s surface can be damaging.
Damaging the final product is undesirable for both the manufacturer and the customer. Zygo, in its portfolio, has three aspheric measurement solutions on offer: Verifire Asphere+, Zygo Compass, and computer-generated holograms.
Computer-generated holograms and the Verifire Asphere+ are compatible with one another, along with some of the other large aperture systems found in Zygo's laser interferometer product line. The Verifire Asphere+, is a fresh take on our legacy VFA a Metro Pro-based instrument, which is our old legacy software package.
Zygo has taken a fresh approach when updating this platform. We have brought it up to Windows 10, implemented our new Mx software, and many of the aspheric utilities have been directly integrated inside of this software package. As a result, the platform is fast, flexible and easy to use, which is the main focus of this new generation of products.
How does the Mx software change the process of measuring aspheres?
The updates from our legacy Metro Pro software have been applied to our Mx software. One of the most significant additions and improvements that make measuring these aspheres a breeze is based on the principle of design files.
Once the information for the aspheric equation is known, it can be loaded into Mx, providing users with an overall SAG table as well as measurability of the aspheric surface. In doing so, the user is given direct feedback on what hardware configuration is recommended to have the lowest number of zones and be the easiest and fastest way to acquire surface data across your asphere.
Essentially the user is taking a spherical wavefront that will be sent out from your transmission sphere.
What updated features will the new platforms include?
The Asphere+ platform includes the same surface processing techniques available on instruments like Zygo’s optical profiler and other laser interferometers.
Another nice feature, the Verifire Asphere+, which is specific to the Asphere, will be the ability to image the individual zones that are independent of each other. Here, if the user is loading multiple zones but is concerned about data quality issues, an individual zone can be loaded to assess the reliability of measurement or vertex uncertainty.
The user can also identify if there will be an excess of vibration that may have presented itself in the measurement, such as someone bumping into the instrument or data quality issues with dropout. As a result, you can know exactly where it is and if it is a good idea to remeasure at that point. It offers secondary assurance that the quality of your data is what has been measured and is being pushed out.
What is the range of part sizes or the maximum part size that you can measure with the VFA+.
If the overall aspheric coefficients that are used to describe a sample’s surface are loaded directly into the Mx, the user can gain an idea of whether it will be measurable or not. It will vary depending on whether it is a convex or concave asphere, as well as the type of surface being analyzed.
Generally, as a rule of thumb, the measurement range would be between 10 millimeters to about eight inches. We can extend beyond eight inches depending on the design, but this is generally where the eight Verifire Asphere+ is really meant to support. Ultimately, it depends on your design.
Does Zygo’s system have the ability to measure more complicated surfaces like free forms?
Using CGHs, freeform optics, optics or potentially more challenging aspheric surfaces such as something that is not rotationally symmetric can be measured. Zygo’s Verifire Asphere+ has integrated CGH mounting that is added into the platform. As a result, it leverages a standard spherical or plano wavefront and then manipulates that wavefront to match the expected design of that aspheric test part.
Not only is this compatible with the Verifire Asphere+, but also works in a horizontal configuration with our standard interferometers.
What is CGH, in principle?
CGH is a computer-generated hologram that is used with a transmission flat or a sphere, accepts that transmission flat or sphere, and then deviates the beam path to match precisely. If the sample part is held in a specific position in Z space relative to your CGH, the wavefront will be an identical match to the design that you are trying to achieve.
This allows users to visualize the deviation from a perfect aspheric surface. It is worth noting that a CGH is not something that can be adapted to as CGH is made specifically to be used with a singular aspheric design. For each one of those complex surfaces, a specific CGH is needed to interface with that design.
Does CGH work for smaller-scale lenses, such as microlenses?
Lenses under 10 millimeters are found on devices like cell phones, automotive cameras, and for LiDAR applications. With this instrument, because of those smaller lenses, it does not necessarily make sense to use a large aperture tool with a lower spatial resolution to capture that surface information.
Image Credit: ShutterStock/sakkmesterke
Zygo has instead taken its optical profiling platform and built a system around some of the problems surrounding microlenses.
Discussing some of those typical measurements that are key and important to the metrology surrounding those, with these smaller lenses, positioning is really important when talking about loading microlenses into small housings. It is essential that the user must get the position of these lenses correct because they tend to be nested either into mechanical datums or other optics that have datums that nest upon each other.
What other features are users to expect with Zygo’s instrumentation?
In addition to measurement acquisition, a full 3D surface map, similar to the Verifire Asphere+, is available where deviation analysis can be carried out. You can also remove the overall form of that aspheric surface and get the texture information which is helpful in relation to the performance of that lens.
This is leveraging one of our 3D surface profilers but with an advanced staging solution so that the surface of the asphere normal is kept to the optical axis of the instrument. That part is rotated and radially translated so that you can restitch and remap the entire surface of that component.
How does the instrumentation account for manufacturing defects?
Manufacturers will aim to generate an overall SAG map of what the aspheric surface looks like, which is a 3D form of the surface being measured.
Using the aspheric equation, you might find a deviation from the aspheric design and ultimately target what that causation could be.
Depending on the application, the user may also be interested in small defect inspection, such as diamond-turning marks. The diamond-turning instrument may then have to be adjusted to minimize this effect, called mid-spatial frequency.
The other thing is surface contamination. Any manufacturing method is going to involve some form of slurry or lubrication, material residue that is left over on your product, or even oils from part handling or cleaning supplies.
Each of these is residual and should be removed so that they are not passed off to the customer. Luckily, these are identifiable with our instrument once the user takes acquisition of the surface, allowing you to perform quality control of products.
Are there any particular tools in the software that enhance the quality control experience for manufacturers?
One of the tools available inside of Mx is a slope capture limit, meaning users can set their own tolerances. This can be a good indication of when you need to rework your molds or to identify if there has been some damage suffered affecting the performance of that lens.
How about with the VFA? Is there a specific sphere you need to use with each part for the VFA+?
As I mentioned with CGHs, everything will be specific, requiring either a transmission sphere or a transmission flat. This is also applicable to the optic being tested. Information like the base radius of curvature and the overall diameter of the part will be a big driver as to what F number transmission sphere is needed to pair up with the surface to have the best chance of 100% measurability on that part.
How do you measure a toric (lens)?
A toric utility is embedded directly into Mx software and supported by the Verifire Asphere+.
With toric surfaces, you have two optical axes: cylindrical optical axes and spherical optical axes. We use a very similar vertex tracking method to your traditional asphere. However, it is a little bit different because of the wavefront coming off of those surfaces.
About Matt Battistoni
Matt Battistoni supports the Sales Department specializing in Zygo Instruments. (Laser Interferometer, Optical Profiler, & Positioning Systems). Matt has been a part of the Zygo team for 6 years now. His first 3 years with Zygo were spent on the applications team focusing on providing instrument-based solutions to his customers’ metrology problems.
About Zygo
ZYGO designs, manufactures, and distributes high-end optical systems and components for metrology and end-user applications. ZYGO's metrology systems are based on optical interferometry measuring displacement, surface figure, and optical wavefront.
Metrology and optical markets for end-user and OEM applications include semiconductor capital equipment, aerospace/defense, automotive, and research.
This information has been sourced, reviewed and adapted from materials provided by Zygo Corporation.
For more information on this source, please visit Zygo Corporation.
Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.