How to Specify a Microlens Array

Transmissive or Reflective: 

An MLA is specified as either a transmissive or a reflective element.  As a replicated optic, the polymer used will generally define the transmission range which typically is in the range of ~400nm to ~2700nm.  Anti-reflection (AR) coatings can be applied to these components to further enhance their transmission efficiency.  The AR coating can be tailored to meet each application’s specific needs. If an AR coating is required, the designer must specify the AR coating’s maximum reflectivity over the operating wavelength range.  If the component is specified as a reflective element, a reflective coating is applied to the MLA to create a micromirror array.  Much like the AR coatings, the reflective coatings can be tailored to meet each application’s operating wavelength range and reflective coating performance requirements.

Lens Shape/Profile: 

The shape of the lens is one of the most critical parameters of the MLA.  There are very few limitations to consider when deciding on the microlens array shape and profile.  For tightly toleranced lens profiles, greyscale laser lithography is used to produce the master.  Greyscale laser lithography allows the flexibility necessary to produce a variety of lens types, sizes and packing configurations.  The following information is required in order to manufacture a custom MLA:

• Lens Packing Configuration:  The packing configuration of the lenses must be provided. Common packing configurations include Round, Square, Rectangular, Hexagonal, and Random (Diffuser).

• Lens Profile (The designer must select from the options below):

1. Spherical, Aspherical, Cylindrical, Acylindrical, Freeform, Toroidal, Mini-Fresnel, Anamorphic, or Biconic
2. Refractive or Diffractive
3. Single-Sided (Plano-Convex and Plano-Concave) or Double-Sided (Bi-Convex, Bi-Concave or Convex/Concave)
4. Maximum Sag (see illustration below): Technological advancements in mastering and replication are continuing to push the boundaries of what is possible.  Recent developments have pushed the largest sag achievable to beyond 100um.    

Illustration Defining Sag

• Lens Pitch (center-to-center distance): The current minimum lens pitch for greyscale laser lithography is ~1um.

• Fill Factor (Lens clear aperture and transition zones):  The “Fill Factor” describes the percentage of the pitch that represents the clear aperture (CA) of the lenslets. The zone between the individual lenses that does not conform to the specifications is referred to as the transition zone (see sketch below).  On the majority of lens designs, the transition zone can be minimized to 1um or less with the fill factor typically reaching 98% or higher.

Illustration Defining Transition Zone

(Not to Scale)

Mechanical and Material Properties:  

The lens shape plays a major role in the design of a custom MLA. However, the mechanical and material properties are also critical. A large range of high quality polymers and substrate materials can be used to produce MLAs, each requiring well defined replication processes in order to meet the most demanding specifications.

• Substrate Materials:  Most MLAs are produced on glass substrates or wafers.  Common optical glasses include N-BK7, Corning EagleXG, Borofloat, D263, as well as several others.  MLAs can also be produced on a variety of plastics, as well as on Silicon, Germanium, and metal substrates.  MLAs can also be replicated directly onto CMOS and VCSEL sensors and chips.

• Substrate Sizes:  The length and width of the substrate is one of the main driving factors in determining the unit pricing.  The replication process used to produce MLAs is very scalable.  The maximum length and width of the substrate are usually limited by the size of the master MLA pattern available to replicate the area.  Wafer-level manufacturing further enhances the cost proposition of replication, as hundreds or thousands of dies can be replicated in a single operation on 1 substrate, to drive down the unit price when going to higher production volumes.

• Substrate Thickness:  The thickness of the substrate factors into several steps of the microlens array manufacturing process.  The use of standard off-the-shelf float glass or fusion drawn glass is a cost effective approach for those applications which do not require micron class thickness or wedge/flatness tolerances.  For more demanding applications, polished glass substrates can be procured. Available glass substrate thicknesses range from 0.1mm to several millimeters thick.

• Polymer Selection:  Microlens arrays are molded into a thin polymer layer that preferentially adheres to the base substrate and not to the mold. Therefore, the optical properties of the polymer material play a critical role in the performance of the MLA.  Many different proprietary replication polymers are used to produce MLAs.  Two of the most important considerations when selecting polymers are refractive index (RI) and environmental requirements.  The refractive index range of the most common replication polymers spans from 1.51 to 1.70 when operating in the visible spectrum.

• Polymer Thickness:  Many custom designs require precision polymer thickness control, which can be achieved by precision in-process thickness measurement and control processes. These measurement and control processes can achieve polymer thickness tolerances to within +/-2 microns.   The nominal replica layer thickness depends on the MLA area and the polymer used, but typical thicknesses are in the range of 10um to 30um.

• Aperture Coated Substrates:  Many designs require the ability to precision align MLAs to custom aperture features patterned onto the glass surface. Semi-automated and automated alignment tools and equipment are available to achieve micron-class alignment accuracy over a 200mm diameter areas and larger.

Profile View of Replicated Coated Substrate

• Alignment Tolerances:  The alignment of fiducial features patterned on both the replica and the substrate, along with custom alignment tools enables micron-class alignment of replicated MLAs to patterned substrates.  This methodology can also be used to align replicated MLAs on both sides of a substrate (double-sided MLA) to similar tolerances.

Optical Performance and Testing:

Microlens arrays cannot be tested via traditional lens testing methods.  In order to evaluate the lenslet profiles, more sophisticated measurement equipment is required.

• Surface Profile Error:  Significant advancements in Confocal Microscopy methods have enabled several companies to develop commercially available confocal measurement microscopes capable of measuring the most challenging lenslet profiles to nanometer resolution.

• Maximum Defect Criterion:  Borrowing heavily from the advancements in semiconductor wafer vision-based defect inspection systems, fully automated Vision Measurement Systems (VMS) coupled with Artificial Intelligence (AI) programs are used to bin defects by defect type and size, based on the individual customer specifications.  The custom automated programs provide a die-level defect map upon completion of the defect inspection of each wafer.  This capability is particularly useful to customers that require hundreds or thousands of dies on a wafer, by allowing them to use the defect map during the “pick and place” step to select only fully conforming dies.

• Energy Density:  MLAs are utilized in many different environments and use conditions.  Several customers utilize MLAs in laser applications.  As the power of the laser source increases, there is a risk that the power density will exceed the polymer’s laser damage threshold. While many replication polymers have very high damage thresholds, for certain applications we recommend that the customer evaluate the performance of one of our sample replicated MLAs (See MLA Sampler Page) to verify that our materials are able to withstand the energy densities required prior to ordering a custom MLA.

• Environmental Requirements:  Depending on the application, the environmental requirements (temperature range, solder reflow compatibility, humidity requirements, UV exposure, etc.) may also play crucial roles in the polymer selection.  In certain cases where a customer has unique environmental requirements, we recommend that they evaluate the performance of our replicated MLA sampler  (See MLA Sampler Page) to verify that our materials are able to withstand the environmental requirements prior to ordering a custom MLA.

NRE/Tooling: 

In most cases, in order to produce the MLA, custom tooling (including masters and stampers) must be produced, and new or modified production processes are required.

• Generation of Final Replicas:  Understanding the total potential volume of your product will help the manufacturer to determine how many generations of tools (stampers) are required.  For feasibility studies, demo products, and other relatively low volume runs, we would recommend the final product to be a third generation (Gen3) replica (for convex lenslets), or a fourth generation (Gen4) replica (for concave lenslets).  For medium to high volume applications (1,000s to 10,000s per year), we would recommend the final product to be a fifth generation (Gen5) replica (for convex lenslets), and a sixth generation (Gen6) replica (for concave lenslets).

The illustration below provides a graphic presentation of the number of replicated wafers that can be produced using our multi-generation stamper tool approach. In this example, 81 Gen4 final replicas can be produced from a single Gen0 master, with each tool producing 3 daughter tools/replicas as shown (3^4 = 81). Depending on the micro-structure details and performance specifications required, it is possible to produce hundreds of tools/replicas from each tool which rapidly increases the number of Gen4 final replicas that can be produced (100^4 = 100 Million!).

For more information please view our Nano-Imprint Template Video

• Master Fabrication and Stamper Production:  The specification and manufacture of the master microlens array mold is one of the most critical steps to a successful MLA project.  Once the final master design is complete, and the master is produced, a careful inspection process is performed to verify the base specifications of the master mold have been achieved. Upon completion, first generation (Gen1) stamping tools (stampers) from the master are produced.  The Gen1 stampers are a mirror image of the master mold. With each stamper generation, the lens curvature changes slightly due to the shrinkage of the polymer material used to produce these stampers. In most cases, the shrinkage of each generation can be calculated, based on known values, to specify the final profile to the mastering source.  However, for MLAs that require highly accurate lenslet form, a shrink trial (see below) is required. The stamper generation production process continues until the desired stamper generation is produced, from which the final replicas will be produced.

• Shrink Trials:  Understanding the curing properties for the many different replication polymers is critically important when designing the microlens arrays masters.  For customers that require high lenslet form accuracy, an additional shrink trial step is added to the mastering process, whereby a trail master is produced, and from it, trial stampers are produced, from which a final shrink trial replica is produced.  This trial replica is then measured to determine the correlation between the master and final replica form.   This additional step adds cost and time to the product development schedule.  However, in many cases, it is necessary to ensure conformance to the customers’ specifications.

High Volume Scalability (Wafer-level Optics): 

See Wafer Level Manufacturing Page

For many of the reasons detailed above, many customers that require high volume components have designed their products to make use of the extensive worldwide wafer-level infrastructure that exists.  This is accomplished by designing a master that includes hundreds or thousands of individual dies, that once replicated, can be singulated (typically diced) into their individual component size.  Common standard wafer sizes include 100mm, 150mm and 200mm diameters, but the replication process also allows for custom sized plates or wafers that can be larger than 500mm round or square.  Wafer-level manufacturing is an exceptionally cost effective way to reduce the recurring costs of these components to levels that are similar to injected molded optics.  After the MLA array is replicated onto the glass wafer, the wafer is typically shipped to system integrators, where the MLA wafer is paired with several other wafers, or stacks of wafers to create complex sensor or emitter systems.