Tooling Process for Microstructures

Optical Engineering and Manufacturing

Step 1. Design

 

This is the typical design process for creating a micro structured component. Using computer design tools the surface is designed. These design tools can be a ray tracing program, such as Fred, Trace Pro, or Zemax,  or a mathematical program, such as MathCAD.

 

The surface is defined into some sort of equation. The usual equation is the aspheric sag equation. C is the curvature (1/R, where R is the radius of a circle), K is the conic constant (is the surface parabolic, hyperbolic, etc?), A4 through A8 are the aspheric coefficients, and r is the distance from the center of the surface.

 

 

The surface is then condensed to a plane. The engineer must decide on the feature depth at this point. The equation below condenses the equation above around a thin surface with thickness d.

 

 

The manufacturer of the master will use these equations to control their machines and make the surfaces.

 

Step 2. Fabrication of the master

 

The drawing and equations must be used to fabricate a master. A master is a piece of relatively soft metal, or other material with the micro structures cut into the metal. It can be used to fabricate parts directly. However, the usual purpose of the master is to produce nickel replicas for use as tool inserts. Microstructures usually cannot be cut into hard materials such as tool steel.

 

Although it is possible to produce some parts directly without using a master, the process is usually identical to making the master. Therefore it is usually more cost effective to make a master and then use the master to make plastic parts.

 

Typical Mastering Processes

 

 

 

 

 

Diamond turning process

A diamond turning machine is an ultra precise lathe. It is placed in an environmentally controlled room, on a vibration isolated surface. Most machines can only rotate about a center axis and produce rotational parts like Fresnel lenses. Others have a flywheel attachment where the spinning surface can be rotated perpendicular to the optical plane. This produces linear parts (our waveguide).

The cutting tools are diamond chips, pre shaped to a wedge. The finest diamonds have tips that are dead sharp or with radii measured in microns. Often the large lead time in starting a job Is ordering the correct diamond for the job. Many shops do not maintain an inventory of the finest diamonds.

A metal blank of copper, brass, or bronze is prepared. The prepared form is a disk with the front surface polished or diamond turned to a finish that is measured in angstroms. The blank is mounted in the diamond turning machine. The diamond spins and cuts the surface into the blank.

Photo lithography

A plate is produced, usually silicon or glass with a thin layer of photo resist. The equation or data file is used to create a mask. The mask is usually several orders of magnitude larger than the part. The mask consists of a reflective or absorptive thin film coating on glass. The mask is larger because it is easier to make millimeter sized features than micron sized features.

The mask is placed in a stepper that has a light source. The stepper projects light through the mask and onto the plate, and uses optics to reduce the image to the desired size. The light pattern reacts with the photo resist to produce a pattern.  Further steps are used to treat the photoresist to create a surface relief pattern with alternate heights. The exposed plate is coated with a thin film of gold, which is conductive.

Laser writing

A material that can be ablated with a laser is used as the substrate. A laser is moved over the surface of the substrate. The exposure time is used to control the burn depth. For larger feature sizes, 0.2 mm for example, the laser cannot be controlled very accurately. Furthermore, the resulting surface is somewhat jagged, where a diamond turned surface is smooth to angstroms.

Typical master materials

 Copper

 Bronze

 Brass

 Electroless nickel

 Aluminum (Used for soft tooling to directly mold parts. )

 Acrylic (For directly cutting prototypes. Other polymers may not be suitable.)

 Photoresist on Glass (photo lithography or laser processes)

 Photoresist on Silicon (photo lithography or laser processes)

 

Step 3. Replication of the master

The master is usually a very expensive and delicate piece of hardware. In many cases the master cannot be used in production. Because it is delicate using it to produce parts may scratch or ruin it after several parts. The master should be replicated in a hard metal. Replication is cheaper than producing the original master. If a master costs $5,000 to $10,000, electroforming will cost $500 to $1000. It becomes cost efficient to make one master to produce replicas, and use the replicas for production.

The master is replicated in nickel.  A layer of nickel is grown on top of the master. After some time, the replica layer is thick enough to separate from the master and be used as a tool. The replica is separated from the master and machined to size, to fit into the mold base.

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Process

Advantage

Disadvantage

 

 

 

Diamond turning

High quality surfaces.

Linear or rotational surfaces only.

Photo lithography

Non rotational, non symmetric patterns. Very tiny structures.

Requires the fabrication of optical masks, can be expensive.

Laser Writing

Non rotational, non symmetric patterns.

Limited to very small structures. Lower quality surface finish.

CNC machining

Low cost and easy to find. Can be used to cut hard metals directly.

Limited to large surfaces (not microstructures).