March 2017 Archives

Photo electroforming has been widely applied as a processing method for precision components in the field of precision electronic components. The photoresist and electrodeposition technologies are most widely utilized in the production of through-hole multilayer print circuit boards among all the electronic components.

Products utilizing the photo electroforming technology in the other field include the following: printing plates, medical equipment as blood filters, outer blades of electronic razors, filters for juicers, various optical slits, diaphragm plates for cameras, electronic microscope meshes, classification filters for analysis, copier meshes, and more. In the field of electronic components, the example include metal deposition masks for thick film IC production, vacuum deposition and sputtering masks, meshes for electron tubes, beam disks for video camera tubes, etc. The technology is applied in products requiring ultra-precision and widely utilized in the field of high technology.

An example of special application is the processed goods with a seamless structure. Specific products of this type are rotary mesh cylinders, printing plates for rotary screens, tobacco bands used in cigarette production, seamless belts for computer terminal equipment, etc.

The most notable application of photo electroforming is the production of fine meshes, micro screens, and microfilters.

You can create a fine mesh in the following steps:

1)Use the sputtering method or similar procedures to apply metal coating over non-conductive substrate metal with lattice groove. Rub off the surface metal except for the groove area. Create a product by electrodepositing metal directly over the lattice-shaped groove and separating the deposited metal.

2)Transfer the lattice pattern on a glass plate over to a resist-applied conductive substrate by light exposure. After developing the negative, perform electroforming. Separate the mandrel and create the product using deposited metal. This is an optimal way of manufacturing fine mesh products.

Photo electroforming allows you to create a fine mesh containing 400 to 800 wires per 1 cm at ±1 µm precision and a screen with tiny holes ranging from several to ten micrometers.

The thickness of electroformed metal affects the minimum hole diameter and the minimum interval between holes. In addition, the thicker the deposited metal, the smaller the holes become.

The shapes and dimensional accuracy of meshes produced by electroforming include the following: a lattice-shaped mesh containing 4 to 80 wires per 1 cm, a fine mesh containing 8,000 to 160,000 holes in 1 cm2, a mesh with different number of vertical and horizontal wires, 40 wires per cm x 80 wires per cm, for example.

(9) Electroforming

Substrates used for electroforming are electrically conductive. To make it easier to separate after electroforming, the resist separation treatment is necessary. Potassium dichromate solution mentioned in previous volumes is used for the resist separation treatment.

If you use a glass substrate, it is necessary to make its surface conductive. To add conductive properties, it is common to form silver, copper, or nickel coating by electroless plating, cathode DC sputtering, or vacuum deposition. Copper and nickel are frequently used for electrodeposition in photo electroforming. The bath composition and electrolysis conditions are same as those for general electroforming. [Fig.1] illustrates the electrodeposition onto a substrate using a photoresist.

To separate the electrodeposited metal from a substrate, use the tip of a knife to peel it off or paste adhesive tape over it and strip the tape. When you separate the metal, be careful not to damage the protective film. The photoresist layer still exists on the substrate even after separating the electrodeposited metal. You can use this substrate repeatedly and produce a number of electrocast products.

[Fig.2] shows how the electrodeposited metal grows. The deposited metal grows up to the height of the resist layer; however, the excess metal deposited after that starts growing in the lateral direction. This lateral growth becomes the margin of error for the pattern. The length of lateral growth on both sides (δ) is almost equal to the thickness of the electrodeposited layer (t). Therefore, this dimensional adjustment must be incorporated into the original drawing.

The thinner resist makes the photo electroforming more accurate. The resist layer thickness varies from 1 to 5 µm depending on the application method. Since the resist layer can be built up to only a given thickness in a single application, a double coating method using two different resists is available if you want to form a thicker layer.

We have seen an electroforming method of depositing a metal layer on a resist-applied substrate up to a certain height and separating the deposited metal. In addition, there is a way to deposit metal higher than the resist layer height. First, electrodeposit metal on a substrate until it becomes several micrometers high. Separate the deposited metal from the substrate and place this layer into another frame. Put this back into the electroforming bath and wait for metal to be deposited on both sides for a certain thickness.

How to remove the resist after separating from the mandrel is different for resist to resist. In general, dip it into alkali solution such as sodium hydroxide or organic solvent as ketones, and brush the resist off from the substrate.

(7') How to apply photoresist (continued)

When you apply a photoresist over a glass substrate, it is common to deposit metals, such as chromium, over the glass surface or to apply silane-based coupling agent.

An infrared lamp or hot air at 60 to 90°C will be used for drying and removing the solvent after applying a photoresist. However, if you dry it too fast, it dries only the surface and leaves bubbles, which will turn into pinholes later or will result in degraded adhesion. [Fig.1], [Fig.2], and [Fig.3] are the illustration of coating by liquid discharge, spin coating, and dip coating, respectively.

(8) Lithographic exposure and development

Place a negative film on top of the base plate with a photosensitive film applied. To improve the adhesion to masks, put this assembly into vacuum contact lithography equipment for light exposure.

Use a carbon arc lamp, a high-pressure mercury lamp, a xenon lamp, or laser light as the light source for exposure.

The photosensitive area of photoresists covers ultraviolet regions and visible parts. Use vacuum contact to adhere the resist-applied surface to the emulsion-applied surface before exposing them to light. [Fig.4] is an image of vacuum contact lithography equipment.

Process the sensitized base material for photoresist development. This processing dissolves and removes the area not sensitized, and hardens the sensitized part to form an insoluble film adhered to the base material. In other words, a shaped film that is difficult to dissolve will be formed on the surface where no electrodeposition will take place in order to protect this area from the electrolyte.

Organic alkali type seems to be more popular than inorganic alkali type for the developing solution. Make sure to adjust the solution's temperature since it affects the developing speed.

Various developing methods are available, such as dipping the negative into developing solution, exposing the negative to vapor of organic solvent and rinsing it off, spraying the developing solution with a spray gun, etc.

After developing the negative, completely remove the residual solvent on the photoresist. In addition, apply heat treatment using hot air or an infrared heater at 60 to 90°C in order to improve the adhesion performance and chemical resistance. (This process is called "baking".)

(5)Pretreatment of base plate

The accuracy of patterns created by photoresist is affected by surface conditions such as roughness and oxides on a base plate where photoresist will be applied. The pretreatment is extremely important because contaminants, such as oil, dust, grease content, and fingerprints, may result in poor adhesion, uneven thickness of the photoresist layer or leave pinholes. The pretreatment is generally performed in the following steps: degreasing -> surface polishing -> water washing -> acid treatment -> water washing -> drying. These steps are same as those for plating pretreatment.

(6)Photoresist

Resins sensitive to ultraviolet and visible light rays are dissolved into photoresists. Once the applied photoresists dry up, they will form a photosensitive film.

For negative resists, a cross-linking reaction of a photosensitive resin is initiated by light exposure, which makes the linear molecule form polymers with a three-dimensional network structure. After the light exposure, these resists become insoluble and resistant to acids and alkalis. Polyvinyl cinnamate is a well-known photosensitive resin.

On the contrary, exposed portions of positive resists become soluble upon exposure to light. In that sense, positive resists are more flexible since they can be exposed to light repeatedly.

Photoresists are available in water-soluble and solvent-based forms. Water-soluble resists are made from casein, gelatin, and polyvinyl alcohol with a mixture of dichromate.

Compared to negative resists, positive resists have lower chemical resistance and adhesion performance to base plates. However, they have better resolving power, which makes them suitable for forming fine patterns.

(7)How to apply photoresist

It is necessary to select an appropriate application method of photoresists according to the processing accuracy. Use a high-resolution photoresist for high accuracy processing. Apply a thin layer evenly over the substrate.

Photoresists are applied in the following methods: 1) dip coating (one by one); 2) spray coating; 3) liquid discharge by nozzle (for continuous sheet); 4) spin coating; 5) roller coating (for application without requiring accuracy), etc.

A film formed by dip coating is only about 1 µm thick. This is too thin for electroforming in terms of corrosion resistance. Even if you make it thicker by repeating the dipping process, the layer becomes unevenly thick. Application by dipping or liquid discharge will make the layer thickness uneven on the top and bottom sides because photoresists are dried with the application plate standing in the vertical direction. However, you can still make the photoresist film evenly distributed if you slowly pull up the application plate from the solution.

Spin coating is a procedure used to deposit an even layer of photoresists on a substrate by centrifugal force of the rotation. An even layer with the thickness of 3 to 5 µm will be deposited if you dry the photoresist using infrared rays during the rotation. In this procedure, secure the base plate onto the spinning disk and rotate the disk for about 80 to 180 spins per minute. It is a popular method because the application layers deposited by this procedure are most accurate and consistent.

(2) Photo electroforming processes

General processes of photo electroforming consist of the following: production of the original drawing → production of the original plate → pretreatment of the base plate → photoresist application → lithographic exposure → development → baking → electroforming → mandrel separation → post-treatment (washing/drying)

The photo electroforming processes start with preparing an original drawing. After the process of creating an original plate by resizing and copying the drawing, you will create a positive or negative photomask called "master pattern". Then, apply photoresist on the base metal surface. Once it dries up, it becomes light sensitive.

Photoresist adheres to the master pattern. When exposed to light, the solubility changes by the action of light. A negative resist is the type of photoresist that the exposed portion becomes insoluble to the photoresist developer. On the other hand, a positive resist is the type of photoresist that the exposed portion becomes soluble to the photoresist developer.

Common photoresist hardens by light exposure and adheres to the substrate. When you develop it after this lithographic exposure, photoresist without electrodeposition will remain on the surface. In other words, photoresist is formed on the protected area (where no electrodeposition takes place).

If you use a metal substrate, you may need to use insulation tape or insulating paint for protecting the back surface and surrounding areas from electrodeposition of the metal. After this treatment, perform the resist separation treatment. Then, electrodeposit a layer of the specified thickness and remove the mandrel. [Fig.1] shows these processes.

(3) Production of original plate

On non-elastic paper or a base sheet, draw a pattern (original image) as accurate as possible, using drawing lines that are two to ten times larger than the actual object.

In general electroforming masks, a consecutive pattern is arranged in a certain interval so that a number of products can be electroformed in a single operation. Produce a negative film (or a negative dry plate) by photographing this original drawing onto a reproduction film (or dry plate) by keeping the same dimensions. A positive film will be made by contact printing from the negative.

(4) Base plate

Commonly, conductive metals, such as copper, nickel, steel, stainless steel, and copper alloy, are used for the base plate. For a base plate requiring high precision, a smooth glass plate with its surface polished is often selected. When you use this type of material, perform electroless plating, vacuum deposition, or sputtering to form a conductive film on the substrate surface.