Photoresists-- the chemistry and application

Paul Endres Chemistry Department Bowling Green State University

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a working definition of a photoresist:

• photolithography and the production of newspapers
• a number of art forms, including the decoration of glass
• the manufacture of printed circuit boards for electronics
• the manufacture of electronic integrated circuits
• the electroforming of metal objects with intricate detail
• the evolving field of nano-manufacturing
  • (making objects with extremely small detail)

Examples of Photoresist Technology

We will also provide specific details for using photoresists in an instructional laboratory.

We will also be providing links to other Web sites that describe photoresists.

Example: the manufacture of Printed Circuit Boards

• A PC board begins with a thin sheet (1/32") of rigid insulating material
• this sheet is laminated with thin layer of copper foil
• the foil is then coated with a layer of photoresist
  • (you could perform these two steps but it is more convenient to buy commercial boards which are already coated with the photoresist.)
• The board is then covered with a photographic negative and exposed to light
• The exposed board is then developed in a suitable solvent or reagent
• The result is a copper foil that is covered by a photochemical mask
  • in most areas the foil is unprotected
  • portions of the foil are protected (masked)
• The exposed copper is now dissolved by immersing the treated board in hot Ferric Chloride or Ammonium Persulfate solution.
• Finally, the board is rinsed and the photoresist mask is removed with a suitable solvent.
• The result is a finished PC board that duplicates the details of the photographic negative.

  Our illustration shows a simple "X" pattern being produced, but a typical printed circuit board produces copper patterns that provide for mounting electrical components and for electrical connections between the components.

  • there are pads for mounting components
  • there are traces (wires) for connecting components
  • there are contacts at the edge which will serve as connectors
• While each board is prepared individually, the same negative can be used to produce an unlimited number of duplicate board.

Electroforming Metal Objects

• We will start with a small sheet of polished stainless steel (1"x2" perhaps)
• We will coat the top with a thin layer of photoresist
• We then expose the photoresist through a suitable negative and develop the image
  • In this case our goal is to have most of the metal masked except for a series of fine lines that form a delicate grid.
• We also coat the back of the metal with a varnish to make it electrically insulating
• We connect a wire to a corner of the work piece and we make it the cathode in an electroplating bath. (Nickel is generally a good choice)
• After 30-45 minutes we will have formed a fine metal grid (0.001-0.002" thick) in the exposed areas
• The work piece is removed and rinsed.
• The electroformed metal grid can be removed by carefully sliding a razor blade across the surface.
• The result is a very fine metal foil with carefully formed details.
• The result is often a metal object with delicate details that could not be produced by more conventional machining techniques.
• Computer based drawing programs can be used to produce intricate images that can be photographed to provide the necessary negatives.
• Stainless steel plays a special role here
  • Stainless steel is an Iron/nickel/Chromium alloy
  • Stainless steel forms a tough layer of Nickel oxide.
  • The oxide is thin enough to be electrically conductive
  • The oxide prevents the plated metal from adhering strongly to the surface of the stainless steel
  • Thus it is relatively easy to remove the electroformed object from the stainless steel sheet.
• We can make much more rigid objects by plating until the metal is thicker.
  • Some electric shavers use electroformed heads (grids) as the shaving heads
  • It would be impossible to drill all the tiny holes and still keep the head thin and flexible
• An alternative is to produce the electroform on an Aluminum or Zinc surface
  • The electroform can be freed by dissolving the Al or Zn in NaOH (which leaves the Ni electroform intact)
• In the jewelry field one often starts with a wax three dimensional object
  • the wax is coated with a thin layer of graphite or silver to make it conductive
  • this object is then electroplated to form a metallic object of the same shape
  • the wax can either be baked out or allowed to remain inside the object
  • (this is an example of electroforming, but it does not involve photoresists)

Sandblasting

We will need a resist that is quite tough and capable of resisting the abrasion associated with sandblasting. A number of recipes are based on gelatin (or gum acacia) which has been made light sensitive with ammonium dichromate. We will instead illustrate the process with a commercial product which is available as sheets.

• We will start with a simple curved surface like the side of a drinking glass
• We will cut a piece of commercial photoresist film with scissors
• we select a photographic positive that represents our desired image
• we expose the photoresist film through the photographic positive
  • we use a commercial UV photographic contact printer
  • an alternative is to cover with a sheet of glass and expose to a quartz halogen lamp or to sunlight.
• We remove the positive and develop the image by washing the photoresist film in warm water
  • the regions exposed to light remain covered with a tough (pink) film
  • the regions that were protected from light dissolve away, and are clear
  • a fairly vigorous spray is best and washing should continue until the clear areas are completely clear.
• The glass is now painted with a special water based glue; this is allowed to dry
• The film is then applied to the glass and lightly rubbed down to eliminate air bubbles.
• Tape is applied at the edges, to protect areas beyond the stencil
• We now carefully sandblast this region of the glass
  • we use a small, hand held sandblasting gun
  • we do this in a cabinet to protect us from the hazards of inhaled sand and glass dust
  • we use tough rubber gloves to protect our hands from hazards
• Finally, we remove the photoresist mask by treating the material with very hot water.

Example: Microscale Joule-Thompson Refrigerators

MMR Technologies -- description of their MicroMiniature Refrigerator" This is described in detail in an article:

author? Rev Sci Instrum. vol, p xxx (198x)

(need a suitable web site , reference, or photograph here)

Integrated Circuits

• Photoresists are capable of reproducing extremely fine detail
  • line widths of 1-3 microns are common in IC manufacture
• One exposure can produce a photomask with thousands or millions of details
• a typical integrated circuit may have several million components
• A large silicon wafer (6-9" in diameter) can be used to process several thousand devices at the same time.
  • The same negative may be moved and used to expose different regions of the silicon surface.
  • Since processing each wafer is expensive, that cost can be distributed over several thousand finished devices.
• A typical IC may require 5-10 steps, each requiring a new set of photoresist masks.
  • At each stage the wafer is treated to modify the silicon
    • Exposure to dopants can create conductive regions of silicon
    • Doped areas can be used to form devices like transistors
    • Exposure to water vapor can convert exposed silicon to SiO2 (insulting layers)
    • Exposure to gaseous silicon compounds can grow an addition layer of Silicon

Lithography and the Development of Photography

Lithography: A printing process that uses a stone surface

• In lithography one starts with a stone surface
  • this is usually a very finely grained stone
  • this stone will absorb water based inks
  • it is in the form of a large flat sheet, polished smooth
• An image is drawn on the stone using a wax or a varnish
  • the stone is coated everywhere except where ink is desired
• The stone is then coated with ink and the excess is wipe off
• A piece of paper is then pressed to the stone
• the exposed areas of stone have absorbed ink
• the ink is transferred to the paper
• the paper now contains an inked version of the image on the stone
• By adding more ink and repeating the process, many copies of the image may be produced
• (The technique was developed around 1825 and is still an important form of printing)

Photolithography (circa 1822)

• Niepce knew that some lithographic varnishes changed on prolonged exposure to light
  • he used as asphalt (petroleum based) varnish (Bitumen of Judea)
  • normally asphalt varnish is soluble in solvents like oil of lavender
  • when exposed to light, varnish layers hardened and became insoluble in these solvents.
• He completely coated a lithographic stone with a thin layer of varnish
  • (To be honest, his work with lithographic stones were not successful. He actually made prints by putting the varnish on zinc plates, developing the image and then etching the plate in acid. This produces fine recesses on the zinc which will hold printing ink.)
• The stone is then covered with a drawing
  • The drawing was done in dark India ink
  • The paper was thin enough to be translucent
• The stone + drawing is then exposed to sunlight (probably several days)
• The stone in then washed with a solvent
  • The varnish under the ink remains unchanged and dissolved
  • The varnish under the rest of the paper hardened and remained
• When ink is applied to the stone, it is retained only in the area where the drawing had lines of dark areas
• When the ink is transferred to paper the result is a faithful copy of the original drawing.

Instructor's Notes-- how to use photoresists

status: minor revisions Nov 2002