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HIGH SCHOOL NANOSCIENCE PROGRAM

Photolithography

 

Background

In this experiment students use light to transfer a pattern onto a surface, ultimately resulting in a network of very small metal wires on a plastic board.  The pattern is transferred by placing a mask with the wire design on a plastic board.  The board is coated with a copper film that is covered with a light-reactive polymer.  The polymer is exposed to UV light through the mask to make a pattern in the polymer.  The metal under the exposed polymer is then chemically etched, leaving only small wires on the surface of the board in a pattern determined by the mask.  Students can then measure resistance as a function of wire length and wire diameter to explore both the positive and negative resistive aspects of making thing small, but close together.  This top-down approach to nanotechnology is commonly used in manufacturing circuit boards for computers and other electronics and students will learn the very simple chemistry and physics at the core of photolithography.

Nanotechnology or the technology of things on the nanometer scale (nanometer = 10-9m), recently burst onto the science scene amid much fanfare. Its predecessor, microtechnology (1 micrometer = micron =
10-6m), dominated the scientific and economic landscape for the past few decades, most notably in the field of computing based on the microchip. One major goal in the silicon chip industry is to increase the speed and performance of their chips. Like the flow of water, the flow of electrons will reach its destination more quickly if the path traveled is shorter. In other words, microchips perform faster when they are smaller.

Two different approaches can be used to make objects on the micro-and nano-scales. The first “bottom up” method builds a structure atom by atom or molecule by molecule, much like building a wall by stacking bricks. The second “top down” method involves cutting down a bulk material to the appropriate size, much as a sculptor would carve a statue out of a large block of marble.

This experiment demonstrates how devices are made using the top-down photolithographic process. Photolithography uses light to transfer a pattern onto a photosensitive (light sensitive) polymer called photoresist (PR). Photoresist often comes in liquid form and can be spread onto a substrate1 (the base foundation) and then hardened by heat. Our circuit boards come with a ready-made layer of hardened photoresist under the black tape.

UV light changes the structure of the photoresist, in our case weakening the internal bonds so that the exposed photoresist is removed by the developer. By covering part of the photoresist with a mask that blocks UV light, portions of the photoresist will not be exposed. This is like using a stencil (mask) with spray paint (UV light) to transfer a pattern onto a substrate. The photoresist can then be developed, much like a photograph, so that only the exposed portions are removed, leaving behind a photoresist film in the form of the desired pattern.

This experiment uses one exposure and one etching step to create a copper pattern. To create a more complicated structure, such as a computer chip, more steps would be involved, but photolithography would still be used to create the desired patterns. Micro-and nano-features can be produced when microscope optics are used in the exposure step. This way, the UV light is focused to a much smaller size.

In addition to affecting the speed of a device, miniaturization also affects the resistance of a device. Resistance is an important factor in a device’s power consumption, as resistive heating is the major source of energy loss. The energy that goes into heating the wire and radiating heat from the wire is not available for doing more useful work. The increase in temperature also hinders the device from working properly (think of a car overheating).
If a voltage is applied across a wire, Ohm’s Law relates the current traveling through a wire to the voltage applied across it:

V = I _ R

Voltage equals current times resistance. The higher the resistance of the wire, the less current will flow through the wire given the same applied voltage.

The resistance of a wire is related to its length (L), cross sectional area (A), and resistivity (r) according to Equation 2:

R = r x (L / A)    

The resistivity is a material property of the wire, while the ration (L/A) is a geometrical factor.  Wires of the same shape made from different metals will have different resistances due to their different resistivities.  Resistivity is a material property because it is dependant on the material of which the wire is made.  The geometry of the wire will also affect its resistance.  A ling wire will have a higher resistance than a short piece of the same wire.  A narrow wire will have a higher resistance than a thick wire of the same length.

The power (watts) lost in the form of heat can be described by the following relation, which is called Joule heating:

P = I x V = I x (I x R) = I2 x R

The larger the resistance is, the greater the power loss.

Resistance is critical to microchips because it is directly related to power loss or power consumption as well as the device’s efficiency.  A shorter wire would have a lower resistance and therefore consume less power, making a more efficient device.  If a wire’s cross-sectional area (width x height) is reduced, the resistance and power consumption increase.  If a miniaturized wire absorbs too much energy and radiates too much heat, it could melt and destroy the microchip board!