Presentation on theme: "Photolithography D. Boolchandani Department of ECE Malaviya National Institute of Technology Jaipur."— Presentation transcript:
Photolithography D. Boolchandani Department of ECE Malaviya National Institute of Technology Jaipur
Photolithography2 Photolithography H In a microelectronic circuit, all the circuit elements (resistors, diodes, transistors, etc.) are formed in the top surface of a wafer (usually silicon). H These circuit elements are interconnected in a complex, controlled, patterned manner. H Consider the simple case of a silicon p-n junction diode with electrical contacts to the p and n sides on the top surface of the wafer.
Photolithography3 Photolithography H Silicon p-n junction diode with both electrical contacts on the top surface of the wafer: n p-type substrate Cross section: Al SiO 2 Top view: H Can you draw the diode symbol on this diagram?
Photolithography4 Photolithography H In order to produce a microelectronic circuit, portions of a silicon wafer must be doped with donors and/or acceptors in a controlled, patterned manner. H Holes or windows must be cut through insulating thin films in a controlled, patterned manner. H Metal interconnections (thin film wires) must be formed in a controlled, patterned manner. H The process by which patterns are transferred to the surface of a wafer is called photolithography.
Photolithography5 Photolithography H Consider the fabrication of a silicon p-n junction diode with both electrical contacts on the top surface of the wafer: n p-type substrate Cross section: Al SiO 2 Top view:
Photolithography6 Photolithography H We start with a bare silicon wafer and oxidize it. (The bottom surface also gets oxidized, but well ignore that.): p-type substrate Cross section: SiO 2 Top view:
Photolithography7 Photolithography H We first need to open a window in the SiO 2 through which we can diffuse a donor dopant (e.g., P) to form the n-type region: p-type substrate Cross section: SiO 2 Top view:
Photolithography8 Photolithography H The starting point for the photolithography process is a mask. H A mask is a glass plate that is coated with an opaque thin film (often a metal thin film such as chromium). H This metal film is patterned in the shape of the features we want to create on the wafer surface.
Photolithography9 Photolithography H For our example, our mask could look like this: glass plate Cross section: opaque metal,e.g.,Cr Top view:
Photolithography10 Photolithography H Recall that we start with a bare silicon wafer and oxidize it. (The bottom surface also gets oxidized, but well ignore that.): p-type substrate Cross section: SiO 2 Top view:
Photolithography11 Photolithography H The wafer is next coated with photoresist. H Photoresist is a light-sensitive polymer. H We will initially consider positive photoresist (more about what this means soon). H Photoresist is usually spun on. H For this step, the wafer is held onto a support chuck by a vacuum. H Photoresist is typically applied in liquid form (dissolved in a solvent). The wafer is spun at high speed (1000 to 6000 rpm) for 20 to 60 seconds to produce a thin, uniform film, typically 0.3 to 2.5 m thick.
Photolithography12 Photolithography H After coating with photoresist, the wafer looks like this: p-type substrate Cross section: Photoresist Top view:
Photolithography13 Photolithography H The wafer is baked at 70 to 90°C (soft bake or pre-bake) to remove solvent from the photoresist and improve adhesion. p-type substrate Cross section: Photoresist Top view:
Photolithography14 Photolithography H The mask is aligned (positioned) as desired on top of the wafer. Mask Cross section: Top view: p-type substrate glass plate
Photolithography15 Photolithography H The photoresist is exposed through the mask with UV light. UV light breaks chemical bonds in the photoresist. Mask Cross section: Top view: p-type substrate glass plate
Photolithography16 Photolithography H The photoresist is developed by immersing the wafer in a chemical solution that removes photoresist that has been exposed to UV light. Cross section: Top view: p-type substrate
Photolithography17 Photolithography H The wafer is baked again, but at a higher temperature (120 to 180°C). This hard bake or post-bake hardens the photoresist. Cross section: Top view: p-type substrate
Photolithography18 Photolithography H The unprotected SiO 2 is removed by etching in a chemical solution containing HF (hydrofluoric acid), or by dry etching in a gaseous plasma, containing CF 4, for example. Cross section: Top view: p-type substrate
Photolithography19 Photolithography H The photoresist has done its job and is now removed (stripped) in a liquid solvent (e.g., acetone) or in a dry O 2 plasma. Cross section: Top view: p-type substrate SiO 2 window
Photolithography20 Photolithography H Phosphorous is next diffused through the window to form an n-type region. The SiO 2 film blocks phosphorus diffusion outside the window. Cross section: Top view: p-type substrate SiO 2 window n-type
Photolithography21 Photolithography H Another photolithography step must be performed in order to open another window in the SiO 2 so we can make an electrical contact to the p-type substrate from the top surface of the wafer. Cross section: Top view: p-type substrate n-type glass plate new mask
Photolithography22 Photolithography H The steps will not be shown in detail, but after photolithography, SiO 2 etching, and photoresist stripping, the wafer structure is shown below. n p-type substrate Cross section: SiO 2 Top view:
Photolithography23 Photolithography H The wafer surface is next coated with aluminum by evaporation or sputtering. The window outlines may still be visible. n p-type substrate Cross section: Al SiO 2 Top view:
Photolithography24 Photolithography H Photolithography is used to pattern photoresist so as to protect the aluminum over the windows: Al SiO 2 n p-type substrate Cross section: Top view:
Photolithography25 Photolithography H What must the mask look like in order to pattern the aluminum film? Assume that were still using positive photoresist. n p-type substrate Cross section: Al SiO 2 Top view:
Photolithography26 Photolithography H The aluminum is etched where it is not protected by photoresist. Wet or dry etchants can be used. n p-type substrate Cross section: Al SiO 2 Top view:
Photolithography27 Photolithography H Then the photoresist is stripped. n p-type substrate Cross section: Al SiO 2 Top view:
Photolithography28 Photolithography H The final step is to anneal (heat treat) the wafer at ~ 450°C in order to improve the electrical contact between the aluminum film and the underlying silicon. n p-type substrate Cross section: Al SiO 2 Top view:
Photolithography29 Photolithography H So far we have only considered positive photoresists. H For positive resists, the resist pattern on the wafer looks just like the pattern on the mask H There are also negative photoresists. H Ultraviolet light crosslinks negative resists, making them less soluble in a developer solution. H For negative resists, the resist pattern on the wafer is the negative of the pattern on the mask.
Photolithography30 Photolithography H In order to align a new pattern to a pattern already on the wafer, alignment marks are used. H Various exposure systems G Contact printing, G Proximity printing, G Projection printing, and G Direct step-on-wafer (step-and-repeat projection).
Photolithography31 Photolithography H A complete photolithography process (photoresist + exposure tool + developing process) can be characterized by the smallest (finest resolution) lines or windows that can be produced on a wafer. H This dimension is called the minimum feature size or minimum linewidth. H The limitations of optical lithography are a consequence of basic physics (diffraction).
Photolithography32 Photolithography H For a single-wavelength projection photo- lithography system, the minimum feature size or minimum linewidth is given by the Rayleigh criterion: is the wavelength. H NA is the numerical aperture, a measure of the light-collecting power of the projection lens. H k depends on the photoresist properties and the quality of the optical system.
Photolithography33 Photolithography H So how do we reduce w min ? H Reduce k. Reduce. H Increase NA.
Photolithography34 Photolithography H Even for the best projection photolithography systems, NA is less than 0.8. H The theoretical limit for k (the lowest value) is about 0.25.
Photolithography35 Photolithography H Lenses with higher NA can produce smaller linewidths. H This linewidth reduction comes at a price. H The depth of focus decreases as NA increases. H Depth of focus is the distance that the wafer can be moved relative to (closer to or farther from) the projection lens and still keep the image in focus on the wafer.
Photolithography36 Photolithography H Depth of focus is given by: Depth of focus decreases (bad) as decreases. H Depth of focus decreases (bad) as NA increases.
Photolithography37 Photolithography H Numerous light sources are (and will be) used for optical lithography:
Photolithography38 Photolithography H Complex devices require the photolithography process to be carried out over 20 times. over 20 mask levels Any dust on the wafer or mask can result in defects. Cleanrooms are required for fabrication of complex devices. H Even if defects occur in only 10% of the chips during each photolithography step, fewer than 50% of the chips will be functional after a seven mask process is completed. H How is this yield calculated?
Photolithography39 Photolithography H Other lithographic techniques will play a role in the future. H Electron beam lithography H Ion beam lithography. H X-ray lithography.