ISAT 436 Micro-/Nanofabrication and Applications Photolithography David J. Lawrence Spring 2004

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.

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?

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.

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:

Photolithography H We start with a bare silicon wafer and oxidize it. (The bottom surface also gets oxidized, but we’ll ignore that.): p-type substrate Cross section: SiO 2 Top view:

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:

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. H See Jaeger, Chapter 2, beginning on page 17.

Photolithography H For our example, our mask could look like this: glass plate Cross section: opaque metal,e.g.,Cr Top view:

Photolithography H A good description of the photolithography process can be found in your textbook. H See Jaeger, Chapter 2, beginning on page 17. H As you review the following presentation of key photolithography process steps, you should continuously refer to Figure 2.1 on page 18 of Jaeger.

Photolithography H Recall that we start with a bare silicon wafer and oxidize it. (The bottom surface also gets oxidized, but we’ll ignore that.): p-type substrate Cross section: SiO 2 Top view:

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.

Photolithography H After coating with photoresist, the wafer looks like this: p-type substrate Cross section: Photoresist Top view:

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:

Photolithography H The mask is “aligned” (positioned) as desired on top of the wafer. Mask Cross section: Top view: p-type substrate glass plate

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

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

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

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

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”

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

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

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:

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:

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:

Photolithography H What must the mask look like in order to pattern the aluminum film? Assume that we’re still using positive photoresist. n p-type substrate Cross section: Al SiO 2 Top view:

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:

Photolithography H Then the photoresist is stripped. n p-type substrate Cross section: Al SiO 2 Top view:

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:

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 Also see Figure 2.2 on page 19 of Jaeger. 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. H See Figure 2.6 on page 24 of Jaeger.

Photolithography H In order to align a new pattern to a pattern already on the wafer, alignment marks are used. H See pages and Figures 2.2 and 2.5 on pages 19 and 23 of Jaeger.

Photolithography H There are numerous etching techniques for the various materials used in microelectronics. These techniques can be divided into two categories: G Wet chemical etching, and G Dry etching. H See pages of Jaeger. H Etching processes can be G Isotropic, or G Anisotropic. H See Figure 2.7 on page 25 of Jaeger.

Photolithography H Photomask fabrication is described on page 28 of Jaeger. H Various exposure systems (“printing techniques”) are described on pages of Jaeger: G Contact printing, G Proximity printing, G Projection printing, and G Direct step-on-wafer (step-and-repeat projection).

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).

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.

Photolithography H So how do we reduce w min ? H Reduce k.  Reduce. H Increase NA.

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.

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.

Photolithography H Depth of focus is given by:  Depth of focus decreases (bad) as decreases. H Depth of focus decreases (bad) as NA increases.

Photolithography H Numerous light sources are (and will be) used for optical lithography:

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?

Photolithography H Other lithographic techniques will play a role in the future. H Electron beam lithography H Ion beam lithography. H X-ray lithography.