ISAT 436ISAT 436Micro-/Nanofabrication Micro-/Nanofabrication

and Applicationsand Applications

PhotolithographyPhotolithography

David J. Lawrence

Spring 2004

PhotolithographyPhotolithography

In a microelectronic circuit, all the circuit elements (resistors, diodes, transistors, etc.) are formed in the top surface of a wafer (usually silicon).

These circuit elements are interconnected in a complex, controlled, patterned manner.

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.

PhotolithographyPhotolithography Silicon p-n junction diode with both electrical contacts on the top

surface of the wafer:

np-type substrate

Cross section:

Al SiO2

Top view:

Can you draw the diode symbol on this diagram?

PhotolithographyPhotolithography 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.

Holes or “windows” must be cut through insulating thin films in a controlled, patterned manner.

Metal “interconnections” (thin film “wires”) must be formed in a controlled, patterned manner.

The process by which patterns are transferred to the surface of a wafer is called photolithography.

PhotolithographyPhotolithography Consider the fabrication of a silicon p-n junction diode with both

electrical contacts on the top surface of the wafer:

np-type substrate

Cross section:

Al SiO2

Top view:

PhotolithographyPhotolithography We start with a bare silicon wafer and oxidize it. (The bottom surface

also gets oxidized, but we’ll ignore that.):

p-type substrateCross section:

SiO2

Top view:

PhotolithographyPhotolithography We first need to open a “window” in the SiO2 through which we can diffuse

a donor dopant (e.g., P) to form the n-type region:

p-type substrateCross section:

SiO2

Top view:

PhotolithographyPhotolithography

The starting point for the photolithography process is a mask.

A mask is a glass plate that is coated with an opaque thin film (often a metal thin film such as chromium).

This metal film is patterned in the shape of the features we want to create on the wafer surface.

See Jaeger, Chapter 2, beginning on page 17.

PhotolithographyPhotolithography For our example, our mask could look like this:

glass plateCross section:

opaque metal,e.g.,Cr

Top view:

PhotolithographyPhotolithography

A good description of the photolithography process can be found in your textbook.

See Jaeger, Chapter 2, beginning on page 17.As you review the following presentation of key

photolithography process steps, you should continuously refer to Figure 2.1 on page 18 of Jaeger.

PhotolithographyPhotolithography 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 substrateCross section:

SiO2

Top view:

PhotolithographyPhotolithography The wafer is next coated with “photoresist”. Photoresist is a light-sensitive polymer. We will initially consider positive photoresist (more

about what this means soon). Photoresist is usually “spun on”. For this step, the wafer is held onto a support chuck

by a vacuum. 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.

PhotolithographyPhotolithography After coating with photoresist, the wafer looks like this:

p-type substrateCross section:

Photoresist

Top view:

PhotolithographyPhotolithography 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 substrateCross section:

Photoresist

Top view:

PhotolithographyPhotolithography The mask is “aligned” (positioned) as desired on top of the wafer.

Mask

Cross section:

Top view:

p-type substrate

glass plate

PhotolithographyPhotolithography 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

PhotolithographyPhotolithography 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

PhotolithographyPhotolithography 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

PhotolithographyPhotolithography The unprotected SiO2 is removed by etching in a chemical

solution containing HF (hydrofluoric acid), or by “dry” etching in a gaseous plasma, containing CF4 , for example.

Cross section:

Top view:

p-type substrate

PhotolithographyPhotolithography The photoresist has done its job and is now removed (“stripped”)

in a liquid solvent (e.g., acetone) or in a “dry” O2 plasma.

Cross section:

Top view:

p-type substrate

SiO2“window”

PhotolithographyPhotolithography Phosphorous is next diffused through the window to form an n-type

region. The SiO2 film blocks phosphorus diffusion outside the window.

Cross section:

Top view:

p-type substrate

SiO2“window”

n-type

PhotolithographyPhotolithography Another photolithography step must be performed in order to open

another window in the SiO2 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 substraten-type

glass platenew mask

PhotolithographyPhotolithography The steps will not be shown in detail, but after photolithography,

SiO2 etching, and photoresist stripping, the wafer structure is shown below.

np-type substrate

Cross section:

SiO2

Top view:

PhotolithographyPhotolithography The wafer surface is next coated with aluminum by evaporation or

sputtering. The window outlines may still be visible.

np-type substrate

Cross section:

Al SiO2

Top view:

PhotolithographyPhotolithography Photolithography is used to pattern photoresist so as to protect the

aluminum over the windows:

Al SiO2

np-type substrate

Cross section:

Top view:

PhotolithographyPhotolithography What must the mask look like in order to pattern the aluminum film?

Assume that we’re still using positive photoresist.

np-type substrate

Cross section:

Al SiO2

Top view:

PhotolithographyPhotolithography The aluminum is etched where it is not protected by photoresist. Wet

or dry etchants can be used.

np-type substrate

Cross section:

Al SiO2

Top view:

PhotolithographyPhotolithography Then the photoresist is stripped.

np-type substrate

Cross section:

Al SiO2

Top view:

PhotolithographyPhotolithography 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.

np-type substrate

Cross section:

Al SiO2

Top view:

PhotolithographyPhotolithography So far we have only considered positive

photoresists. For positive resists, the resist pattern on the

wafer looks just like the pattern on the mask. Also see Figure 2.2 on page 19 of Jaeger. There are also negative photoresists. Ultraviolet light crosslinks negative resists, making

them less soluble in a developer solution. For negative resists, the resist pattern on the

wafer is the negative of the pattern on the mask. See Figure 2.6 on page 24 of Jaeger.

PhotolithographyPhotolithography

In order to align a new pattern to a pattern already on the wafer, alignment marks are used.

See pages 22-23 and Figures 2.2 and 2.5 on pages 19 and 23 of Jaeger.

PhotolithographyPhotolithographyThere are numerous etching techniques for the

various materials used in microelectronics. These techniques can be divided into two categories: Wet chemical etching, and Dry etching.

See pages 25-27 of Jaeger.Etching processes can be

Isotropic, or Anisotropic.

See Figure 2.7 on page 25 of Jaeger.

PhotolithographyPhotolithography Photomask fabrication is described on page 28 of Jaeger.

Various exposure systems (“printing techniques”) are described on pages 28-36 of Jaeger: Contact printing, Proximity printing, Projection printing, and Direct step-on-wafer (step-and-repeat projection).

PhotolithographyPhotolithography

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.

This dimension is called the minimum feature size or minimum linewidth.

The limitations of optical lithography are a consequence of basic physics (diffraction).

PhotolithographyPhotolithography For a single-wavelength projection photo-

lithography system, the minimum feature size or minimum linewidth is given by the Rayleigh criterion:

is the wavelength.NA is the numerical aperture, a measure of the

light-collecting power of the projection lens. k depends on the photoresist properties and the

“quality’ of the optical system.

NAkFw

min

PhotolithographyPhotolithography

So how do we reduce wmin ?

Reduce k.Reduce . Increase NA.

NAkFw

min

PhotolithographyPhotolithography

Even for the best projection photolithography systems, NA is less than 0.8.

The theoretical limit for k (the lowest value) is about 0.25.

NAkFw

min

PhotolithographyPhotolithography

Lenses with higher NA can produce smaller linewidths.

This linewidth reduction comes at a price.The depth of focus decreases as NA increases.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.

NAkFw

min

PhotolithographyPhotolithography

Depth of focus is given by:

2)(6.0NA

DF

Depth of focus decreases (bad) as decreases.Depth of focus decreases (bad) as NA increases.

PhotolithographyPhotolithography Numerous light sources are (and will be) used for

optical lithography:

Light Source

(nm)

wmin (nm)

DF (nm)

g-line (Hg lamp) 436 311 850

i-line (Hg lamp) 365 260 730

KrF laser 248 175 500

ArF laser 193 140 400

F2 laser 157 112 320

PhotolithographyPhotolithography 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.

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.

How is this yield calculated?

PhotolithographyPhotolithography

Other lithographic techniques will play a role in the future.

Electron beam lithography Ion beam lithography.X-ray lithography.