Page 120 - Sami Franssila Introduction to Microfabrication

9

Optical Lithography

Lithography work flow consists of the following major We will discuss lithography first from a tool point of
steps when viewed from the point of view of the wafer: view, and then from a pattern point of view: the shape
and size of patterns that can be printed on the wafer.
1. Photosensitive film (photoresist) application
2. Alignment of mask and wafer 9.1 LITHOGRAPHY TOOLS (ALIGNMENT
3. Exposure of the photoresist AND EXPOSURE)
4. Development of patterns.
The simplest lithographic technique is contact lithog-
raphy: the photomask and the resist-covered wafer are
The alternative view is that of information flow; this brought into intimate contact, and exposed. The resolu-
will be discussed in Chapter 10 in conjunction with tion is determined by mask dimensions and diffraction
lithography simulation. at mask edges. Extremely small patterns can be made
Optical lithography is basically photography. The in theory but making photomasks with submicron fea-
original image to be transferred, the photomask, which tures is prohibitively expensive. Damage to mask is
corresponds to the negative in photography, is set frequent when the mask and the wafer are brought into
in a mask-aligner/exposure tool. It is aligned to the contact, which makes contact printing not very produc-
photoresist-coated wafer, and exposed by UV radiation tion worthy.
(Figure 9.1). Exposure changes photoresist solubility, Proximity lithography is a modification of contact
which enables selective removal of resist in the develop- lithography: a small gap, for example, 3 to 50 µm is
ment step. In positive resists, the exposed areas become left between the mask and the wafer. The wavefront
more soluble in the developer, and in negative resists, traversing the mask is diffracted by the mask patterns,
the exposed parts become insoluble. and Fresnel diffraction formulae have to be used
This resist pattern can be used as an etch mask. Pho- to estimate resolution. Both contact and proximity
toresist is removed after etching. The patterning process lithography are done in one and the same machine: the
continues with new doping and deposition steps, and gap between the mask and the wafer is an adjustable
new lithographic steps. Layers have to be aligned to parameter, with values from zero up (Figure 9.2).
each other, as in multiple exposure photography. Over- Contact/proximity lithography systems are 1X: the
image is the same size as the original. The role of
lay of successive layers is a critical factor in lithography,
optical system I (Figure 9.1) is then to provide uniform
not only in resolution.
illumination. Optical system II does not exist.
There are three rather different elements in the optical
In projection optical systems, the optical system II of
lithography process:
Figure 9.1 is the key element: it provides an image of
the mask on the wafer. Reduction optics can be used,
• Optics: radiation generation, propagation, focusing, and this is a great improvement over 1X systems. With
diffraction, interference; 5X reduction projection optics, the original photomask
• Chemistry: photochemical reactions in the resist, features can be made rather large, for example, 1 µm for
development; 0.2 µm final feature size. Fraunhofer far-field diffraction
• Mechanics: mask-to-wafer alignment. governs the optics of projection systems.

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)