Phase Shifting Mask

A phase-shifting mask is employed to diminish the problems arising from wavelength or depth of focus (DOF). The basic concept of the phase-shifting mask is illustrated in figure 4.12. At the conventional transmission mask (Figure 4.12a), the electric field q has the same phase at every transparent area. The limited resolution and diffraction of the optical system spread the electric field q at the wafer as intensity I is proportional to the square of the electric field shown by the line. Interference between waves diffracted by the consecutive spots in mask enhances the field between them.

The phase-shifting layer that covers adjacent spot in mask reverses the sign of the electric field but intensity at the mask is unchanged as shown in figure 4.12(b). The electric field of these images shown by the dotted line can be canceled at the wafer. Subsequently, images that are projected close to each other can be separated completely. A 180° phase change produces when a transparent layer of thickness d = X / 2 (q - 1), where q is the refraction index and X is the wavelength, covers one mask as shown in figure 4.12(b).

Basic concept of Phase Shifting Mask

Fig. 4.12 Basic concept of Phase Shifting Mask

Photoresist

To get an authentic recording of the geometry of mask over the substrate surface the resist should fulfill following conditions:

  • • Uniform film formation
  • • Resolution
  • • Good adhesion to the substrate
  • • Proper resistance to diy and wet etch processes

A photoresist is a radiation-sensitive compound that forms a Polymer film in radiation. The film is photosensitive or capable or reacting with the photolysis product of added compound so that the solubility in developer solution increases or decreases significantly by exposure to UY radiation. According to the solubility changes that take place, photoresists are termed negative or positive. Materials which are solidified, less soluble in a developer solution by radiance produce a negative pattern of the mask and are called negative photoresists. Wherein positive resists, the exposed region becomes more soluble by radiance so more readily removed in the developing process. The net result is that the patterns formed in the positive resist on the wafer are same as those on the mask and the patterns etched are the reverse of the mask patterns for negative resists as the exposed regions become less soluble. The change in solubility is due to a set of chemical reactions which occur as the photon scatters and loses energy in the resist polymer material. Figure 4.13 shows lithographic transfer process.

Lithographic transfer process

Fig. 4.13 Lithographic transfer process

A positive photoresist comprises of three ingredients: (a) photosensitive compound, (b) base resin, (c) organic solvent. The photosensitive compound is insoluble in the developer solution earlier to UY radiance. During radiation the photosensitive compound in the exposed pattern areas changes its chemical structure by absorbing the radiation energy (free energy) and converts into a more soluble species. Bonds are then broken within the resin itself that increases its solubility. After that the exposed areas are removed by the developer solution. In positive resists exposed section of the resist is not required for the final integrated circuit pattern. This is known as shadow printing. Few positive electron resists are PMMA, and poly-butene-sulfone also known as PBS.

Negative photoresists are polymers pooled with a photosensitive compound i.e. it consist of a chemically inert film-forming component along with a photoactive agent. During exposure the photosensitive compound absorbs the free energy and converts it into chemical energy for initiating a chain reaction in which photoactive agent releases nitrogen gas on exposure to light and the radicals generated in this reaction react with the C=C and C=0 double bonds within the polymer that causes crosslinking of the polymer molecules. The cross-linked polymer becomes hard due to higher molecular weight so insoluble in the developer solution. The unexposed portions are removed after development processing. Dining this process, the cross-linked polymer molecules tend to swell as they now have a higher molecular weight, and therefore distort the pattern on the resist. The major drawback of a negative photoresist is limiting the resolution as the resist absorbs developer solvent and swells. Common negative electron resists is Poly-glycidyl-methacrylate- co-ethyl-acrylate abbreviated COP.

Positive photoresist (left) and negative photoresist (right)

Fig. 4.14 Positive photoresist (left) and negative photoresist (right): Exposure response curve and cross section of the resist image after development

The left portion of figure 4.14 shows the exposure response curve for a positive resist. It should be noted that even prior to exposure the resist has a finite solubility in the developer solution. At a threshold energy (ET) the resist becomes completely soluble so ET corresponds to the sensitivity of the photoresist. The contrast ratio (y) has dependent on ET as given:

Where E} is the energy obtained by sketching the tangent at ET to reach 100% resist thickness as shown in figure 4.14. A larger у indicates rapid dissolution of the resist with an incremental rise of exposure energy that resulted a sharper image. The image cross section depicted in (figure 4.13) illustrates that the edges of the resist image are generally blurred due to diffraction.

The right portion of Figure 4.14 illustrate an analogous situation but for a negative photoresist. Here the sensitivity of a negative photoresist is defined as the energy required retaining 50% of the original resist film thickness in the exposed region. Table 4.1 lists some of the common resists used in IC Technology.

Table 4.1 Common resists used in IC Technology.

Lithography

Name

Type

Sensitivity

V

Optical

Kodak 747

Negative

9 mJ/cm2

1.9

AZ-1350J

Positive

90 mJ/cm2

1.4

PR102

Positive

140 mJ/cm2

1.9

e-beam

COP

Negative

0.3 pC/cm2

0.45

GeSe

Negative

80 pC/cm2

3.5

PBS

Positive

1 pC/cm2

0.35

PMMA

Positive

50 pC/cm2

1.0

X-ray

COP

Negative

175 mJ/cm2

0.45

DCOPA

Negative

10 mJ/cm2

0.65

PBS

Positive

95 mJ/cm2

0.50

PMMA

Positive

1000 mJ/cm2

1.0

 
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