**3.2.2 Using calixarene negative EB resist**

10 Recent Advances in Nanofabrication Techniques and Applications

Figure 3.3 shows SEM images of ZEP520 pit patterns drawn at an exposure dosage of around 180 C/cm2. The figure shows that the minimum pit arrays were drawn with a minimum pit diameter of <20 nm at a pitch of 40 nm x 60 nm [13]. We could not form higherpacked pit patterns than this. Furthermore, the pit size drastically changed, with a fluctuation of about 18 nm in this pitch arrays pattern and about 11 nm in the case of a pitch of 100 nm x 60 nm (Fig. 3. 4). This indicates that the pit array pattern with a pitch of 40 nm x 60 nm is the limit for ZEP520. This pattern corresponds to about 270 Gb/in2. But it is not practically usable because the fluctuations are too large. In addition, the minimum pit diameter is about 7 nm in Fig. 3. 3. Comparing with Monte Carlo Simulation result, the pit diameter is larger than simulated size

Fig. 3.3. SEM images of ultrahigh packed pit resist pattern using ZEP520 (180 C/cm2, 30

Fig. 3.4. Variations of ZEP520 resist pit width in ultrahigh packed pit arrays (a) with a pitch

of 100 nm x 50 nm and (b) 60 nm x 40 nm for exposure dosage at 30 kV.

of 4 nm. This is caused by capillary force between pit and developer.

kV), (a) pitch of 60 nm x of 50 nm, (b) 60 nm x 40 nm.

**3.2 EB drawing [7, 13]** 

**3.2.1 Using ZEP520 positive EB resist** 

Figure 3.5 shows SEM images of ultrahigh-packed dot arrays resist patterns (a) at a pitch of 30 nm x 30 nm, and (b) 30 nm x 25 nm. The exposure dosage was 40 mC/in2. In these experiments, we succeeded in obtaining high packed dot arrays pattern (Fig. 3. 5(b)). The dot size was 11 to 14 nm in diameter. The size fluctuation was about 2 or 3 nm and almost constant in the range of 30 to 45 mC/cm2. This resist is very suitable to nano-fabrication (Fig. 3. 6).

Calixarene resist, however, has the drawback that its sensitivity is too low for mass production purposes. It takes much time to draw the dot arrays pattern over large sample. We have to develop a new resist with sensitivity higher by 2 orders of magnitude. The reason for its low sensitivity is in related with its molecule size. A lot of electrons are required to change the calixarene molecule to large molecular (weight: >several 10000s) for insolubility by linking many calixarene molecular (weight: about 600).

Fig. 3.5. SEM images of ultrahigh packed dot resist pattern using calixarene (14mC/cm2, 30kV), (a) pitch of 30 nm x 30 nm, (b) 30 nm x 25 nm.

Fig. 3.6. Variations of the calixarene dot size in ultra-high-packed dot arrays; (a) pitch of 30 nm x 30 nm and (b) 25 nm x 25 nm.

### **3.3 Consideration of the different limitations in ZEP520 and calixarene**

The difference between the limitations has been investigated using the exposure intensity distribution (EID) and EDD in Monte Carlo simulation. The EID functions were determined by measuring the widths of one-line patterns drawn under various exposure dosages. We obtained the change of the required exposure dosage for the line-width, and then the EID from the change.

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 13

The possibility to achieve very fine dot arrays with a pitch of less than 20 nm x 20 nm using 30 keV EB drawing with calixarene resist has been investigated. In order to form such a pattern, the resist thickness dependence on dot size and packing has been studied. In this section, EB drawing with extremely thin film for very highly packed dot arrays formation is described. As the experimental results, it is demonstrated to form higher packed dot arrays pattern with a pitch of 20 nm x 20 nm and 18 nm x 18 nm in 13-nm-thick resist using 30 keV electron beam.

In order to check the resist thickness dependence on formation of very fine pitch dot arrays using 30 keV electrons, the EDD was calculated using Monte Carlo simulation. From the simulation, the possibility to achieve a very fine pitch dot arrays is studied. It is very important to study the EDD because the resist dot formation occurs by linking the molecula enhanced by the energy in a case of using negative resist. The EDD function is represented as Eq. (3. 1), assuming that the distribution consists of 2 Gaussian distributions as described in last section.

Figure 4. 1 shows the EDDs with PMMA resist thickness of 15 nm, 70 nm and 200 nm on Si substrate with 30 keV electrons using Monte Carlo simulation. From the result, the σ1 values are about 2 nm, 3 nm and 8 nm at a resist thickness of 15 nm, 70 nm and 200 nm, respectively. Figure 4. 2 shows a variation of the σ1 value for the resist thicknesses. As the resist thickness decreases, the σ1 value becomes small. We can estimate the σ1 value of about 1.5 nm at a resist thickness of 10 nm. This means that it is possible to form finer pitch dot arrays than that in previous section using such a thin film. Therefore, it is very important to


Fig. 4.1. Monte Carlo calculated result of energy deposition distributions (EDDs) for various

Resist thickness 70nm 15nm

30keV Bottom PMMA 5x105 electrons

200nm

At least, the miniaturization of the dot size, the σ1 value in the first term is very crucial.

use resist film as thin as possible for formation of very fine pitch dot arrays.

Fig. 4.2. A variation of the σ1 value in EDD for the resist thicknesses.

resist thickness of PMMA on Si substrate.

**4. Challenge of formation of less than 20 nm x 20 nm very fine pitch dot arrays using 30 keV EB drawing with thin calixarene resists [5, 16, 17]** 

**4.1 Electron scattering in thin resist film for fine dot arrays [16]** 

The EID is defined by Eq. (3.1), assuming that the distribution is Gaussian. The 1st and 2nd terms in Eq. (3.1) represent the energy depositions due to electron forward scattering (FS) and backward scattering (BS), respectively.

$$E(r) = C\_1 \exp(-\frac{r^2}{\sigma\_1^2}) + C\_2 \exp(-\frac{r^2}{\sigma\_2^2}) \tag{3.1}$$

Fig. 3.7. EID functions measured by using ZEP520 and calixarene EB-drawing at 30 keV.

For the miniaturization of the bit size, the 1 value in the 1st term is crucial. In the experiments, the exposure dosages were changed from 10 C/cm2 to 5 C/cm2. Then, we obtained the EIDs using ZEP520 and calixarene, as shown in Fig. 3. 7. Figure 3. 7 shows the 1st term in detail. When we roughly fit the EID function to Eq. (3.1), the 1 values were about 5 nm and 20 nm in calixarene and ZEP520, respectively. In the Monte Carlo simulation, the 1 values were about 2 nm and 3 nm at a resist thickness of 15 nm and 70 nm, respectively (Fig. 4. 1). Although the values were calculated under the condition of 30-keV electron incidence on PMMA resist on a Si substrate and are slightly smaller than those using calixarene and ZEP520, we consider that they are roughly the same as those using PMMA because the resist thicknesses are very thin. When the electron probe size of 2 nm and the molecular size of 1 and 3 nm in calixarene and ZEP520, respectively, are considered, the experimental value becomes 13 nm larger than the estimated value in ZEP520, while the values are almost same in calixarene. This may be due to the molecular size, structure of the ZEP520 resist and capillary force in the development. The size of ZEP520 is a few nm assuming to be spherical. Sometimes, ZEP520 may be in a chain structure when the molecule is not solved after EB exposure. This comparison indicates that the smallest pattern in EB writing may be determined by the resist's molecular size, structure and resist type.

The EID is defined by Eq. (3.1), assuming that the distribution is Gaussian. The 1st and 2nd terms in Eq. (3.1) represent the energy depositions due to electron forward scattering (FS)

**)exp()exp()(** <sup>2</sup>

2

Fig. 3.7. EID functions measured by using ZEP520 and calixarene EB-drawing at 30 keV.

the 1st term in detail. When we roughly fit the EID function to Eq. (3.1), the

experiments, the exposure dosages were changed from 10 C/cm2 to 5 C/cm2. Then, we obtained the EIDs using ZEP520 and calixarene, as shown in Fig. 3. 7. Figure 3. 7 shows

were about 5 nm and 20 nm in calixarene and ZEP520, respectively. In the Monte Carlo

nm, respectively (Fig. 4. 1). Although the values were calculated under the condition of 30-keV electron incidence on PMMA resist on a Si substrate and are slightly smaller than those using calixarene and ZEP520, we consider that they are roughly the same as those using PMMA because the resist thicknesses are very thin. When the electron probe size of 2 nm and the molecular size of 1 and 3 nm in calixarene and ZEP520, respectively, are considered, the experimental value becomes 13 nm larger than the estimated value in ZEP520, while the values are almost same in calixarene. This may be due to the molecular size, structure of the ZEP520 resist and capillary force in the development. The size of ZEP520 is a few nm assuming to be spherical. Sometimes, ZEP520 may be in a chain structure when the molecule is not solved after EB exposure. This comparison indicates that the smallest pattern in EB writing may be determined by the resist's molecular size,

1 values were about 2 nm and 3 nm at a resist thickness of 15 nm and 70

1 value in the 1st term is crucial. In the

1 values

1

2 2 1

2

2

*<sup>r</sup> <sup>C</sup> <sup>r</sup> CrE* (3.1)

and backward scattering (BS), respectively.

For the miniaturization of the bit size, the

structure and resist type.

simulation, the

#### **4. Challenge of formation of less than 20 nm x 20 nm very fine pitch dot arrays using 30 keV EB drawing with thin calixarene resists [5, 16, 17]**

The possibility to achieve very fine dot arrays with a pitch of less than 20 nm x 20 nm using 30 keV EB drawing with calixarene resist has been investigated. In order to form such a pattern, the resist thickness dependence on dot size and packing has been studied. In this section, EB drawing with extremely thin film for very highly packed dot arrays formation is described. As the experimental results, it is demonstrated to form higher packed dot arrays pattern with a pitch of 20 nm x 20 nm and 18 nm x 18 nm in 13-nm-thick resist using 30 keV electron beam.

#### **4.1 Electron scattering in thin resist film for fine dot arrays [16]**

In order to check the resist thickness dependence on formation of very fine pitch dot arrays using 30 keV electrons, the EDD was calculated using Monte Carlo simulation. From the simulation, the possibility to achieve a very fine pitch dot arrays is studied. It is very important to study the EDD because the resist dot formation occurs by linking the molecula enhanced by the energy in a case of using negative resist. The EDD function is represented as Eq. (3. 1), assuming that the distribution consists of 2 Gaussian distributions as described in last section. At least, the miniaturization of the dot size, the σ1 value in the first term is very crucial.

Figure 4. 1 shows the EDDs with PMMA resist thickness of 15 nm, 70 nm and 200 nm on Si substrate with 30 keV electrons using Monte Carlo simulation. From the result, the σ1 values are about 2 nm, 3 nm and 8 nm at a resist thickness of 15 nm, 70 nm and 200 nm, respectively. Figure 4. 2 shows a variation of the σ1 value for the resist thicknesses. As the resist thickness decreases, the σ1 value becomes small. We can estimate the σ1 value of about 1.5 nm at a resist thickness of 10 nm. This means that it is possible to form finer pitch dot arrays than that in previous section using such a thin film. Therefore, it is very important to use resist film as thin as possible for formation of very fine pitch dot arrays.

Fig. 4.1. Monte Carlo calculated result of energy deposition distributions (EDDs) for various resist thickness of PMMA on Si substrate.

Fig. 4.2. A variation of the σ1 value in EDD for the resist thicknesses.

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 15

Fig. 4.5. Variations of average calixarene dot size with thicknesses for various exposure dosages.

Fig. 4.6. SEM images of 20nm x 20nm pitched calixarene resist dot arrays formed by 30keV

From Fig. 4. 4, the variations of average calixarene dot size for various thicknesses and exposure dosages were obtained as shown in Fig. 4. 5. The figure shows that the dot diameter decreases with not only thickness but also exposure dosage. Although the diameter variation with exposure dosage means that proximity effect occurs in the EB writing, the thin resist layer contributes to get small drawing probe. In order to draw the 20 nm x 20 nm pitch dot arrays pattern, we need a dot size of 15 nm at least. From Fig. 4. 5, it is neccessary to choose an exposure dosage of < 16 mC/cm2 and a resist thickness of < 13 nm

Figure 4. 6 shows the result of the 20 nm x 20 nm pitch dot arrays patterns drawing with a thickness of 11.8-14.7 nm at some exposure dosages. The exposure dosages were 14 mC/cm2, 16 mC/cm2 and 18 mC/cm2. At a dosage of 14 mC/cm2, there are some vacancies as defects. It may be caused by that the dosage is not enough to make the resist molecular link. When using commercial developer, the insufficient exposed resist part was solved so

EB drawing on Si substrate (1.6 Tb/in2) in 9 shots/dot drawing.

for the fine pitch arrays formation.
