**2. Monte Carlo simulation of electron scattering in solid for EB lithography [8]**

Electron scattering in the resist and substrate is described based on its scattering angle, mean free path, energy loss, etc. Trajectories of incident electrons and energy deposition distributions (EDDs) in the resist are calculated. From the EDD, EB drawn resist dot profiles are estimated. The formation of nanometer sized pattern for electron energy, resist thickness and resist type can be studied. The EDD in 100 nm-thick resist on Si substrate were calculated for small pattern drawing. The calculations show that 4 nm-wide pattern will be formed when resist thickness is less than 30 nm. Furthermore, a negative resist is more suitable than positive resist by the estimation of a shape of the EDD.

### **2.1 Calculation model for Monte Carlo simulation of electron scattering**

For the treatment of electron elastic scattering, the screened Rutherford scattering model [11] is employed as follow,

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 5

where the *Z* is the thickness of sub-layer and the *r* is increment in radius direction, From

Monte Carlo simulation has been excuted in energetic electrons impinging in thin film of Si covered with resist material. By using uniform random numbers between 0 and 1, the

,we can calculate the energy loss *E* due to scattering of the electrons with atoms in the sample along its trajectory. The trajectory of the electron was traced till its energy slowed down to 50 eV. PMMA *CHO* 5 82 with a compound of carbon(C), hydrogen(H) and oxygen(O) was used as the typical resist. In the simulation, we use random sampling method to determine the scattering center, the step length and use a new coordinate conversion method [12] for calculating the trajectories of electrons. The initial energies of the incident electrons are taken to be 30 keV and 10 keV. The scattering trajectories of electrons with different incident beam energies in the resist material was used with the same as PMMA resist layer on Si target are shown in Figs. 2. 1- 2. 4. In the simulation, the thickness of the resist layer of 100 nm and the number of incident electrons of 500 was used. With incident energy of 30 keV, the penetration depth was about 3.5μm and lateral range was about 1.5 μm in Si (Fig. 2. 1). In the resist layer, the electron scattering was expanded only to about 20 nm in radius direction (Fig. 2. 2). Although using 10 keV incident electrons can diffuse as deep as 0.5 μm into the sample (Fig. 2. 3) but lateral range was about 50 nm in the resist layer which is larger than that of 30 keV (Fig. 2. 4). It indicated that as the energy decreases, the electrons scattering lateral range is expanded in

*EDD r z E r z V N* , ( , )/ <sup>0</sup> (2.8)

can be calculated by using Eq. (2.4). Using Eqs. (2.5) and (2.6)

the volume, the EDD function is given by following equation,

where the *N*0 is total number of incident electron.

**2.2 Simulation results and discussion 2.2.1 Description and electron trajectories** 

> and

the thin resist layer at the top.

Fig. 2.1. Electron scattering trajectories at incident energy 30 keV

scattering angles

$$\frac{d\sigma\_i}{d\Omega} = \frac{e^4 Z\_i \left(Z\_i + 1\right)}{4E^2 \left(1 - \cos\theta + 2\,\beta\right)}\tag{2.1}$$

where the *β* is the screening parameter which is given by

$$\beta = \frac{1}{4} \left( \frac{1.12 \,\lambda\_0 \text{h}}{2 \,\pi p} \right), \,\,\lambda\_0 = Z^{1/3} \,/\, 0.885 a\_0 \tag{2.2}$$

where the e is electronic charge, th the is scattering angle, the *a0* is Bohr radius, the *h* is Planck's constant and the *p* is the electron momentum. Step length is calculated based on the electron mean free path . The is given by

$$
\Lambda = \frac{1}{n\sigma} = \frac{A}{N\rho\sigma} \tag{2.3}
$$

where the n is volume density of atoms, the is total cross section calculated from differential scattering cross section, the N is Avogadro's number, the A is atomic weight and the is mass density. Scattering angle and azimuthal angle can be obtained using the following equations:

$$\theta = \cos^{-1}\left(1 - \frac{2\beta R\_1}{1 - \beta - R\_1}\right), \text{ } \phi = 2\pi R\_2 \tag{2.4}$$

where the *R1* and *R2* are independent equidistributed random number between 0 and 1. Since the electron suffers scattering along its trajectory, it continuously loses its kinetic energy along its trajectory. In Monte Carlo simulation, incident electron is slowing down following Bethe's formula, which is a good empirical method of calculating this energy loss in electron solid interaction. The Bethe's approximation is given by

$$-\frac{dE}{ds} = \frac{2\pi e^4}{E} \sum\_i n\_i Z\_i \ln\left(\frac{1.166E}{J\_i}\right) \tag{2.5}$$

where the *ni* is volume density of atoms, the *Ji* is mean ionization energy of atom i . The terminal energy of the nth scattering is:

$$E\_{n+1} = E\_n - \left| dE \,/\, ds \right|\_{E\_n} \cdot \Lambda\_n \tag{2.6}$$

where the *En* is the energy of the (n-1)th scattering, the *n* is step length, and the / *En dE ds* is the energy loss rate which can be obtained from Eq. (2.5).

The EDD is an important parameter in consideration of EBL. In order to calculate the EDD in resist, we use cylindrical coordination system. We divide the resist layer along Z-axis into several thin sub-layers. The EDD was calculated in a radius-depth coordination system. This means that the resist layer was devided into many small concentric rings. The simulation was excuted to calculate the total energies *Er z* (,) in every unit ring for EDD function. The ring volume *V* is given by following equation,

$$
\Delta V = \left(\pi \left(r + \Delta r\right)^2 - \pi r^2\right) \cdot \Delta Z \tag{2.7}
$$

where the *Z* is the thickness of sub-layer and the *r* is increment in radius direction, From the volume, the EDD function is given by following equation,

$$\text{EDD}(r, z) = \text{E}(r, z) / \left(\Delta V \cdot N\_0\right) \tag{2.8}$$

where the *N*0 is total number of incident electron.

#### **2.2 Simulation results and discussion 2.2.1 Description and electron trajectories**

4 Recent Advances in Nanofabrication Techniques and Applications

2

*<sup>i</sup> i i d eZ Z*

*h p* 

<sup>2</sup> cos 1 1

1 1

*d E* 

<sup>0</sup> 1 1.12 4 2

 

where the *β* is the screening parameter which is given by

the energy loss rate which can be obtained from Eq. (2.5).

ring volume *V* is given by following equation,

in electron solid interaction. The Bethe's approximation is given by

where the e is electronic charge, th the

the 

following equations:

the electron mean free path . The is given by

is mass density. Scattering angle

terminal energy of the nth scattering is:

<sup>4</sup>

4 (1 cos 2 )

, 1/3 

Planck's constant and the *p* is the electron momentum. Step length is calculated based on

1 *A n N* 

where the n is volume density of atoms, the is total cross section calculated from differential scattering cross section, the N is Avogadro's number, the A is atomic weight and

and azimuthal angle

1

 2

, 2

*R R*

where the *R1* and *R2* are independent equidistributed random number between 0 and 1. Since the electron suffers scattering along its trajectory, it continuously loses its kinetic energy along its trajectory. In Monte Carlo simulation, incident electron is slowing down following Bethe's formula, which is a good empirical method of calculating this energy loss

> <sup>4</sup> 2 1.166 *i i* ln *i i*

*dE e <sup>E</sup> n Z ds E J*

where the *ni* is volume density of atoms, the *Ji* is mean ionization energy of atom i . The

<sup>1</sup> / *<sup>n</sup>*

where the *En* is the energy of the (n-1)th scattering, the *n* is step length, and the / *En*

The EDD is an important parameter in consideration of EBL. In order to calculate the EDD in resist, we use cylindrical coordination system. We divide the resist layer along Z-axis into several thin sub-layers. The EDD was calculated in a radius-depth coordination system. This means that the resist layer was devided into many small concentric rings. The simulation was excuted to calculate the total energies *Er z* (,) in every unit ring for EDD function. The

<sup>2</sup> <sup>2</sup> *V rr r Z* ( )

 

1

0 0 *Z a* /0.885 (2.2)

is scattering angle, the *a0* is Bohr radius, the *h* is

(2.3)

(2.5)

(2.7)

*E E dE ds n n <sup>E</sup> <sup>n</sup>* (2.6)

can be obtained using the

*R* (2.4)

*dE ds* is

(2.1)

> Monte Carlo simulation has been excuted in energetic electrons impinging in thin film of Si covered with resist material. By using uniform random numbers between 0 and 1, the scattering angles and can be calculated by using Eq. (2.4). Using Eqs. (2.5) and (2.6) ,we can calculate the energy loss *E* due to scattering of the electrons with atoms in the sample along its trajectory. The trajectory of the electron was traced till its energy slowed down to 50 eV. PMMA *CHO* 5 82 with a compound of carbon(C), hydrogen(H) and oxygen(O) was used as the typical resist. In the simulation, we use random sampling method to determine the scattering center, the step length and use a new coordinate conversion method [12] for calculating the trajectories of electrons. The initial energies of the incident electrons are taken to be 30 keV and 10 keV. The scattering trajectories of electrons with different incident beam energies in the resist material was used with the same as PMMA resist layer on Si target are shown in Figs. 2. 1- 2. 4. In the simulation, the thickness of the resist layer of 100 nm and the number of incident electrons of 500 was used. With incident energy of 30 keV, the penetration depth was about 3.5μm and lateral range was about 1.5 μm in Si (Fig. 2. 1). In the resist layer, the electron scattering was expanded only to about 20 nm in radius direction (Fig. 2. 2). Although using 10 keV incident electrons can diffuse as deep as 0.5 μm into the sample (Fig. 2. 3) but lateral range was about 50 nm in the resist layer which is larger than that of 30 keV (Fig. 2. 4). It indicated that as the energy decreases, the electrons scattering lateral range is expanded in the thin resist layer at the top.

Fig. 2.1. Electron scattering trajectories at incident energy 30 keV

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 7

in the resist layer of various depths 10 nm, 50 nm and 100 nm. It can be clearly seen that the shallower the depth from the surface of the resist, the narrower and the shaper the EDD. Fig. 2.6 shows the relationship between resist depth and standard deviation σ of the EDD

<sup>2</sup>

It indicates that small pattern could be produced by using thin resist. It can effectively

exp 2 2

1 1

0 2

(2.9)

*r r*

 

assuming that the EDD is approximated by Gauss distribution.

*EDD r*

reduce proximity effects and thus greatly improve resolution.

Fig. 2.5. Energy deposition distribution of different depth of resist.

Fig. 2.6. The relationship between resist depth and width of dot.

**2.2.3 Consideration for resist development based on EDD** 

Resist development is assumed as the resist molecule are solved and linked over the critical energy density so that they are soluble and insoluble in positive and negative resists, respectively. Figs. 2. 7(a)-(d) show the area over the critical energy density of 28.125 <sup>3</sup> *keV cm* / – 0.5 <sup>3</sup> *keV cm* / . It is clear that small pattern formation is possible by selecting large critical energy density, which corresponds to small exposure dosage in experiment. In the positive resist, however, it is very important to solve the top layer at

.

Fig. 2.2. Trajectories in resist (100 nm) at incident energy 30 keV.

Fig. 2.3. Electron scattering trajectories at incident energy 10 keV.

Fig. 2.4. Trajectories in resist (100 nm) at incident energy 10 keV.

#### **2.2.2 Energy deposition distribution (EDD)**

The energy deposition density at various positions in depths and radius in the thin resist was calculated. The thickness of the resist was 100 nm, and the incident energy was 30 keV. The *Z* and *r* were 2nm, and the number of electrons was 30000. Fig. 2. 5 shows the EDD

.

Fig. 2.2. Trajectories in resist (100 nm) at incident energy 30 keV.

Fig. 2.3. Electron scattering trajectories at incident energy 10 keV.

Fig. 2.4. Trajectories in resist (100 nm) at incident energy 10 keV.

The energy deposition density at various positions in depths and radius in the thin resist was calculated. The thickness of the resist was 100 nm, and the incident energy was 30 keV. The *Z* and *r* were 2nm, and the number of electrons was 30000. Fig. 2. 5 shows the EDD

**2.2.2 Energy deposition distribution (EDD)** 

in the resist layer of various depths 10 nm, 50 nm and 100 nm. It can be clearly seen that the shallower the depth from the surface of the resist, the narrower and the shaper the EDD. Fig. 2.6 shows the relationship between resist depth and standard deviation σ of the EDD assuming that the EDD is approximated by Gauss distribution.

$$EDD(r) = \frac{1}{\sqrt{2\pi}} \cdot \frac{1}{\sigma} \exp\left(-\frac{\left(r - r\_0\right)^2}{2\sigma^2}\right) \tag{2.9}$$

It indicates that small pattern could be produced by using thin resist. It can effectively reduce proximity effects and thus greatly improve resolution.

Fig. 2.5. Energy deposition distribution of different depth of resist.

Fig. 2.6. The relationship between resist depth and width of dot.

#### **2.2.3 Consideration for resist development based on EDD**

Resist development is assumed as the resist molecule are solved and linked over the critical energy density so that they are soluble and insoluble in positive and negative resists, respectively. Figs. 2. 7(a)-(d) show the area over the critical energy density of 28.125 <sup>3</sup> *keV cm* / – 0.5 <sup>3</sup> *keV cm* / . It is clear that small pattern formation is possible by selecting large critical energy density, which corresponds to small exposure dosage in experiment. In the positive resist, however, it is very important to solve the top layer at

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 9

I have already reported key technologies which enable ultrahigh packing over 1 Tb/in2: (1) the use of an EB drawing system with a fine probe and large probe current, (2) the thinning of the resist to prevent the spread of incident electron scattering in the resist, and (3) the design of a highly packed pattern with a hexagonal or centered rectangular lattice structure to avoid the proximity effect (Fig. 3. 1). The EB drawing system consists of a HR-SEM (JSM6500F, JEOL, Ltd.) and a writing controller (Tokyo Technology Co., Ltd.) (Fig. 3. 2). The drawing was done on a resist coating on a piece of Si placed on the XY table, which was not moved during the EB writing. This type system is very suitable for checking the limit of the drawing pattern size, because the stage position error can be neglected. The system provides a high probe current of 2 nA at a resolution of 2 nm. We used the system under a probe current of 100 pA and an acceleration voltage of 30 kV. In the drawing, the address resolutions were 10 nm and 2.5 nm for ZEP520 and calixarene [14, 15] resists, respectively. Development was carried out using the commercial developers ZED-N50 (MIBK+IPA) and ZEP-RD (xylene) for 210 sec and 180 sec for ZEP520 and calixarene, respectively. In particular, we adjusted the focus to get the fine

In order to achieve 1-Tb/in2 storage patterned media, we carried out experiments to confirm whether EB writing can form very fine pit or dot arrays with a pitch of <60 nm or not. At first, we coated the resist on a piece of Si substrate with a thickness of 70 nm and 15 nm in ZEP520 and calixarene, respectively. After pre-baking, we drew ultrahigh-packed pit and dot patterns with a pitch of 20-60 nm. After developing and rinsing the resists, we checked the drawn patterns using the same HR-SEM. Thinning allows not only to prevent electron scattering extension in the resist, but also to avoid electron charging. We determined the minimum thickness at which there is sufficient contrast and no damage to the resist during SEM observation. The thickness of 70 nm in ZEP520 resist was decided by electron damage and contrast signal in SEM. I could not obtain the high magnitude SEM image in a thickness

Fig. 3.1. Bit arrangement for highly packed bits array; (a) rectangular lattice pattern and (b)

**3.1 EB-drawing system [7]** 

range of less than 70 nm.

centered rectangular lattice pattern used here.

Fig. 3.2. Overview image of conventional EB drawing system.

probe on the sample surface at a magnification of 250k-200k.

first. The energy densities of the top layer do not reach to the critical energy density in Figs. 2. 7(a)-(c). As the result, no patterning occurs in the energy region as shown in Figs. 2. 7(e)-(g). When the critical energy density is less than 0.5 <sup>3</sup> *keV cm* / , the hole pattern appears as Fig. 2. 7(h). The hole diameter increases with the depth in the resist layer. In our experiment, however, the small diameter of about 4 nm disappears. This may be caused by capillary force. The minimum diameter of about 7 nm was obtained in previous experiment using ZEP520 positive resist [7]. On the other hand, Figs. 2. 7(i)-(l) show the developed resist profiles at various critical energy densities. As the linked molecule is remained on the substrate based on negative resist development mechanism, nanometersized patterns are formed as shown in Fig. 2.7(i), although the height of the resist pattern is not complete and short. It is clear that the smaller pattern size is obtained by selecting the heigher critical energy density,but the hight of the resist pattern decreases as the critical energy density increases. Therefore, negative resist is very suitable to form nanometer-sized pattern.

Fig. 2.7. Simulated resist profiles at various critical energies; (a) resist region over 28.125 keV/cm2, (b) 12.5 keV/cm2, (c) 6.25 keV/cm2, (d) 0.5 keV/cm2, (e)-(h) positive resist region remained by development, (i)-(l) remained negative resist regions.

### **3. Formation of highly packed fine pit and dot arrays using EB drawing with positive and negative resists [7, 13]**

Fine bit arrays formation for an ultrahigh density optical and magnetic recordings has experimentally been studied using EB drawing with a high resolution scanning electron microscope (HR-SEM), a drawing controller [7] and positive and negative EB resists. As experimental results, calixarene negative EB resist is very suitable to form an ultrahigh packed resist dot arrays pattern, comparing with ZEP520 positive resist as same as described in previous section. We obtained very fine dot arrays with a diameter of <15 nm, and 2-dimensional pitch of <30 nm.

#### **3.1 EB-drawing system [7]**

8 Recent Advances in Nanofabrication Techniques and Applications

first. The energy densities of the top layer do not reach to the critical energy density in Figs. 2. 7(a)-(c). As the result, no patterning occurs in the energy region as shown in Figs. 2. 7(e)-(g). When the critical energy density is less than 0.5 <sup>3</sup> *keV cm* / , the hole pattern appears as Fig. 2. 7(h). The hole diameter increases with the depth in the resist layer. In our experiment, however, the small diameter of about 4 nm disappears. This may be caused by capillary force. The minimum diameter of about 7 nm was obtained in previous experiment using ZEP520 positive resist [7]. On the other hand, Figs. 2. 7(i)-(l) show the developed resist profiles at various critical energy densities. As the linked molecule is remained on the substrate based on negative resist development mechanism, nanometersized patterns are formed as shown in Fig. 2.7(i), although the height of the resist pattern is not complete and short. It is clear that the smaller pattern size is obtained by selecting the heigher critical energy density,but the hight of the resist pattern decreases as the critical energy density increases. Therefore, negative resist is very suitable to form

Fig. 2.7. Simulated resist profiles at various critical energies; (a) resist region over 28.125 keV/cm2, (b) 12.5 keV/cm2, (c) 6.25 keV/cm2, (d) 0.5 keV/cm2, (e)-(h) positive resist region

**3. Formation of highly packed fine pit and dot arrays using EB drawing with** 

Fine bit arrays formation for an ultrahigh density optical and magnetic recordings has experimentally been studied using EB drawing with a high resolution scanning electron microscope (HR-SEM), a drawing controller [7] and positive and negative EB resists. As experimental results, calixarene negative EB resist is very suitable to form an ultrahigh packed resist dot arrays pattern, comparing with ZEP520 positive resist as same as described in previous section. We obtained very fine dot arrays with a diameter of <15 nm,

remained by development, (i)-(l) remained negative resist regions.

**positive and negative resists [7, 13]** 

and 2-dimensional pitch of <30 nm.

nanometer-sized pattern.

I have already reported key technologies which enable ultrahigh packing over 1 Tb/in2: (1) the use of an EB drawing system with a fine probe and large probe current, (2) the thinning of the resist to prevent the spread of incident electron scattering in the resist, and (3) the design of a highly packed pattern with a hexagonal or centered rectangular lattice structure to avoid the proximity effect (Fig. 3. 1). The EB drawing system consists of a HR-SEM (JSM6500F, JEOL, Ltd.) and a writing controller (Tokyo Technology Co., Ltd.) (Fig. 3. 2). The drawing was done on a resist coating on a piece of Si placed on the XY table, which was not moved during the EB writing. This type system is very suitable for checking the limit of the drawing pattern size, because the stage position error can be neglected. The system provides a high probe current of 2 nA at a resolution of 2 nm. We used the system under a probe current of 100 pA and an acceleration voltage of 30 kV. In the drawing, the address resolutions were 10 nm and 2.5 nm for ZEP520 and calixarene [14, 15] resists, respectively. Development was carried out using the commercial developers ZED-N50 (MIBK+IPA) and ZEP-RD (xylene) for 210 sec and 180 sec for ZEP520 and calixarene, respectively. In particular, we adjusted the focus to get the fine probe on the sample surface at a magnification of 250k-200k.

In order to achieve 1-Tb/in2 storage patterned media, we carried out experiments to confirm whether EB writing can form very fine pit or dot arrays with a pitch of <60 nm or not. At first, we coated the resist on a piece of Si substrate with a thickness of 70 nm and 15 nm in ZEP520 and calixarene, respectively. After pre-baking, we drew ultrahigh-packed pit and dot patterns with a pitch of 20-60 nm. After developing and rinsing the resists, we checked the drawn patterns using the same HR-SEM. Thinning allows not only to prevent electron scattering extension in the resist, but also to avoid electron charging. We determined the minimum thickness at which there is sufficient contrast and no damage to the resist during SEM observation. The thickness of 70 nm in ZEP520 resist was decided by electron damage and contrast signal in SEM. I could not obtain the high magnitude SEM image in a thickness range of less than 70 nm.

Fig. 3.1. Bit arrangement for highly packed bits array; (a) rectangular lattice pattern and (b) centered rectangular lattice pattern used here.

Fig. 3.2. Overview image of conventional EB drawing system.

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 11

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.

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

Fig. 3.5. SEM images of ultrahigh packed dot resist pattern using calixarene (14mC/cm2,

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

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

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

insolubility by linking many calixarene molecular (weight: about 600).

30kV), (a) pitch of 30 nm x 30 nm, (b) 30 nm x 25 nm.

nm x 30 nm and (b) 25 nm x 25 nm.

from the change.

**3.2.2 Using calixarene negative EB resist** 

3. 6).

#### **3.2 EB drawing [7, 13] 3.2.1 Using ZEP520 positive EB resist**

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

Fig. 3.3. SEM images of ultrahigh packed pit resist pattern using ZEP520 (180 C/cm2, 30 kV), (a) pitch of 60 nm x of 50 nm, (b) 60 nm x 40 nm.

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.
