**4. Lithography**

Lithography is employed to define patterns inside a target material. Using an FIB, a multitude of processing techniques exists to achieve this goal. Similar to photolithography it can be achieved by exposing a resist material using the ion beam. However, with ions, patterns can also be defined by physically sputtering the target atoms (FIB milling), by triggering chemical reactions inside an adsorbed layer of a precursor gas (gas-assisted processing) and by ion implantation.

Among these techniques the most prominent are FIB milling and gas-assisted processing. They are employed for optical mask repair (Yasaka et al., 2008), circuit editing (CE) (Boit et al., 2008), transmission electron microscope (TEM) sample preparation and rapid prototyping (Persson et al., 2010). We will discuss these techniques only briefly since they are already well described and reviewed elsewhere (Reyntjens & Puers, 2001) (Utke et al., 2008). Instead we will focus on the less prominent FIB patterning techniques.

In the following sections we will review the work carried out on resist-based IBL and discuss the reasons for its failure as well as its chance of resurrection. Further we will present our findings on patterning using ion implantation with a focus on 3D nano patterning. Finally, we will introduce a new technique, called direct hard mask patterning (DHP) which combine the advantages of FIB milling with the speed of resist-based lithography.

#### **4.1 FIB milling and gas-assisted processing**

When an ion hits the target, surface elastic and inelastic scattering processes will take place. While inelastic processes are responsible for the generation of photons and secondary electrons, elastic scattering will transfer kinetic energy from the ion to the target atoms (Orloff et al., 2002). This kinetic energy transfer will cause the displacement of the target

Focused Ion Beam Lithography 39

The typical process flow for resist-based IBL is identical to EBL and is illustrated in Figure 8. The pattern definition is performed by the chemical modification of the resist irradiated by ions. The key elements in the process are thus the employed resist and its interaction with

1. Resist application 3. Pattern writing 5. RIE pattern transfer

2. Pre exposure bake 4. Resist devellopement 6. Resist striping

The resist-based IBL was developed after EBL and thus most resist materials employed in IBL were first employed for EBL and then found suitable for IBL. However, not every material suitable for EBL can be employed for IBL without restriction. Due to the different interaction of ions with resists compared to electrons, the resist properties may change

How a resist material behaves under ion beam irradiation largely depends on the form of energy deposition. Ions with a low mass to energy ratio deposit their energy mainly by electronic effects. For ions with high mass to energy, the energy deposition is mainly due to

In the case of electronic energy deposition, resist materials behave similar to what is known from electron beam lithography. The energy dissipated into the resist following exposure leads to chemical damage of the polymer bonds, such as chain scission for positive resist and cross-linking in case of negative resist (Ansari et al., 2004). However, the ion resist interaction is much stronger for ions and will thus result in increased resist sensitivity. E.g., spin on glass (SOG) was found to be 500 times more sensitive to 30keV Ga ions than to

Although most resists are more sensitive to ion irradiation than to electron irradiation, this does not imply a higher patterning speed. The practical patterning speed depends on a large number of parameters including source brightness properties of the employed optics and required resolution. Depending on the specifications, either EBL or IBL may be faster. Due

Fig. 7. Schematic illustration of the FIB-induced etching process

**4.2 Resist-based lithography** 

the beam employed for exposure.

significantly.

Fig. 8. Typical process flow for resist-based IBL

nuclear stopping (Gowa et al., 2010).

electrons (Taniguchi et al., 2006).

atoms and trigger a recoil cascade whereby kinetic energy is transferred from one atom to another by elastic scattering processes.

When the recoil cascade induced by the incident ion reaches the surface the target atoms at the surface may gather sufficient energy to leave the surface and enter the surrounding vacuum. The atoms are then either re-deposited or removed from the vacuum chamber by the pumping system. This material removal process is called milling.

How many target atoms a single ion is able to remove is largely dependent on the target material, the ions species, its energy and the angle of incidence of the ion beam. E.g. a 30keV Ga ion may eject 2 to 3 Si atoms. A 25keV Ga ion may also eject 23 Au atoms (Utke et al., 2008).

This milling process can be easily applied to almost any material and permits patterning down to the sub-100 nm regime. Due to its large material independence and high achievable resolution this process is commercially employed for transmission electron microscope (TEM) sample preparation. Due its ease of application and versatility, this process is often employed for rapid prototyping in research. Due to the high ion doses required for patterning only small areas can be patterned by FIB milling if processing times ought to remain within reasonable limits.

The energy transfer from the incident ion to the target may also be transferred to molecules adsorbed on the target surface. This may trigger a chemical reaction. Possible chemical reactions include the decomposition of adsorbed molecules and reaction of adsorbed molecules with the target atoms. This FIB-induced reaction process is illustrated in Figure 7. The energy transfer mechanism from the ion to the adsorbed molecule is not yet fully understood. However, it is commonly agreed that it can be mainly attributed to the same recoil cascade which causes milling. Alternative explanations include secondary electron and local heating.

The decomposition products may be solid and thus deposited on the target surface, or volatile and thus be removed by the pumping system. By the choice of appropriate substances one may induce the local deposition of specific materials, e.g., metals may be deposited from appropriate metal organic precursors. This process is called gas-assisted deposition (GAD).

GAD is commercially employed for rewiring of integrated circuits in circuit editing (CE) (Boit et al., 2008), to protect the specimen surface of TEM samples during preparation by FIB milling and to correct void defects on photo masks (Boit et al., 2008).

The energy transferred to the target surface may trigger a reaction of the precursor with the target material. If the reaction product is volatile this will cause local etching of the target. This is mainly achieved by supplying light, reactive species such as halogens or halogen compound onto the surface. This process is called gas-assisted etching (GAE).

The etching efficiency of the GAE process is dependent on the target material. Thus, this can be employed to locally remove one specific material. E.g., the addition of XeF2 will increase the removal rate of SiO2 by a factor of nine compared to FIB milling while the removal rate of most metals (e.g., Al) will not be altered. This can be employed to remove a SiO2 dielectric layer while mostly keeping Al interconnects intact. GAE is employed for selective removal of material in CE and for the selective removal of Cr in photo mask repair (Utke et al., 2008).

The chemical etching will also increase the removal rate compared to milling. This may be employed to increase processing speed and to minimize contamination and amorphization of the target material.

atoms and trigger a recoil cascade whereby kinetic energy is transferred from one atom to

When the recoil cascade induced by the incident ion reaches the surface the target atoms at the surface may gather sufficient energy to leave the surface and enter the surrounding vacuum. The atoms are then either re-deposited or removed from the vacuum chamber by

How many target atoms a single ion is able to remove is largely dependent on the target material, the ions species, its energy and the angle of incidence of the ion beam. E.g. a 30keV Ga ion may eject 2 to 3 Si atoms. A 25keV Ga ion may also eject 23 Au atoms (Utke et al., 2008). This milling process can be easily applied to almost any material and permits patterning down to the sub-100 nm regime. Due to its large material independence and high achievable resolution this process is commercially employed for transmission electron microscope (TEM) sample preparation. Due its ease of application and versatility, this process is often employed for rapid prototyping in research. Due to the high ion doses required for patterning only small areas can be patterned by FIB milling if processing times ought to

The energy transfer from the incident ion to the target may also be transferred to molecules adsorbed on the target surface. This may trigger a chemical reaction. Possible chemical reactions include the decomposition of adsorbed molecules and reaction of adsorbed molecules with the target atoms. This FIB-induced reaction process is illustrated in Figure 7. The energy transfer mechanism from the ion to the adsorbed molecule is not yet fully understood. However, it is commonly agreed that it can be mainly attributed to the same recoil cascade which causes milling. Alternative explanations include secondary electron

The decomposition products may be solid and thus deposited on the target surface, or volatile and thus be removed by the pumping system. By the choice of appropriate substances one may induce the local deposition of specific materials, e.g., metals may be deposited from appropriate metal organic precursors. This process is called gas-assisted

GAD is commercially employed for rewiring of integrated circuits in circuit editing (CE) (Boit et al., 2008), to protect the specimen surface of TEM samples during preparation by FIB

The energy transferred to the target surface may trigger a reaction of the precursor with the target material. If the reaction product is volatile this will cause local etching of the target. This is mainly achieved by supplying light, reactive species such as halogens or halogen

The etching efficiency of the GAE process is dependent on the target material. Thus, this can be employed to locally remove one specific material. E.g., the addition of XeF2 will increase the removal rate of SiO2 by a factor of nine compared to FIB milling while the removal rate of most metals (e.g., Al) will not be altered. This can be employed to remove a SiO2 dielectric layer while mostly keeping Al interconnects intact. GAE is employed for selective removal of material in CE and for the selective removal of Cr in photo mask repair (Utke et al., 2008). The chemical etching will also increase the removal rate compared to milling. This may be employed to increase processing speed and to minimize contamination and amorphization

milling and to correct void defects on photo masks (Boit et al., 2008).

compound onto the surface. This process is called gas-assisted etching (GAE).

the pumping system. This material removal process is called milling.

another by elastic scattering processes.

remain within reasonable limits.

and local heating.

deposition (GAD).

of the target material.

Fig. 7. Schematic illustration of the FIB-induced etching process

#### **4.2 Resist-based lithography**

The typical process flow for resist-based IBL is identical to EBL and is illustrated in Figure 8. The pattern definition is performed by the chemical modification of the resist irradiated by ions. The key elements in the process are thus the employed resist and its interaction with the beam employed for exposure.

Fig. 8. Typical process flow for resist-based IBL

The resist-based IBL was developed after EBL and thus most resist materials employed in IBL were first employed for EBL and then found suitable for IBL. However, not every material suitable for EBL can be employed for IBL without restriction. Due to the different interaction of ions with resists compared to electrons, the resist properties may change significantly.

How a resist material behaves under ion beam irradiation largely depends on the form of energy deposition. Ions with a low mass to energy ratio deposit their energy mainly by electronic effects. For ions with high mass to energy, the energy deposition is mainly due to nuclear stopping (Gowa et al., 2010).

In the case of electronic energy deposition, resist materials behave similar to what is known from electron beam lithography. The energy dissipated into the resist following exposure leads to chemical damage of the polymer bonds, such as chain scission for positive resist and cross-linking in case of negative resist (Ansari et al., 2004). However, the ion resist interaction is much stronger for ions and will thus result in increased resist sensitivity. E.g., spin on glass (SOG) was found to be 500 times more sensitive to 30keV Ga ions than to electrons (Taniguchi et al., 2006).

Although most resists are more sensitive to ion irradiation than to electron irradiation, this does not imply a higher patterning speed. The practical patterning speed depends on a large number of parameters including source brightness properties of the employed optics and required resolution. Depending on the specifications, either EBL or IBL may be faster. Due

Focused Ion Beam Lithography 41

which the ions come to rest is sufficiently narrow and if a sufficient number of ions are implanted they may form a thin layer of highly doped substrate material and locally change

For the very common material system gallium on silicon a SRIM (Ziegler, 2004) simulation quickly reveals a projected range of 28 nm at 30keV ion energy. Starting from an ion dose of 2 10^15 cm¯² (Chekurov et al., 2009), a change in the reactive ion etcher (RIE) etch speed of Si inside doped areas is noticed for fluorine-based plasmas. Only recently it was discovered that by modulating the ion dose, the time the highly doped layer is able to withstand the

The dependence of the etch depth from the applied Ga dose may be employed as an effective way for 3D patterning. The process flow is illustrated in Figure 9. It consists of two steps: (i) Implantation and (ii) Pattern transfer using RIE. The key parameters for the process are: (i) The implantable Ga quantity in dependence of the scan parameters and (ii) The dependence of the etch depth on the implanted Ga quantity and on the etch parameters. For

1. Ga implantation

We measured the implantable Ga in dependence of the scan parameters by using EDX. Since EDX can only measure the relative Ga content, a method was needed to calculate the implanted absolute Ga quantity. We found that the dependence of the measured Ga quantity from the applied Ga quantity fitted well to the exponential function in eq. 1. Herby �� denotes the measured Ga quantity,��is the applied Ga quantity and A and B are fit

Under the assumption that for low ion doses the implanted Ga dose is proportional to the applied Ga dose one can calculate the proportionality factor between measured and applied Ga dose from the fit parameters. The implanted Ga quantity in physical units�� can thus be

�� � <sup>1</sup>

The measured Ga quantity in dependence of the applied Ga quantity for different ion energies is shown in Fig. 12. We find that at low implantation doses, the measured implantation dose is proportional to the applied Ga dose, while at higher implantation doses, the implanted Ga is also removed due to sputtering and the implanted ion dose saturates. The maximum implantable Ga quantity is dependent on the ion energy and becomes higher with increasing energy. The implantation efficiency for 30keV Ga ions is

�� � ����� (1)

�� �� (2)

2. RIE pattern transfer

its chemical properties.

Fig. 9. Process flow

parameters.

calculated using eq. 2.

etching can also be modulated (Henry et al., 2010).

effective patterning these parameters have to be optimized.

to the availability of higher current densities and the variable shape beam (VSB) writers EBL usually wins this battle.

For low-energy and high-mass ions resist behavior may change severely compared to EBL. It was shown that, for low-energy Ga ions, several positive resists behave as negative resists when irradiated with a high fluency or high flux ion beam (Gowa et al., 2010). This behavior is attributed to cross-linking induced by the radicals liberated by the incident ions (Gowa et al., 2010)**.** 

At sufficient ion doses also ion implantation into the resist may be an issue, but can also be employed for patterning. Sufficiently high concentrations of Ga implanted into a resist can protect it from being developed or etched. This fact was employed in combination with a DNQ/Novolak-based resist to permit positive/negative patterning in one exposure step. Weather a feature is exposed positively or negatively is solely dependent on the ion dose (K. Arshak et al., 2004).

During IBL a significant part of the employed ions may be implanted into the substrate. This may be unacceptable in occasions where the substrate is sensitive to defects or doping, e.g., in semiconductor device fabrication. The issue may be circumvented by employing a doublelayer resist system, whereby the second layer acts as an ion absorber (Hillmann, 2001).

Beside 2D patterning, resist-based 3D patterning using a FIB was also demonstrated (Hillmann, 2001) (Taniguchi et al., 2006). For this purpose, a positive resist is employed and the depth to which the resist is removed by the developer is dose dependent. Due to the absence of the proximity effect (Hillmann, 2001) this technique permits fast 3D nano patterning with high lateral resolution.

Light ions with large energies may pass thick resist materials with little deviation from their trajectory thus permitting the creation of high aspect ratio features. Due to the large impact of the ion energy and mass, the penetration depth of ions into the resist must always be considered and the resist thickness must be chosen appropriately.

At high resist sensitivities, e.g., when using chemically amplified resists (CARs) resolution and edge roughness may be limited by shot noise (Rau, 1998). Rau found that using a CAR feature printing down to an average of 7 ions was possible, but very unreliable. At an average dose of 28 ions per feature, 98% of all features were printed.

Beside classical organic resists also alternative materials are employed for FIB lithography. The change in the crystalline phase of MoO3 and WO3 (Hashimoto, 1998) was successfully used for patterning. The intermixing of the Ag2Se/GeSe2 bilayer system caused by ion bombardment proved also to be a viable patterning technique (Wagner, 1981)**.** Selfdevelopment was shown using two materials, namely AlF3 (Gierak et al., 1997) and nitrocellulose (Harakawa, 1986).

Resist-based focused IBL was studied extensively in the 1980s (Gierak et al., 1997). FIB devices for resist-based FIB lithography were put on the market and the impact of ion irradiation on resist materials was investigated. In the end, IBL could not supersede EBL and the commercialization efforts for these specialized devices were stopped by most companies. The introduction of multi beam and high-resolution FIB systems (Elmar Platzgummer & Hans Loeschner 2009) might now lead to a renascence of resist-based FIB lithography.

#### **4.3 3D patterning by ion implantation**

During ion bombardment, ions are implanted into the irradiated substrate. Depending on their mass and energy they will come to a rest in deeper or shallower regions. If the depth in

to the availability of higher current densities and the variable shape beam (VSB) writers EBL

For low-energy and high-mass ions resist behavior may change severely compared to EBL. It was shown that, for low-energy Ga ions, several positive resists behave as negative resists when irradiated with a high fluency or high flux ion beam (Gowa et al., 2010). This behavior is attributed to cross-linking induced by the radicals liberated by the incident ions (Gowa et

At sufficient ion doses also ion implantation into the resist may be an issue, but can also be employed for patterning. Sufficiently high concentrations of Ga implanted into a resist can protect it from being developed or etched. This fact was employed in combination with a DNQ/Novolak-based resist to permit positive/negative patterning in one exposure step. Weather a feature is exposed positively or negatively is solely dependent on the ion dose (K.

During IBL a significant part of the employed ions may be implanted into the substrate. This may be unacceptable in occasions where the substrate is sensitive to defects or doping, e.g., in semiconductor device fabrication. The issue may be circumvented by employing a double-

Beside 2D patterning, resist-based 3D patterning using a FIB was also demonstrated (Hillmann, 2001) (Taniguchi et al., 2006). For this purpose, a positive resist is employed and the depth to which the resist is removed by the developer is dose dependent. Due to the absence of the proximity effect (Hillmann, 2001) this technique permits fast 3D nano

Light ions with large energies may pass thick resist materials with little deviation from their trajectory thus permitting the creation of high aspect ratio features. Due to the large impact of the ion energy and mass, the penetration depth of ions into the resist must always be

At high resist sensitivities, e.g., when using chemically amplified resists (CARs) resolution and edge roughness may be limited by shot noise (Rau, 1998). Rau found that using a CAR feature printing down to an average of 7 ions was possible, but very unreliable. At an

Beside classical organic resists also alternative materials are employed for FIB lithography. The change in the crystalline phase of MoO3 and WO3 (Hashimoto, 1998) was successfully used for patterning. The intermixing of the Ag2Se/GeSe2 bilayer system caused by ion bombardment proved also to be a viable patterning technique (Wagner, 1981)**.** Selfdevelopment was shown using two materials, namely AlF3 (Gierak et al., 1997) and

Resist-based focused IBL was studied extensively in the 1980s (Gierak et al., 1997). FIB devices for resist-based FIB lithography were put on the market and the impact of ion irradiation on resist materials was investigated. In the end, IBL could not supersede EBL and the commercialization efforts for these specialized devices were stopped by most companies. The introduction of multi beam and high-resolution FIB systems (Elmar Platzgummer & Hans

During ion bombardment, ions are implanted into the irradiated substrate. Depending on their mass and energy they will come to a rest in deeper or shallower regions. If the depth in

Loeschner 2009) might now lead to a renascence of resist-based FIB lithography.

layer resist system, whereby the second layer acts as an ion absorber (Hillmann, 2001).

considered and the resist thickness must be chosen appropriately.

average dose of 28 ions per feature, 98% of all features were printed.

usually wins this battle.

Arshak et al., 2004).

patterning with high lateral resolution.

nitrocellulose (Harakawa, 1986).

**4.3 3D patterning by ion implantation** 

al., 2010)**.** 

which the ions come to rest is sufficiently narrow and if a sufficient number of ions are implanted they may form a thin layer of highly doped substrate material and locally change its chemical properties.

For the very common material system gallium on silicon a SRIM (Ziegler, 2004) simulation quickly reveals a projected range of 28 nm at 30keV ion energy. Starting from an ion dose of 2 10^15 cm¯² (Chekurov et al., 2009), a change in the reactive ion etcher (RIE) etch speed of Si inside doped areas is noticed for fluorine-based plasmas. Only recently it was discovered that by modulating the ion dose, the time the highly doped layer is able to withstand the etching can also be modulated (Henry et al., 2010).

The dependence of the etch depth from the applied Ga dose may be employed as an effective way for 3D patterning. The process flow is illustrated in Figure 9. It consists of two steps: (i) Implantation and (ii) Pattern transfer using RIE. The key parameters for the process are: (i) The implantable Ga quantity in dependence of the scan parameters and (ii) The dependence of the etch depth on the implanted Ga quantity and on the etch parameters. For effective patterning these parameters have to be optimized.

#### Fig. 9. Process flow

We measured the implantable Ga in dependence of the scan parameters by using EDX. Since EDX can only measure the relative Ga content, a method was needed to calculate the implanted absolute Ga quantity. We found that the dependence of the measured Ga quantity from the applied Ga quantity fitted well to the exponential function in eq. 1. Herby �� denotes the measured Ga quantity,��is the applied Ga quantity and A and B are fit parameters.

Under the assumption that for low ion doses the implanted Ga dose is proportional to the applied Ga dose one can calculate the proportionality factor between measured and applied Ga dose from the fit parameters. The implanted Ga quantity in physical units�� can thus be calculated using eq. 2.

$$d\_m = B e^{Ad\_l} \tag{1}$$

$$D\_m = \frac{1}{AB} \, d\_i \tag{2}$$

The measured Ga quantity in dependence of the applied Ga quantity for different ion energies is shown in Fig. 12. We find that at low implantation doses, the measured implantation dose is proportional to the applied Ga dose, while at higher implantation doses, the implanted Ga is also removed due to sputtering and the implanted ion dose saturates. The maximum implantable Ga quantity is dependent on the ion energy and becomes higher with increasing energy. The implantation efficiency for 30keV Ga ions is

Focused Ion Beam Lithography 43

Fig. 11. Impact of the implanted ion dose on the etch depth. The etch gas was composed of

Fig. 12. AFM image of a micro lens created by ion implantation and subsequent RIE pattern

Beside speed, resolution is also among the key properties of a lithographic technique. In the presented lithography technique it is mainly limited by the current distribution of the ion beam and the ion sample interaction. We find that with our Canion 31 Ga LMIS we can

As we have learned, in IBL the proximity effect is absent or negligible. This makes IBL very attractive for 3D nano patterning and possibly the only 3D nano patterning technique with sufficient throughput. The most popular workaround, namely EBL multilevel patterning is a good option if only a few height levels are required. However, it cannot provide real 3D

We conclude that 3D patterning by ion implantation and subsequent RIE etch is a promising patterning technique. It permits the creation of real 3D nano patterns not feasible with other methods. If combined with nano imprint lithography 3D nano patterns may be economically

easily achieve line/space patterns with 50 nm HP as shown in Fig. 13.

Ar and SF6.

transfer

patterns.

created and replicated.

summarized in Table 4. One will tend to maximise the implantation efficiency and thus choose sufficiently low implantation doses.

Besides the impact of the ion energy, we also measured the influence of scanning speed and ion current on the implanted dose (not shown). At doses below 100pC/µm² and 30keV ion energy we find that the effect of these parameters on the implanted Ga quantity is negligible. In practice, one will necessarily avoid higher implantation doses due to the low implantation efficiency.

Fig. 10. Impact of the applied Ga dose on the effectively implanted ion dose. The indicated measured ion dose shown was calculated from the measured relative ion dose as described in the text.


Table 4. Effect of the ion dose on the implantation efficiency at an ion energy of 30keV

For the RIE pattern transfer, the dependence of the etching depth on the applied Ga dose is of importance. We measured the dependence for three gas compositions, namely SF6 + Ar, SF6 + O2 and SF6 + SiCl4. The first of these gas compositions was found to be the most interesting for 3D patterning. The SF6 + SiCl4 gas composition was found to be useful to suppress the masking effect of implanted Ga.

The resulting etch depth in dependence on the applied Ga dose for the SF6+Ar plasma is shown in Fig. 11. We find that depending on the plasma composition the sensitivity of the etch depth on the applied Ga dose may be modulated. To optimize the overall process for speed one will tend to choose a gas composition which minimizes the required implantation dose. However, the process with the highest dose sensitivity will also exhibit the highest sensitivity against dose variations. Thus, in practice, one will have to make a trade-off between patterning speed and precise height control.

summarized in Table 4. One will tend to maximise the implantation efficiency and thus

Besides the impact of the ion energy, we also measured the influence of scanning speed and ion current on the implanted dose (not shown). At doses below 100pC/µm² and 30keV ion energy we find that the effect of these parameters on the implanted Ga quantity is negligible. In practice, one will necessarily avoid higher implantation doses due to the low

Fig. 10. Impact of the applied Ga dose on the effectively implanted ion dose. The indicated measured ion dose shown was calculated from the measured relative ion dose as described

**Ion dose Implantation** 

For the RIE pattern transfer, the dependence of the etching depth on the applied Ga dose is of importance. We measured the dependence for three gas compositions, namely SF6 + Ar, SF6 + O2 and SF6 + SiCl4. The first of these gas compositions was found to be the most interesting for 3D patterning. The SF6 + SiCl4 gas composition was found to be useful to

The resulting etch depth in dependence on the applied Ga dose for the SF6+Ar plasma is shown in Fig. 11. We find that depending on the plasma composition the sensitivity of the etch depth on the applied Ga dose may be modulated. To optimize the overall process for speed one will tend to choose a gas composition which minimizes the required implantation dose. However, the process with the highest dose sensitivity will also exhibit the highest sensitivity against dose variations. Thus, in practice, one will have to make a trade-off

100pC/µm² 75% 200pC/µm² 60% 300pC/µm² 50% 500pC/µm² 36% Table 4. Effect of the ion dose on the implantation efficiency at an ion energy of 30keV

**efficiency** 

choose sufficiently low implantation doses.

suppress the masking effect of implanted Ga.

between patterning speed and precise height control.

implantation efficiency.

in the text.

Fig. 11. Impact of the implanted ion dose on the etch depth. The etch gas was composed of Ar and SF6.

Fig. 12. AFM image of a micro lens created by ion implantation and subsequent RIE pattern transfer

Beside speed, resolution is also among the key properties of a lithographic technique. In the presented lithography technique it is mainly limited by the current distribution of the ion beam and the ion sample interaction. We find that with our Canion 31 Ga LMIS we can easily achieve line/space patterns with 50 nm HP as shown in Fig. 13.

As we have learned, in IBL the proximity effect is absent or negligible. This makes IBL very attractive for 3D nano patterning and possibly the only 3D nano patterning technique with sufficient throughput. The most popular workaround, namely EBL multilevel patterning is a good option if only a few height levels are required. However, it cannot provide real 3D patterns.

We conclude that 3D patterning by ion implantation and subsequent RIE etch is a promising patterning technique. It permits the creation of real 3D nano patterns not feasible with other methods. If combined with nano imprint lithography 3D nano patterns may be economically created and replicated.

Focused Ion Beam Lithography 45

a) Ruthenium b) Aluminium doped zinc oxide (AZO)

Fig. 15. AFM images of two hard materials after pattern definition. The patterning process was identical; however, the resulting pattern quality differs significantly. The nominal hard

To quantify the impact of the FIB milling induced roughening, the roughness in dependence of the milling depth was measured for a number of materials. The resulting curves are shown in Fig. 16. Both AZO and Ta masks show excellent flatness after patterning. For Ru, the roughness is found to increase abruptly when the mask thickness approaches zero.

Fig. 16. Impact of ion milling on the roughness of hard mask materials. The hard mask

For the RIE transfer-etch it is important to choose both the hard mask material and etching chemistry appropriately. Further, it must be considered that during FIB milling the employed ion species is implanted into the substrate and can disturb the etching process. The etching chemistry must etch the substrate material and the implanted ions but must not

Ga implanted into an Si substrate acts as an etch stop for the routinely employed fluorinebased RIE chemistries. We found that our routinely employed Si etch recipes could not be employed for the transfer-etch due to the masking effect of the implanted Ga. Chlorine is known to be an etchant for Ga, thus we investigated the impact of the addition of a chlorine

mask thickness before patterning was 10 nm in both cases.

thickness before milling was 10±2 nm in all cases.

source to a well-established fluorine-based RIE recipe.

remove the hard mask material.

Fig. 13. 50 nm half-pitch lines/spaces. Scanning electron microscope (SEM) image of an implanted and etched lines/spaces pattern with 50 nm half-pit*c*h.

#### **4.4 Direct patterning of hard mask layers**

Direct hard mask patterning is an alternative to resist-based patterning and direct FIB milling. To our knowledge, lithography by direct patterning of hard mask layers is a completely new technique.

Compared to resist-based methods, the employment of inorganic mask layers permits a larger flexibility in terms of patterned materials and the patterning of pre-structured substrates with relatively few patterning steps. This technique is not as flexible and straightforward as direct patterning; however, it is significantly faster. Thus, this patterning technique can be seen as a speed/complexity trade-off.

Fig. 14. Process flow for direct hard mask patterning

The direct hard mask patterning technique consists of four steps: (i) Hard mask application, (ii) Pattern definition, (iii) Pattern transfer-etch and (iv) Hard mask removal. For hard mask application standard sputtering, ALD or other methods may be employed. The hard mask removal may be performed in a dedicated etching step, e.g., by wet etching or combined with the RIE pattern transfer step.

For the pattern definition the material properties during FIB patterning are of outstanding importance. Fig. 15 shows the same pattern in two hard mask materials: AZO and Ru on Si. While the pattern in the AZO mask is clearly defined, the pattern in the Ru hard mask is heavily distorted by the surface roughening induced by the FIB milling. In this case, we believe this is due to the formation of Ru island films on the Si substrate.

Fig. 13. 50 nm half-pitch lines/spaces. Scanning electron microscope (SEM) image of an

Direct hard mask patterning is an alternative to resist-based patterning and direct FIB milling. To our knowledge, lithography by direct patterning of hard mask layers is a

Compared to resist-based methods, the employment of inorganic mask layers permits a larger flexibility in terms of patterned materials and the patterning of pre-structured substrates with relatively few patterning steps. This technique is not as flexible and straightforward as direct patterning; however, it is significantly faster. Thus, this patterning

The direct hard mask patterning technique consists of four steps: (i) Hard mask application, (ii) Pattern definition, (iii) Pattern transfer-etch and (iv) Hard mask removal. For hard mask application standard sputtering, ALD or other methods may be employed. The hard mask removal may be performed in a dedicated etching step, e.g., by wet etching or combined

For the pattern definition the material properties during FIB patterning are of outstanding importance. Fig. 15 shows the same pattern in two hard mask materials: AZO and Ru on Si. While the pattern in the AZO mask is clearly defined, the pattern in the Ru hard mask is heavily distorted by the surface roughening induced by the FIB milling. In this case, we

4. Hard mask removal

3. RIE pattern transfer

implanted and etched lines/spaces pattern with 50 nm half-pit*c*h.

**4.4 Direct patterning of hard mask layers** 

technique can be seen as a speed/complexity trade-off.

Fig. 14. Process flow for direct hard mask patterning

with the RIE pattern transfer step.

2. FIB pattern definition

believe this is due to the formation of Ru island films on the Si substrate.

1. Hard mask application

completely new technique.

Fig. 15. AFM images of two hard materials after pattern definition. The patterning process was identical; however, the resulting pattern quality differs significantly. The nominal hard mask thickness before patterning was 10 nm in both cases.

To quantify the impact of the FIB milling induced roughening, the roughness in dependence of the milling depth was measured for a number of materials. The resulting curves are shown in Fig. 16. Both AZO and Ta masks show excellent flatness after patterning. For Ru, the roughness is found to increase abruptly when the mask thickness approaches zero.

Fig. 16. Impact of ion milling on the roughness of hard mask materials. The hard mask thickness before milling was 10±2 nm in all cases.

For the RIE transfer-etch it is important to choose both the hard mask material and etching chemistry appropriately. Further, it must be considered that during FIB milling the employed ion species is implanted into the substrate and can disturb the etching process. The etching chemistry must etch the substrate material and the implanted ions but must not remove the hard mask material.

Ga implanted into an Si substrate acts as an etch stop for the routinely employed fluorinebased RIE chemistries. We found that our routinely employed Si etch recipes could not be employed for the transfer-etch due to the masking effect of the implanted Ga. Chlorine is known to be an etchant for Ga, thus we investigated the impact of the addition of a chlorine source to a well-established fluorine-based RIE recipe.

Focused Ion Beam Lithography 47

structuring approach. With the emergence of helium ion beams, a new tool with new structuring capabilities is on the market and allows new applications. In this work also several structuring approaches have been discussed including (i) FIB milling (ii) FIBinduced gas-assisted processes (iii) 3D patterning by ion implantation and (iv) patterning my milling of hard mask layers. Due to the versatility of these approaches an increasing amount of applications of IBL for optical systems, sensor devices, in the modification and custom-trimming of microelectronic circuitry as well as in the 'classical' fields of photomask repair and defect analysis of cross-sections may be expected. With the neon ion beam systems on the verge of commercial introduction, this is surely going to remain an exciting field of research. With the multi-beam ion systems reaching maturity, interest in IBL from

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**6. References** 

The impact of the addition of chlorine on the masking capability of Ga implanted into Si is shown in Fig. 17. While for low SiCl4 concentration the Ga still shows a significant masking effect, we find that the addition of 30% SiCl4 is sufficient to completely suppress the impact of the implanted Ga on the final pattern.

Fig. 17. Impact of the addition of SiCl4 to the masking capability of Ga implanted by FIB into Si in a SF6-based RIE process

The addition of chlorine solves the issue of Ga implantation. However, it negatively impacts the available pool of hard mask materials. Now the hard mask material must not only resist etching by a standard fluorine-based RIE process but also to the added chlorine species. As shown above, Ta exhibits excellent properties during the pattern definition process. However, it does not resist our chlorine-containing etch recipe and thus cannot be employed.

In terms of resolution, the direct hard mask patterning process compares very favourably to FIB direct milling. Since the pattern in the thin hard mask layer is transferred into a thicker layer, slopes with low steepness are imaged into slopes with higher steepness. Thus patterning close to the beam diameter becomes possible. Also the influence of the beam tails and mechanical deformation due to milling-induced strain is minimized. We found that lines down to 40 nm half pith are obtainable in a 10 nm thick Ni hard mask layer.

We conclude that the direct patterning of hard mask materials and subsequent pattern transfer by RIE can help to speed up patterning compared to direct FIB milling. We believe that this method is useful for a number of applications including patterning on uneven surfaces.
