ION BEAM ETCHING METHOD AND ION BEAM ETCHING APPARATUS

FIELD OF THE INVENTION

The present invention relates to manufactur- ing of three dimensional (3D) nanostructures, particularly nanostructures having dimensions in the range of below 100 run.

BACKGROUND OF THE INVENTION There is a variety of different techniques available for the fabrication of three dimensional

(3D) nano-sized structures of different materials. For example, UV Lithography has conventionally been widely used especially in the microelectronics industry. There is, however, a fundamental limit below which the structural dimensions can not be reduced. By using deep UV light this limit is somewhere close to 100 run.

More advanced methods based on Electron Beam Lithography (EBL) are capable of providing even smaller dimensions and have been applied for example to the fabrication of 5 - 7 niti wide etched lines on a silicon substrate. However, when evaporating metallic structures through masks made with EBL, the limit is higher, around 20 - 50 niti. The disadvantage of EBL is that it is a rather slow method.

Focused Ion Beam technique (FIB) is a method conceptually similar to EBL except that the electrons are replaced with focused heavy ions. FIB is successfully used for very localised etching and cutting, however, rather on a micrometer scale than on nano- scale. To the best of authors' knowledge, no 3D structures with dimensions significantly below the 100 ran range fabricated by FIB have been reported. In addition, FIB equipment is very expensive. On the other hand, for industrial fabrication purposes FIB is ex-

tremely slow and not compatible at all with the concept of mass production.

The inventors have earlier disclosed the basic principle of a totally new approach, where the di- mensions of the prefabricated nano-sized structures are reduced by ion beam sputtering (Applied Physics A 79, 1769 - 1773, 2004) . The principle is to expose the nanostructure to be processed to a wide beam of low energy, medium mass ions at a glancing angle of inci- dence. The energy of the ions colliding the surface layer of the target causes the surface atoms to be ejected. This physical process is adjusted by carefully selecting the process parameters like ion energy and mass, the angle of incidence of the ion beam and the ion beam current intensity. The sample is rotated about the azimuth angle in order to assure isotropic etching. The process can be performed by commercially available ion beam sources and vacuum systems added by a sample manipulator for tilting and rotating pur- poses. The inventors have shown that by this kind of process structures even below 10 nm can be produced with high accuracy and providing an extreme surface smoothness (Nano Letters, Vol. 5, No. 6, 1029 - 1033, 2005) . Despite the promising preliminary results achieved, the difficulty in applying this technique to different kinds of materials and structure shapes and dimensions is in finding the optimal set of process parameters. Not even the theory of conventional ion beam sputtering of macroscopic 2D structures is fully known and naturally it is much more complicated to predict the results of a process of nanomachining 3D structures of sub-100 nm dimensions.

PURPOSE OF THE INVENTION The purpose of the present method is to provide a new, well controllable method and apparatus for processing three dimensional nanostructures, particu-

larly structures in the sub-100 nm scale, with high accuracy and with nanometer-scale surface roughness.

SUMMARY OF THE INVENTION The method in accordance with the present invention is characterised by what is presented in claim 1. The apparatus according to the present invention is characterised by what is presented in claim 9.

The present invention firstly concerns an ion beam etching method for processing three dimensional (3D) nanostructures preferably having dimensions in the sub-100 nm range. In the method, a pre-fabricated nanostructure formed on a substrate and preferably having substantially the desired final shape of the nanostructure is etched three dimensionally by bombarding the nanostructure in vacuum conditions by a beam of low-energy medium-mass ions at a glancing incident angle with respect to the substrate while optionally rotating the nanostructure about an axis nor- mal to the substrate. The etching according to the present invention can have two purposes. At first, the purpose can be to reduce a prefabricated sample three dimensionally to the final size and shape of the nanostructure by a well controllable ion beam etching process which allows producing dimensions clearly below most of the conventional fabrication methods. On the other hand, the etching process of the invention can be used just for smoothing the surface of the sample fabricated with some traditional processing method like photo and e-beam lithography, LIGA or nanoim- printing. The etching is based on a physical sputtering process wherein the surface atoms of the target are ejected due to collisions with energetic ions impinging the sample. As a difference to two dimensional (2D), i.e. planar structures like planar films or stripes with a thickness substantially smaller than the length and

width of the stripe, three dimensional means herein nanostructures having also height dimensions substantially in the same scale as the lateral dimension (s) . Examples of such 3D structures include different kinds of wires, beams, islands and combinations thereof. The incident angle means an angle between the substrate normal and the beam longitudinal axis. The tilted incident angle of the ion beam as well as the rotation of the sample about the azimuth angle is needed for isotropic etching, i.e. the same etch rates for both lateral and vertical surfaces. Also, because due to the rotation about an axis normal to the substrate each point of the sample will be bombarded from various directions, the roughness of an etched surface is very small. "Normal to the substrate" means herein a direction substantially perpendicular to the main plane of the substrate. With typical planar substrates, for example round wafers, this is an unambiguous definition. With non-planar substrates this di- rection is the extension direction of the nanostruc- ture from the substrate. The expression "optionally rotating" means that it is naturally also possible to keep the nanostructure stationary and bombard it from one direction only. This kind of direction-selective etching allows controllable modification of the nanostructure shape.

According to the invention, the energy per mass of the ions is within a range of 0.0025 - 0.0225 keV/amu where amu means atomic mass unit (1.66-10 ^"2 ' ^' kg) , and the incident angle of the ion beam with respect to the substrate is within a range of 30 - 50 degrees, preferably about 40 degrees. This combination of interrelated parameters was surprisingly found to ensure an effective but however gentle and well con- trollable etching process providing isotropic etching and producing an extremely low surface roughness. This set of parameters is also very generic being applica-

ble to a wide variety of different materials. Both of those parameters are discussed in more detail in the following.

Impinging the target with an ion beam perpen- dicularly with respect to the target surface usually causes ions to penetrate into the target rather deeply from a nanomachining point of view. This complicates the controllability of the etching process. In addition, the surface roughness resulted from perpendicu- lar bombardment is typically in the range of at least several nanometres. If there are defects in the material of the target, the roughness becomes even worse due to faster etching around the defects. Thus, a tilted angle suitable at the same time for surfaces with different directions is needed. According to prior art technology relating to ion beam sputtering processes in general, an incident angle of 55 - 60 degrees have been considered to provide most effective sputtering. Similar angle was also disclosed in the inventors' publication mentioned in the introduction. Lower angles have been known to significantly decrease the sputtering rate. Against this background it is surprising that, when striving for effective and homogeneous etching of 3D nanostructures, the optimal in- cident angle, in combination with those other parameter specifications given in the claims, was found to be around 40 degrees.

Traditionally, in 2D sputtering processes, high energy ions having energies of several keV are used. Referring to what is said above about the penetration depth and controllability and understanding also the fact that the higher is the energy the higher is the probability for damages, gentle processing of the sample necessitates lower ion energy. This also contributes to producing surface smoothness in a nanometer range. On the other hand, the penetration depth D depends on the energy E exponentially: D ~ exp(-

E/F) , F being a material constant. Thus, already relatively small differences of the ion energy from that of the prior art solutions can change the etching not only quantitatively but also qualitatively. Naturally, too low energy produces no etching at all. In addition, other factors affecting the lower limit of the ion energy are: 1) below about 1 keV the ion beam is very wide, basically leading to inhomogeneous etching; 2) with too low ion energy, e.g. below about 1 keV for Ar, the ion plasma is not stable; 3) when lowering the ion energy, the probability for re-deposition of the sputtered material increases when the knocked-out atoms fall down in the vicinity of the sample instead of being totally removed. All these would suggest higher energies than those defined in the claims. However, when combined with the other parameters defined in the claims, the optimal ion energies were found to be unexpectedly low.

In addition to the kinetic energy dependent on the accelerating voltage and the ion charge, also the ion mass influences the process of ejecting the surface atoms of the sample because the probability of the target atom removal depends on the momentum of the incident ion. Thus, also the mass has to be taken into consideration when selecting the ion type and energy. This is the reason for determining the energy in relation to the mass of the ion. As a compromise between a reasonable sputtering rate, preventing damaging of the surface and availability of different ion types, me- dium-mass ions are preferably used. The range of 0.0025 - 0.0225 keV/amu, corresponding to 0.1 - 0.9 keV for Ar as an example of a suitable ion type, together with those other parameters specified in the claims, was found optimal for most of the materials to be etched and the ion types used.

Preferably, the energy per mass of the ions is within a range of 0.0100 - 0.0150 keV/amu, more

preferably about 0.0125 keV/amu, which has been found a suitable value for a great variety of sample materials and ion types.

It is important to note that, so far, no com- prehensive theory concerning the mechanism of ion beam assisted etching of the nanostructures has been created. Hence, despite of some suggestive theoretical predictions, optimising the interrelated process parameters is very much based on experimental studies. As usual in nanotechnology, also in this case the final conclusions resulted in a parameter set essentially differing from what a person skilled in the ion beam technique applied to macroscopic objects could expect. Preferably, the aspect ratio of the prefabricated sample satisfies the condition — ≥ 1 , w wherein h and w are height and width of the structure, correspondingly. This condition relates to the need for preserving the original shape of the pre- fabricated sample during the etching process. Preserving the shape greatly simplifies the entire manufacturing process when already the pre-fabricated nanos- tructure can have the desired final geometry. Unlike in the earlier disclosed experiments of the inventors, the inventors found that, with the angle of incidence and the energy per mass as defined above, having the parts of the sample at least as high as they are wide most effectively ensures preserving the initial shape of the sample with minor geometry modifications during the etching process. Naturally, if preservation of the original shape of the processed nanostructure is not required, the ratio between the height and the width can be of any value.

The current intensity of the ion beam in the method according to the present invention is preferably within a range of 10 - 20 μA/cm ^z . The lower limit

of this range unexpectedly found empirically comes from a need for sufficient dynamic cleaning of the surface. By dynamic cleaning we mean a process of preventing growth of any additional material on the proc- essed surface. For example, because there always is some amount of residual oxygen in the vacuum chamber, an oxide layer would easily form on a metal surface without sufficient ion beam current density. On the other hand, too high current density would lead to un- desired surface damage and/or overheating, resulting in poor control of the etching process .

In order to ensure a sufficiently large processing area, i.e. an area at the level of the substrate surface in which the etching can occur, and thus to enable simultaneous processing of a plurality of nanostructures on a single substrate, the diameter of the ion beam impinging the nanostructure is preferably at least about 10 mm. A large beam also helps to equalise the ion flux intensity through the beam. The beam width depends on the equipment properties. It can be adjusted to some extent by selection of the acceleration voltage: the higher is the voltage the narrower is the beam. Depending on the substrate, it is thus possible to have a beam cross section even compa- rable to the substrate size. The intensity, however, usually has Gaussian or some other uneven distribution. Therefore, the processing area can be further increased as well as the homogeneity of the ion flux intensity and thus the homogeneity of the etching rate enhanced by sweeping the ion beam with respect to the substrate in a direction perpendicular to the beam longitudinal axis over a larger area during the processing. The sweeping can be executed either by sweeping the beam or by moving the substrate carrying the nanostructure.

In ion sputtering, charging of the target ( = sample) might cause damaging of the nanostructure ex-

posed to the ion beam due to electric discharge of the accumulated positive charges. Thus, the nanostructure is preferably electrically grounded in order to prevent charge accumulation in the sample. Grounding can be done, for example, by multiple bonding with metal wires connecting the sample to a metallic, grounded sample holder.

The charge accumulation can also be prevented by using a conductive and grounded substrate of semi- conductor or metallic material on which the sample is fabricated. A conductive and grounded substrate effectively decreases the charging of the sample.

Especially in the case where the sample is fabricated on an insulating substrate, it is prefer- able to bring additional electrons to the vicinity of the nanostructure in order to neutralise the charging of the sample due to the charged ions of the bombarding beam. In one preferred embodiment, the extra electrons in the form of an electron cloud are arranged by means of a heated metallic filament, e.g. a tungsten wire, integrated to the equipment.

The inventors have found the method of the present invention applicable to different metals like bismuth, aluminium and tin, to inorganic insulators like aluminium oxide, mica and silicon oxide, to silicon as an example of semiconductors as well as to organic PMMA resist. The etch rates are different for different materials making the etching process very selective. The smallest dimensions of structures fab- ricated with nanometer scale accuracy are in the range of 5 run. Attainable surface roughness has proven to be about 1 ran. Due to the gentle process, also very fragile nanostructures like single electron transistors can be etched by the method of the present invention. The present invention also concerns an ion beam etching apparatus for three dimensional etching of sub-100 nm pre-fabricated nanostructures formed on

a planar substrate by an ion beam. The apparatus comprises a vacuum chamber, a sample manipulator mounted in the vacuum chamber and including a sample holder for clamping the nanostructure thereon, and an ion beam source arranged for emitting a beam of low-energy medium-mass ions towards the sample holder at a glancing incident angle with respect to a substrate mounted on it. The sample manipulator is capable of rotating the sample holder and thus a substrate laying on it about an axis normal to the substrate.

According to the invention, the ion beam source is capable of emitting ions having energy per mass within a range of 0.0025 - 0.0225 keV/amu, and the incident angle of the ion beam with respect to a substrate placed on the sample holder is within a range of 30 - 50 degrees, preferably about 40 degrees. These properties of the apparatus together with properly selected sample geometry enable a well controllable and gentle isotropic etching process producing high quality of the etched surface.

The ion beam etching apparatus is preferably capable of producing a current intensity of the ion beam within a range of 10 - 20 μA/cm ² . This has been found a suitable range for providing effective dynamic cleaning of the processed surface.

The ion beam source is preferably capable of producing an ion beam having a diameter of at least 10 mm in order to ensure a sufficiently large processing area and thus simultaneous etching of a plurality of nanostructures on the same substrate.

The apparatus comprises preferably also means for sweeping the ion beam with respect to the substrate in a direction perpendicular to the beam longitudinal axis in order to enlarge the processing area and improve the homogeneity of the ion flux intensity in the processing area. This feature enables simulta-

neous and uniform etching of a large number of nanos- tructures on a single substrate.

Preferably, the apparatus also comprises a heated metallic filament for bringing additional elec- trons to the vicinity of the sample holder in order to neutralise charging of the nanostructure due to charged ions of the bombarding ion beam.

To summarize, the present invention enables fabrication of even sub-10 run nanostructures control- lably and using a low cost equipment consisting of an ion beam source and vacuum system added by a sample manipulator. In addition to the accuracy of the dimensions, the processed surface also has excellent smoothness. Contrary to, for example, the FIB method, a large number of nanostructures formed on a single substrate can be processed simultaneously with a uniform etch rate throughout the substrate enabling industrial mass production. The method and apparatus of the present invention are also very flexible. Pre- fabrication of the sample can be performed with any known fabrication method and the method can be applied to most of the materials used in micro- and nanoelec- tronics. Also the ion type can be selected freely as long as the specifications in accordance with the claims are followed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, which are included to provide a further understanding of the invention and constitute a part of this specification, together with the description explain the principles of the invention.

Figure 1 shows a schematic figure of an ion beam etching apparatus in accordance with the present invention.

Figure 2 represents an example of nanostruc- tures to be processed by the method of the present invention.

Figures 3a and 3b are AFM (Atomic Force Mi- croscope) images of a nanostructure before and after an etching process according to the present invention.

Figure 4 shows AFM images of a nanostructure before and after several etching sessions according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments and examples relating to the present invention, which are illustrated in the accompanying figures .

The ion beam etching apparatus 1 of figure 1 includes a vacuum chamber 2 connected to a vacuum pump 3, an ion gun 4 producing the ion beam 5 needed in etching, and a sample manipulator 6 including a grounded sample holder 7. The ion gun 4 and the sample manipulator are arranged in such a configuration that the ion beam reaches the substrate 8 placed on the sample holder 7 at a glancing angle α of about 40 degrees with respect to the substrate normal N. Prefera- bly the apparatus also includes means for adjusting this angle. The sample manipulator also has means for rotating the sample holder around the axis N normal to the substrate surface and, in consequence thereof, for rotating the sample around the azimuth angle. A beam sweeping system 9 is joined to the ion gun for sweeping the beam in a direction perpendicular to the beam axis for enlarging the etching area. The apparatus also comprises a charge neutraliser 10 based on a heated metallic filament mounted in the vacuum chamber within the vicinity of the sample holder for emitting electrons 11 towards the sample for neutralising the charge accumulated in the sample due to the charged

ions. The apparatus naturally also comprises normal control means not specific to the present invention and thus not presented in the figure. For example, there can be an ammeter connected to the sample ma- nipulator for measuring the ion current to be recalculated (integrated) into the ion dose. The whole equipment is preferably controllable by a computer.

A piece of a conductive substrate 8 on which an array of nanowires 12 is formed is shown in figure 2. The height h of the cross section of the wire is greater than or comparable to the width w. This together with properly selected process parameters ensures that the shape of the cross section remains substantially unchanged during the etching process. Re- lating to the scale, it is important to note that for illustrating purposes the length of the wires is greatly reduced from what it typically is in reality. The substrate 8 is preferably made of at least partly conducting material in order to electrically connect the nanowires to the sample holder via the substrate. Grounding prevents charge accumulation in the nanos- tructures, which otherwise might happen due to the charge of the bombarding ions.

AFM images 3a and 3b show a nanowire made of aluminium before and after etching by the method according to the present invention. It can be seen in the figures that, in addition to the reduction of the dimensions, also the smoothness of the nanowire surface is greatly improved. In fact, experiments have shown that a roughness of even about 1 nm can be achieved by the present invention.

Figure 4 shows a nanostructure before and after one, two and three etching sessions according to the present invention. Also in this case one can clearly see both the reduction in size of the structure and the decreasing of the surface roughness.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.