Electron-beam lithography

The electron beam lithography ( ESL, English electron beam lithography often than e-beam lithography for short) is in the micro- and semiconductor technology, a special method for structuring an electron - sensitive layer ( engl. resist, in analogy called for photolithography and photoresist ). The process belongs to the group of next-generation lithography and is closely related to the ion beam lithography. By " exposure " with an electron beam, the resist is chemically modified so that it can be released locally (developer) and a patterned resist layer is formed. The structure can then be transferred to a layer of another material, for example by etching an underlying layer, or by selective deposition of a material on the resist. The main advantage of the method is that structures with significantly smaller dimensions ( in the nanometer range ) can be prepared as in photolithography.

The method has great importance in the manufacture of microelectronic circuits for modern electronic devices, and is mainly used in the manufacture of photomasks used in the photolithography. It can also be used as a maskless lithography method for patterning layers or wafers in the prototype or small batch production. Electron beam lithography is also traded in large-scale production as the successor method for today (as of 2011 ) photolithography based on excimer laser. The long process times to current techniques, in which the electron beam is scanned over the substrate, for example, but are not economical and also lead to technical problems, such as instability in the electron beam.

• 4.1 Maskless techniques 4.1.1 Raster and Vector scanning principle
• 4.1.2 beam shape

• 4.2.1 SCALPEL

Energy loss of electrons in matter

Scattering

Electrons are particles having a relatively low mass compared with the atomic nuclei. Take electrons from an incident beam ( the primary electrons ) and high energy (typically 10-50 keV) to the resist, they undergo in the material in both a forward and a back-scattering. Of the forward scatter is defined as a deflection of the electrons is less than 90 ° in the direction of incidence. Scattering effects, inter alia, a widening of the beam diameter and thus leads to an effective degradation of the resolving power, but this is less than by the generation of secondary electrons. Sometimes, the primary electrons are scattered at an angle of more than 90 degrees, that is, they will not be scattered in the substrate. These electrons are called backscattered electrons (English backscattered electrons ) and have the same effect as long-range lens scattering effects (german lens flare ) of optical projection systems. A sufficiently large dose of backscattered electrons can cause significantly greater than the beam cross section in the focus to a full exposure of an area.

Production of secondary electrons

In addition to the elastic scattering in the resist and the substrate, the primary electrons experience when entering or passing through a material such as the resist or inelastic scattering collisions with other electron (e.g. the electrons of the lattice atoms ). In such a collision, the primary electrons lose energy by a momentum transfer from the incident electron to the other electron, and may be described by the relation, wherein the distance of closest approach between the electrons and the speed of the incident electrons is. The energy which is transmitted by the collision can be described about the connection, wherein the mass of the electron, the elementary charge and the electron energy corresponding to is. By the integration over all values ​​of between the lowest bond energy and the incident energy is obtained as a result that the total cross-section of a collision is inversely proportional to the incident power and proportional. In general, the result is substantially inversely proportional to the binding energy.

Resolution

Unlike the optical lithography, the electron beam lithography is practically limited by the wavelength of the particles used. Thus, the wavelength of electrons is located at an energy of about 25 keV at about 8 picometers, which is about one-twelfth of the hydrogen diameter. The resolving power therefore depends rather on the beam diameter used, which in turn is limited by the electron source, the aberration of the electron optical system and the interactions in a highly collimated electron beam. With today's electron optics electron beams can be produced with cross sections of a few nanometers. However, the practical resolution limit is not solely determined by the beam diameter, but also by forward scattering in the resist and especially the secondary electrons, which move in the resist. The forward scattering can be reduced by using higher electron energies, and thin resist layers, the generation of secondary electrons is, however, inevitable. The distance of a secondary electron is not a generally predictable value, but a statistical parameter which can be determined from many experiments or Monte Carlo simulations with energy less than 1 eV. This is necessary because the the peak in the energy distribution of secondary electrons well below 10 eV liegt.Wiederholbarkeit and control in the practical resolution limit often require the consideration of influences that do not match the image formation are related, for example, resist development and intermolecular forces.

Write time

The minimum exposure time for a given area at a given radiation dose is described by the following formula:

For example, the minimum exposure time for an area of ​​1 cm 2, a dose of 10-3 C/cm2 and a beam current of 10-9 A is about 106 s (about 12 days). This minimum write time does not include the time for movement of the substrate holder, the beam is blanked and other possible technical corrections and adjustments during writing. To cover the 700 cm2 surface of a 300 mm silicon wafer, the minimum write time would be at 7108 seconds, about 22 years longer. It is clear that in this case the flow rate presents a serious limitation for electron beam lithography, in particular upon exposure of dense patterns on a large area. Direct writing method with only one beam are therefore not suitable for high volume production. Because for a single wafer to expose a pattern with a sub-100 -nm resolution with the electron beam lithography, several days would be required, as a rule, in comparison, need of today's 193-nm photolithography systems less than a minute for the same task.

Proximity effect

The smallest structures produced by electron beam lithography are usually insulated structures, since the production of closely together standing structures (usually lines ) by the proximity effect ( Germanized by engl proximity effect: German, neighborhood effect '. ) Is difficult. The proximity effect describes the crosstalk of electrons in the exposure, that is, electrons that were " intended " to expose a certain structure, outshine the provided field and contribute to the exposure of adjacent areas at. This leads to an increase in the written structures effectively expands its image and resulting in a reduction of the contrast, i.e., the difference between maximum and minimum intensity in a range. Therefore, the exposure and release of dense structures is more difficult to control. Most resists, it is therefore difficult to produce lines and spaces of less than 25 nm; the lower limit is currently 20 nm

The main reason for the proximity effects is the scattering of electrons due to the electrical interaction of the negatively charged electrons with each other. The problem can be reduced by a previously calculated correction of the exposure function. It allows a dose distribution as close as possible to the desired dose

Charging

Applies a high-energy electron beam to a substrate, this stop part of the electrons. Since electrons are charged particles, they tend to charge the substrate negative if they are not quickly discharged towards ground. For low- conductive substrates, such as a silicon wafer, this usually poses no problem Unlike the case or not is poorly conducting substrates, such as quartz substrates used for photomasks. Often a negative charge in the substrate is associated with a positive counter charge on the surface, which is caused mainly by the secondary electron emission into the vacuum. The range for the emission of low energy secondary electrons ( the largest component of the free electrons in the resist -substrate system ), can contribute to the charge, is between 0 and 50 nm below the surface. The charging of the resist and the substrate is general not repeatable and therefore difficult to compensate. Positive charges here are not as bad as negative charge, since the latter can deflect the electron beam during the exposure of the desired position.

Similar effects also occur in the scanning electron microscopy, where they lead to a loss of contrast and lower resolution. There, one makes do with the application of a thin conductive layer on the sample. In the ESL are those conductive layer above or below the resist is usually of limited use, since high-energy (50 keV or more) electron beams pass through most of the layers relatively freely and to continue to accumulate in the substrate. In low-energy beams of use, however, is quite effective and meaningful.

Electron beam lithography systems

Electron beam lithography systems consist essentially of the following components of the electron source, the electron- optical system and the deflection and projection unit ( focusing). Due to the linear arrangement of the components, the entire structure is also referred to as a column.

Systems with lower resolution can be used Thermionic, usually based on lanthanum hexaboride ( LaB6 ). Systems with higher resolution, however, require field emission sources, such as heated W/ZrO2, for lower power consumption and enhanced intensity. This thermal field emission sources, despite their slightly larger beam size to cold emission sources is preferred because they offer better stability when writing for a long time ( several hours).

For the concentration and focus of the electron specific plant parts are required, which are often referred to in analogy to optics and lens system. In ESL systems, both electrostatic and magnetic lenses can be used. However, electrostatic lenses show a greater aberration and are thus not suitable for fine focusing. Because currently there are no techniques for the production of achromatic electron lenses, so that electron beams are required with an extremely narrow energy dispersion for fine focusing.

Typically electrostatic systems are used for very small deflection of the electron beam, larger beam deflections require electromagnetic systems. And because of the inaccuracy of the finite number of exposure steps, the exposure field on the order of 100 to 1000 microns. Larger samples require a movement of the substrate support (English stage or chuck ), which must satisfy very high requirements in terms of stringing reduction of patterns and aligning a pattern on a plane with respect to the previous level, cf overlay ( semiconductor technology).

The used for commercial applications ESL systems are earmarked, for example, for photomask manufacturing, and very expensive ( about U.S. $ 4 million ). For devices for research applications is, however, often modified electron microscopes, the comparatively low cost (less than 100 thousand USD) have been converted into an ESL system. This is also reflected in the achievable results down, so could be mapped smaller of 10 nm and with the dedicated systems already feature sizes. With research devices based on electron microscopes, however, only sizes of about 20 nm can be imaged.

Resists

One of the first and today still resists are used as short-chain and long-chain polymethyl methacrylate (PMMA, sensitivity at 100 keV approximately 0.8-0.9 C/cm2 ). This is usually a Einkomponentenresist. In contrast, there are similar to normal photoresists also multi-component resists, in which not only the electron-sensitive component, for example, substances are added which provide a stronger networking of the resist after exposure (so-called chemically amplified resists ).

Further searching for electron-sensitive resists or chemically stable in order to allow, for example, shorter exposure times. The electron-sensitive resists include, inter alia, hydrogen silsesquioxane ( engl. hydrogen silsesquioxane, HSQ, about 1 C/cm2 @ 100 keV) or calixarenes (about 10 C/cm2 and greater @ 100 keV). Unlike these two PMMA resists are negative resists, that is, the exposed regions remain after development of the resist on the wafer.

Method

ESL systems include both maskless and mask-based method. Both groups of methods can be further divided into various sub- techniques.

The maskless method, ie direct writing with a guided electron beam, can also the strategy of the beam deflection are classified according to both the beam shape as. Older systems use Gaussian beam -shaped electron beams, which is guided over the substrate (scanning mode). Newer systems use shaped beams, ie beams which via a mask a desired geometric cross section was impressed, and their diversion to different positions in the " Title Block " (vector - scan mode ).

The mask-based method similar to the conventional photolithography. Even with the ESL, there are specific proximity irradiation techniques such as the 1:1 projection or projections in which the structures of the mask is reduced.

Maskless techniques

• Reflective electron beam lithography ( REBL )
• Multibeam systems
• More mask-based method

For an immediate write the information into the resist, an electron beam is imaged without a mask. For this purpose the beam is guided in accordance with the desired mask pattern on the substrate. The required deflection is accomplished through electrostatic interactions between the electrons.

Raster and vector scanning principle

The raster scan principle, the electron beam line by line over the exposure field. This is similar to the beam guide to a CRT monitor or a scanning electron microscope. The exposure of the structures takes place through selective switching on and off the electron beam. The XY - table of the substrate support is moved continuously in the rule.

In contrast, in vector scan principle, the beam is deflected selectively onto the structure to be exposed in the exposure field and is written there into a meandering or spiral motion of the electron beam. After all the structures were exposed in the deflection field, the XY table moves to the next position, similar to the step-and -repeat process today's photolithography systems takes place. This principle brings, particularly in less structured areas a distinct advantage in terms of processing time with them.

Beam shape

When both raster and vector scan principle in differently shaped electron beams can be used. They are divided into several types in terms of energy distribution ( in the beam cross -section):

The beam shape is generated by an aperture or aperture plates structured. The latter can be thought of as a simple pinhole with finite geometry.

Mask -based techniques

The maskless techniques have a major disadvantage, the long write time per wafer. To make the ESL for mass production attractive, alternative techniques have been developed, such as multi- beam writer. But are also interesting mask-based techniques, as they are already used in the conventional photolithography. Electron beams are bidding against light exposure a significant advantage, they show due to their very small wavelength ( de Broglie wavelength) no practically relevant diffraction effects which would interfere with the transfer of structures from a mask into the resist.

The exposure is a shadow projection of the mask structures using a parallel electron beam. The masks are either transmission masks, in which the structures " punched out " were (English stencil mask ), or masks, in which an electron- transparent substrate on an absorbing layer was deposited and patterned similar to popular photo masks. After exposure, the exposure field is moved in a step-and -repeat process to the next exposure position on the wafer or substrate.

SCALPEL

SCALPEL ( Scattering with Angular Limitation Projection Electron -beam Lithography ) is another mask-based technique that uses a diffusion mask on a transparent film for electrons. Similar to the conventional photolithography certain parts are shadowed by the electron beam through the mask. For this purpose, a scattering layer is used, which deflects impinging electrons strongly. You will then be hidden through an aperture. The advantage of the diffusing layer compared to an absorption of the electrons is on the one hand in the significantly lower charging, on the other, a lower heating of the mask.

Example of a process flow

The following example shows reference to the figures, as a nano- metal bridge can be made for certain break junction experiments using electron beam lithography. For the experiment is an elastic substrate made ​​of bronze sheet necessary as well as a free-standing metal bridge with a predetermined breaking point, which slowly tearing during bending of the substrate. The structure used in the basic research of the generation of single-atom contacts. To prepare suitable in this application, especially the raster electron beam lithography, as only a few samples are needed as research objects and the required structure size is normally not reached with a 100 -nm-wide constriction with optical lithography process.

Step 2: sacrificial layer of polyimide deposited by spin coating and heating process.

Step 3: applying a soft electron beam resist as a buffer layer ( spin coating drying).

Step 4: applying a hard PMMA resist as vapor deposition mask ( spin coating drying).

Step 5: exposure to the electron beam of a scanning electron microscope (SEM).

Step 6: developing in a developing liquid, which dissolves the exposed areas ( positive resist ).

Step 7: vapor deposition of metal such as aluminum. The buffer layer leads to well-defined metal edges without touching the paint.

Step 8: lift-off of the unwanted metal in a solvent ( acetone).

Step 9: Cropping the nanobridge by partially removing the sacrificial layer in reactive -ion plasma ( RIE).

Ready nanobridge of aluminum on the polyimide substrate, viewed in the SEM ( artificially colored).

Defects

Despite the high resolution of electron beam lithography generation of defects of users is often not considered. The defects that occur can be divided into two categories: data-related and physical defects.

Data-related defects in turn can be divided into two subgroups. Fade-out or deflection errors occur when the electron beam is not deflected correctly. However shape defect occur (English shaping errors) in systems with variable beam shape when the wrong shape is projected onto the sample. These errors can be either from the electron optical control hardware or resulting input data. As might be expected, this larger data sets are more prone to data-related defects.

Physical defects are varied and include effects such as the electrostatic charging of the sample ( positive and negative), back-scattering of electrons dose error, fogging ( long-range reflection of the backscattered electrons) outgassing of the resist, dirt and beam expander. As the time for the direct writing slightly more hours may take (even a day), are more likely to occur randomly occurring errors. Again, larger amounts of data more susceptible to defects.

Future developments

In order to solve the problems associated with the secondary electron generation, it will be essential to use the low-energy electrons for the exposure of the resist. The energy of the electrons should ideally have on the order of a few electron volts. This has already been shown in a study using a ELS- system based on a scanning tunneling microscope. It was found that electrons can penetrate with energies less than 12 eV in a 50 nm thick polymer photoresist. The disadvantage of the use of low-energy electrons is that the spread of the electron beam is difficult to prevent in the photoresist. In addition, the design of the electron beam system for low beam energies and high resolution is difficult because the Coulomb repulsion between the electrons becomes important.

An alternative is the use of very high energy ( at least 100 keV) in order to achieve removal of material by sputtering. This phenomenon has been widely observed in the transmission electron microscopy. However, it involves a very inefficient process, due to the inefficient transmission of impulses from the electron beam to the material. This results in a slow process with a much longer exposure time than in the conventional electron beam lithography. In addition to damaging the substrate high beam energies.

In order to achieve a shorter exposure time and thus an economical throughput of at least 10 wafers per hour in the manufacture of microelectronic circuits, approaches have been studied for several years, in which a plurality of electron beams (English multi - beam lithography ) are used simultaneously. In such multi beam writers should allow much less than one hour for the exposure of a 300 -mm wafer 10,000 or more beams of process times.