Utilize These Six Secret Electron Beam Lithography Techniques

Utilize These Six Secret Electron Beam Lithography Techniques

Photolithography masks can be made using EBL, which is used in many different applications. In addition to being time-consuming, EBL requires that the pattern be written consecutively. To reduce the writing time, a variety of methods are employed. The acceleration voltage applied by EBL instruments used in industrial applications is typically very high (50 kV).

However, more cost-effective instruments are used in many research environments. However, they are not very fast and are usually optimized for high-resolution writing. Even for limited resolution applications, they are generally not considered suitable for writing large-scale structures with high pattern density. As the authors demonstrate in this paper using the Raith e LiNE EBL instrument, optimizing the writing parameters can reduce the writing time by over 40 times compared to standard instrument settings.

Photolithography masks with high precision have been made using the optimization procedure by the authors. The instrument software estimated a write time of about 14 days for the most commonly used settings. When compared to conventional chrome masks, our pattern definition is significantly better.

Several methods of mask-free printing. The most common are electron beam lithography (EBL), direct laser writing 1,2, and interference lithography. Besides focusing on beam lithography and dip-pen lithography, other techniques are becoming more and more important today.

As a result of its ability to write patterns with high resolution down to a few nanometers, EBL is widely used in many nanotechnology-related research fields. EBL writing exposes a resistor with a tightly focused beam of electrons, which is developed later. In order to create the final structure, the resist pattern can be processed in a variety of ways. EBL is not diffraction-limited under typical operating conditions because the wavelength of the electrons is in the range of picometers or less. The resists and the subsequent processing steps are the main obstacles to achieving high resolution in an EBL system. Because of its extremely high resolution (line widths can be as small as 10 nm), poly(methyl methacrylate) is one of the most widely used resists.

Most of the patterning time is spent on resist exposure, stage movement (for structures larger than a single write-field), and electron beam settling in electron lithography. If you’re using EBL software to ensure that the beam is stable at each new location, then the settling time is typically built into the software. The maximum achievable beam current is physically limited by space charge effects. When using serial exposure with only one beam, the patterning time will be limited by this value. Because the overall beam current can be higher, shaped beams and multi-beam exposure tools can be faster.

Both the exposure time and idle time should be minimized in order to increase write speed on any EBL device. When exposed to a 50 kV electron beam, the sensitivity of newer resists, such as the negative tone resist SU-8, has been reported to be as high as 3.6 C/cm2.

Look at these secret techniques for improving electron beam lithography systems:

How to find out how much acceleration you have

Resistance clearance dose increases as acceleration voltage increases. Why? Because the forward-scattered electrons are more efficient at energy transfer to the resistor at lower acceleration voltages (10 kV), resulting in lower clearance dose requirements but at the cost of a broader incident beam spot and rougher line surfaces.

Additionally, the amount of clearance dose required is highly dependent on the developer type and development process used.

Collimation Aperture Size Selection

To collimate and current-limit the electron beam, a beamline with interchangeable apertures can be used to collimate the beam. A collimating aperture of 120 microns diameter was used to increase the beam current, allowing more electrons from the filament to reach the sample. Standard features of an electron microscope include a collimating aperture in the electron column. Basically, it’s a way to change the numerical aperture of the beam. As a result, smaller apertures result in a smaller numerical aperture and, therefore, a larger depth field. Despite the fact that Raith was unable to provide precise information on the numerical aperture, they did state that the field of depth typically changed from approximately 10 to 1–2 m when switching between the 30 and 120 m diameter apertures.

electron-beam lithography system - Helmholtz-Zentrum Berlin (HZB)

The high current mode should be enabled.

Raith offers a “high current” mode in which the condenser lens’ focusing properties are altered, resulting in a smaller, parallel beam. The beam current will be approximately doubled in this mode. Due to the effects of space charge, the final resolution will be slightly reduced, but the smaller, parallel beam will increase the depth of focus. Using the collimating aperture with a diameter of 120 m and an acceleration voltage of 10 kV, we measured a beam current of 6.8 nA.

Enter the field size

The typical size of a write field is 100 m 100 m. Since we used a very large write-field (the maximum is 2000 m 2000 m), we could reproduce the pattern with a 100-fold reduction in the number of write fields. Since the sample stage will move and settle faster, this will reduce the sample stage’s moving and settling time by 100.

Using larger write fields has a number of drawbacks. Since the digital-to-analog converter (DAC) of the pattern generator has a limited addressable resolution (16 bits in our case), the minimum step size is reduced. Although the EBL’s addressable step size is limited, it is still very small for write fields 1000 m 1000 m. We can calculate the minimum addressable step size for the 16-bit DAC used in the Raith e LiNE and the large write fields.

Write mode

Within one write-field, there are generally two ways to move the beam around raster and vector scan. Using a raster scan is the simplest method, but it takes longer. As the beam passes over areas that need to be exposed, it is unblanked. Technically more difficult, the vector scan directs the beam to each area that needs to be exposed and only scans over the areas that need to be exposed. The amount of time that can be saved by using a vector scan is highly dependent on the type of pattern that is being created, so keep this in mind. 

Step size and beam speed

This is done in the e LiNE design software using the GDSII Raith lithography module. When it comes to integrated circuits, the GDSII format is a common one to use. In addition to bitmaps and other pattern file formats, other file types can also be imported. A bitmap can be imported into e LiNE software using the “bitmap as reference” mode. It is also possible to use the “bitmap as reference” format when using the line or meander mode. The beam will scan the entire write field, un blanking and blanking for exposure, respectively.