Electron lithography or electron beam lithography is a method of nano lithography using an electron beam .
Content
Principle of the method
An electron beam sharply focused with magnetic lenses on the surface of a polymer layer ( resist ) sensitive to electron irradiation draws on it an image that is detected after processing the resist in the developer. Processing with an electron beam of a resist changes the degree of solubility of the polymer in the solvent (developer). The surface areas with the image recorded on them are cleaned of the resist with the help of a developer. Through the resulting windows in a resist film, a vacuum deposition of a suitable material, such as titanium or metal nitride or ion etching, is performed. At the last stage of the technological process, the non-exposed radiation resist is also washed off with another solvent. The electron beam is moved along the surface by a computer by changing currents in deflecting magnetic systems. In some installations, the shape and size of the electron beam spot change. At the output of a multi-stage technological process, a mask is obtained for use in photolithography and other nanotechnological processes, for example, in reactive ion etching technology.
Electron lithography makes it possible at the current level of technology development in record experimental installations to obtain structures with a resolution of less than 1 nm , unattainable for hard ultraviolet radiation, due to the shorter de Broglie wavelength of electrons compared to light [1] (see Wave mechanics ).
Electronic lithography is the main method of obtaining masks for use in subsequent photolithography in the manufacture of monolithic microcircuits [2] [3] (including masks for projection photolithography in the mass production of ultra-large microcircuits ).
An alternative way to create masks is laser technology [4] , but this technology has a lower resolution [5] .
Also, electronic lithography, which has low productivity, is used in the production of single copies of electronic components, in cases where nanometer resolution is required, in industry and in scientific research.
Electronic Lithography Resolution
The resolution of the details of the picture during recording is affected by both the size of the electron beam and the processes of interaction of the electron beam with the resist. [6]
Electron Beam Size
The diameter of the electron beam several factors influence: the size of the electron source and scaling factor of the electronic focusing system . These parameters are related by the formula:
- .
Electron wavelength depends on accelerating potential and equals nm For an accelerating voltage of 10 kV, the electron wavelength is 12.2 pm, and, accordingly, the resolution of the system, limited by diffraction, is:
- ,
Where - half the beam focus angle.
In real systems, magnetic lenses are spherical and chromatic aberration. Spherical aberration occurs due to different focal lengths for electrons moving along the axis and at the periphery of the beam. The spread of electron velocities in the beam leads to chromatic aberration - electrons with different initial velocities are focused at different distances.
To reduce spherical aberration, an aperture limitation of the beam is used - diaphragms that cut off peripheral electrons. But when the beam is diaphragmed, its current decreases.
Thus, the resolution determined by the properties of the electron beam has the form:
- .
The figure shows the dependence of the beam size on the focus angle, taking into account all types of distortion of the beam sizes.
Decrease in resolution due to nonlinear processes in the interaction of an electron beam with a resist
The final resolution of electron lithography is determined not only by the diameter of the focused beam, but also by the nature of its interaction with the resist layer. The collision of the electrons of the primary, high-energy electron beam (red line) with the atoms of the resist material gives rise to a damped avalanche of secondary knocked-out electrons (blue lines) in it, secondary electrons parasitically “illuminate” the resist. As a result, the exposed spot in the resist film is several times larger in size relative to the diameter of the electron beam.
To reduce the energy of the avalanche of secondary electrons, and, accordingly, reduce the size of the exposure spot, it is necessary to reduce the energy of the beam electrons, that is, to reduce the accelerating voltage of the electron gun. But when the accelerating voltage decreases, the beam focusing worsens. Therefore, a compromise value of the accelerating voltage is practically chosen - to ensure the best resolution with the applied resist layer thickness and its properties.
Principles for recording a pattern on a sample [7]
Currently (2015), the recording of a latent image in a resist film on a sample surface can be carried out by three possible methods:
- raster way;
- vector way;
- recording by an electron beam with a varying size and shape of a focused spot.
- Raster recording
This type of recording is similar to reading (recording) an image on a TV screen, where an electron beam sequentially (line by line) runs around every point on the screen. In places where necessary, the beam exhibits a resist, at the remaining points the electron beam is blocked by blocking the electron gun, although scanning (changing the current in the deflection system) continues.
- Vector Record
The electron beam is fed only to those places where exposure is required, and does not go to places not subject to exposure. Therefore, the entire exposure process is much faster than with the raster recording method.
- Electron beam recording with varying size and shape of the electron beam
In this case, the recording is “a big brushstroke,” according to the terminology of artists. Since any picture can be drawn using rectangles, there is no need to rasterize the picture into elementary pixels , it is enough to change the shape and size of the focused beam, from a small rectangle to a large one. Recording in this case is even faster than in the vector method.
Electronic Lithography Systems
Electronic lithography systems for commercial applications cost around $ 4 million or more. For scientific research, an electron microscope is usually used, converted into an electronic lithography system using relatively cheap additional devices (the total cost of such an installation is <$ 100 thousand). These modified systems made it possible to draw lines with a width of about 20 nm since the 1990s. Meanwhile, modern specialized equipment will allow obtaining a resolution better than 10 nm.
Manufacturers
Electronic lithography is used to create masks for photolithography ( photomasks ), while systems with a single electron beam are traditionally used. Similar systems were produced by the companies: Applied Materials, Leica, Hitachi, Toshiba, JEOL , Etec [8] [9] [10] .
Since the mid-2010s, several manufacturers of electronic lithography plants have been offering multi-beam systems for creating photomasks for the production of monolithic microcircuits [11] , while manufacturers also declare them as installations for directly recording a pattern on large substrates (maskless lithography), since they have great performance , compared with single-beam installations, and therefore can compete with the traditional photolithographic method in the production of small batches of microcircuits [12] :
- Mapper Lithography (Netherlands)
- IMS Nanofabrication AG (Vienna, Austria)
- KLA-Tencor Corp. (Milpitas, California) - Reflective Electron Beam Lithography (REBL) technology
- Elionix, Japan
The table shows, as an example, the characteristics of the Elionix ELS-F125 installation [13] (typical installation parameters with a single beam):
| Electron source - cathode of the electron gun | ZrO 2 / W - heating element |
| The diameter of the electron beam at the width of the half-intensity | 1.7 nm at 125 kV |
| Minimum line width | about 5 nm at 125 kV |
| Electron beam current | 5 pA ... 100 nA |
| Accelerating voltage | 125 kV, 100 kV, 50 kV, 25 kV |
| Recordable Size | 3000 μm x 3000 μm (maximum), 100 μm x 100 μm (minimum) |
| Beam Positioning Accuracy | 0.01 nm |
| The maximum size of the processed plate | 20 cm (200 mm plates and 200 mm masks) |
See also
- Photolithography
- Ion beam lithography
- Nanoprint lithography
Literature
- Abroyan I.A., Andronov A.N., Titov A.I. Physical basis of electronic and ionic technology. - M .: Higher school, 1984. - 320 p.
- Electron beam technology in the manufacture of microelectronic devices / Brewer J. R .. - M .: Radio and communications, 1984. - 336 p.
- Valiev K. A. Microelectronics: Achievements and Ways of Development / Valiev K. A. .. - M .: Nauka, 1986. - 141 p.
- Valiev, K.A .; Rakov, A.V. Physical foundations of submicron lithography in microelectronics. - M .: Nauka, 1984 .-- 352 p.
- Popov V.F., Gorin Yu.N. Processes and installations of electron-ion technology. - M .: Higher. school, 1988 .-- 255 p. - ISBN 5-06-001480-0 .
- Vinogradov M.I., Maishev Yu.P. Vacuum processes and equipment of ion - and electron-beam technology. - M .: Engineering, 1989 .-- 56 p. - ISBN 5-217-00726-5 .
Notes
- ↑ McCord, MA 2 // SPIE Handbook of Microlithography, Micromachining and Microfabrication . - 2000.
- ↑ Principles of Lithography, Third Edition, SPIE Press, 2011 ISBN 978-0-8194-8324-9 7.4 Electron-beam lithography and mask writers "For two decades, the MEBES systems were the primary beam writers used to make photomasks"
- ↑ Syed Rizvi , Handbook of Photomask Manufacturing Technology (inaccessible link) , Taylor & Francis, 2005, ISBN 978-0-8247-5374-0 . Sergey Babin 3. Mask Writers: An Overview, 3.1 Introduction. "For decades, the unique features of EBL systems - easily programmable computer control, high accuracy, and relatively high throughput - have positioned these systems as the main tools to fabricate critical masks."
- ↑ Hwaiyu Geng Semiconductor manufacturing handbook. ISBN 978-0-07-146965-4 , McGraw-Hill Handbooks 2005, doi: 10.1036 / 0071445595 . Section 8.2.2 Pattern generation ( Charles Howard , DuPont) “The other pattern generation alternative is a laser-based system”
- ↑ Peter Buck (DuPont Photomasks), Optical lithography: The future of mask manufacturing? (link unavailable) , Microlithography World volume 11 issue 3, PennWell Publishing, Aug 2002 (p 22): “Optical mask lithography systems are restricted in resolution, just like wafer steppers, to roughly 3/4 of the exposure wavelength. Accordingly, they do not exhibit the <100nm resolution possible for VSB / electron lithography / systems. ”
- ↑ SPIE Handbook of Microlithography, Micromachining and Microfabrication Volume 1: Microlithography, http://www.cnf.cornell.edu/cnf_spie2.html#2.2.6
- ↑ Syed Rizvi , Handbook of Photomask Manufacturing Technology (inaccessible link) , Taylor & Francis, 2005, ISBN 978-0-8247-5374-0 . 3.3 Vector Scan Systems, pages 60-6
- ↑ Mask Data Format Standardization / DuPont Photomasks, 2001
- ↑ Applied scrambles to hold lead in e-beam photomask tools / EETimes, 2001-07-27
- ↑ SPIE Handbook of Microlithography, Micromachining and Microfabrication. Volume 1: Microlithography Chapter 2, E Beam Lithography
- ↑ http://semiengineering.com/5-disruptive-mask-technologies/ “In 2015, photomask vendors could begin to make a gradual transition from single-beam e-beam tools to a new class of multi-beam mask writers.”
- ↑ Peter Clarke . TSMC set to receive Matrix 13,000 e-beam litho machine (English) , EETimes (2/17/2012). Released January 10, 2014. “There are at least three potential suppliers of the maskless e-beam technology: IMS Nanofabrication AG (Vienna, Austria), KLA-Tencor Corp. (Milpitas, Calif.) With its Reflective Electron Beam Lithography (REBL) system and Mapper Lithography. ”
- ↑ Electron Beam Lithography (EBL) | ELS-F125 | ELIONIX unopened (inaccessible link) . Date of treatment December 20, 2015. Archived on February 6, 2016.