rus
Issue #3/2015
M.Shibata, T.Obata, H.Ohyi
Nanodevice fabrication technologies with advanced high resolution electron beam lithography systems
Nanodevice fabrication technologies with advanced high resolution electron beam lithography systems
Crestec Corporation developed a high resolution electron beam lithography system CABL-UH. The machine concept, specification and writing performance of the system and potential applications are discussed in this article.
Теги: electron beam lithography nanodevice fabrication производство наноустройств электронно-лучевая литография
Nanotechnology is one of the most active technologies that generates industrial revolutions in many fields such as IT, biotechnology, energy, ecology and so on. Crestec Corporation contributes to the growth of the nanotechnology with development of electron beam lithography (EBL) systems, which play the leading role in nanodevice fabrications because of capability of single-nano-scale pattern formation. Crestec developed dedicated point beam EBL systems, XYZ stage model of CABL-9000C (up to 50 kV), CABL-UH (up to 130 kV) and X-theta stage model of CEBR-3000 series. Crestec is also developing massively parallel EBLs with and without a mask for high volume manufacturing.
In this article, the machine concept, specifications and writing performances of the CABL-UH series and potential applications are discussed.
Machine concept
CABL-UH series were developed to realize the high resolution writing, high stability and high reliability. To achieve the high resolution fabrication, the high acceleration voltage of up to 130 kV is adopted and a beam spot size is reduced by the single-stage acceleration electron gun which suppresses Coulomb blur [1]. On the other hand, to achieve high stability and reliability, micro-discharge is the main problem for the electron gun. In principle a micro-discharge happens more often in a single-stage acceleration electron gun than in a multi-stage electron gun, but our electron gun is made with high-work-function materials and is microdischarge-free. In addition, a stray magnetic field, temperature variation and vibration are controlled without a dedicated shielding room because these disturbances sometimes affect writing performance. Owing to above mentioned design, uptime of more than 90% is feasible in CABL-UH.
Specifications
The standard specification of the CABL-UH series is shown in Table. Crestec has three types of CABL-UH90, CABL-UH110 and CABL-UH130 and the maximum acceleration voltage is 90, 110 and 130 kV, respectively. The acceleration voltage can be changed by 5 kV step. The minimum beam spot size is 1.6 nm at 130 kV as shown in fig.1. At the beam current of 2 nA, the spot size is still less than 2 nm. The minimum line width is 7 nm at 130 kV. Stitching and overlay accuracy are 20 nm at 60 micrometer field and 35 nm at 120 micrometer field, respectively. Beam current stability is better than plus-minus 0.5% for ten hours, although actual data show plus-minus 0.08% for 48 hours (fig.2). Beam position stability is better than plus-minus 30 nm for five hours.
Applications of CABL-UH electron beam lithography with negative-tone resists
The narrow point beam can resolve the very small and precise structures, which offers the smallest structure for nano-scale devices such as FET devices, MOS devices, photonic crystals, optical modulators and nano-imprint molds, etc. Compared with the optical lithography methods, CABL-UH affords to fabricate the single-nano-scale structures without any additional processes such as a Lithography-Etching method. We can directly draw fine structures of several nanometers. We demonstrate some exposure examples with conventional electron beam resists.
Isolated lines and line-and-space (L/S) patterns specify the exposure resolution. We perform the exposure tests with them. A 20-nanometer thick Calixarene is a good candidate to show the critical line width of the exposure. We have resolved the minimum line width of 7 nm (fig.3a). This critical dimension would be reduced with a thinner resist.
We also examine an HSQ negative-tone resist. Resist thickness is 30 nm. No prebaking is undertaken so as to have a good pattern resolution. The exposure condition is following. The acceleration voltage and the beam current are 110 kV and 60 pA, respectively. The L/S lithography patterns, which are very useful for producing LSIs and photonic devices, are demonstrated. Ten-nanometer half-pitch patterns are clearly obtained (fig.3b). We often succeed in resolving sub-ten-nanometer half-pitch L/S patterns (fig.3c). It looks that narrow pitch patterns have been affected by a proximity effect, and the contrast looks worse, but it is still good for applications of dry etching. Proximity effect correction (PEC) is available for the CABL-UH.
Applications of CABL-UH electron beam lithography with positive-tone resists
The positive-tone resists are commonly used for EBL. Because the electron beam for taking secondary electron beam image would degrade the resist during observation, we cannot judge the real gap width, but the resist thickness is fairly precise. We demonstrate the lithography patterns with relatively thicker positive-tone resists. The electron beam in CABL-UH is less forward-scattering because of the high acceleration voltage, and we can form a very deep but straight structure, which are useful for deep reactive ion etching (deep RIE) process and conventional lift-off process.
ZEP-520A positive-tone resist (Nihon ZEON) is spin-coat on a silicon wafer surface, followed by pre-baking. The resist thickness is 1.4 micrometers. The larger acceleration voltage of 130 kV is promising for making deep and straight gaps (fig.4a). The current is 200 pA. The beam diameter would be broadened by a Coulomb effect, but still the gap at the bottom is measured as 80 nm. The deep RIE process can form deep micro-fin structures.
The tee-shaped gate (T-gate) pattern is a good application of CABL-UH. The T-gate is useful for high-power and high-speed transistors. A large massive metal sits on a wire. Both are electrically connected each other. The cutting cross section looks a tee shape. The massive metal transmits a high frequency signal while the wire depletes electron in a high electron mobility transistor (HEMT). This pattern usually needs a several steps of exposure but we offer a single step of the lithography method with CABL-UH.
A ZEP-520A resist covers a bare surface of a wafer. Buffer layer of a Polymethylglutarimide (PMGI) resist separates two ZEP-520A layers. Using different development method for each ZEP-520A layer, we can form a step structure by changing the dose times. We expose a stronger dose at the center of a line, and the wafer surface shows up. The electron beam is restricted as 20 pA, which allow us to fabricate so narrow gate width as 50 nm. The top layer of a ZEP-520A resist is developed by a commercial liquid developer. The bottom layer of a ZEP-520A resist can be influenced with the same liquid but is still protected by a buffer layer resist of PMGI. After developing the buffer layer, the critical layer of ZEP-520A is finally removed (fig.4b). After proper etching treatment of surface cleaning, we can deposit gate metals.
Summary
Crestec developed an EBL system, CABL-UH series with high resolution, high stability and high reliability. Writing results for both negative- and positive-tone resists show that CABL-UH has potential in a wide range of application fields such as FET devices, MOS devices, photonic crystals, optical modulators, nano-imprint molds and HEMT.
In this article, the machine concept, specifications and writing performances of the CABL-UH series and potential applications are discussed.
Machine concept
CABL-UH series were developed to realize the high resolution writing, high stability and high reliability. To achieve the high resolution fabrication, the high acceleration voltage of up to 130 kV is adopted and a beam spot size is reduced by the single-stage acceleration electron gun which suppresses Coulomb blur [1]. On the other hand, to achieve high stability and reliability, micro-discharge is the main problem for the electron gun. In principle a micro-discharge happens more often in a single-stage acceleration electron gun than in a multi-stage electron gun, but our electron gun is made with high-work-function materials and is microdischarge-free. In addition, a stray magnetic field, temperature variation and vibration are controlled without a dedicated shielding room because these disturbances sometimes affect writing performance. Owing to above mentioned design, uptime of more than 90% is feasible in CABL-UH.
Specifications
The standard specification of the CABL-UH series is shown in Table. Crestec has three types of CABL-UH90, CABL-UH110 and CABL-UH130 and the maximum acceleration voltage is 90, 110 and 130 kV, respectively. The acceleration voltage can be changed by 5 kV step. The minimum beam spot size is 1.6 nm at 130 kV as shown in fig.1. At the beam current of 2 nA, the spot size is still less than 2 nm. The minimum line width is 7 nm at 130 kV. Stitching and overlay accuracy are 20 nm at 60 micrometer field and 35 nm at 120 micrometer field, respectively. Beam current stability is better than plus-minus 0.5% for ten hours, although actual data show plus-minus 0.08% for 48 hours (fig.2). Beam position stability is better than plus-minus 30 nm for five hours.
Applications of CABL-UH electron beam lithography with negative-tone resists
The narrow point beam can resolve the very small and precise structures, which offers the smallest structure for nano-scale devices such as FET devices, MOS devices, photonic crystals, optical modulators and nano-imprint molds, etc. Compared with the optical lithography methods, CABL-UH affords to fabricate the single-nano-scale structures without any additional processes such as a Lithography-Etching method. We can directly draw fine structures of several nanometers. We demonstrate some exposure examples with conventional electron beam resists.
Isolated lines and line-and-space (L/S) patterns specify the exposure resolution. We perform the exposure tests with them. A 20-nanometer thick Calixarene is a good candidate to show the critical line width of the exposure. We have resolved the minimum line width of 7 nm (fig.3a). This critical dimension would be reduced with a thinner resist.
We also examine an HSQ negative-tone resist. Resist thickness is 30 nm. No prebaking is undertaken so as to have a good pattern resolution. The exposure condition is following. The acceleration voltage and the beam current are 110 kV and 60 pA, respectively. The L/S lithography patterns, which are very useful for producing LSIs and photonic devices, are demonstrated. Ten-nanometer half-pitch patterns are clearly obtained (fig.3b). We often succeed in resolving sub-ten-nanometer half-pitch L/S patterns (fig.3c). It looks that narrow pitch patterns have been affected by a proximity effect, and the contrast looks worse, but it is still good for applications of dry etching. Proximity effect correction (PEC) is available for the CABL-UH.
Applications of CABL-UH electron beam lithography with positive-tone resists
The positive-tone resists are commonly used for EBL. Because the electron beam for taking secondary electron beam image would degrade the resist during observation, we cannot judge the real gap width, but the resist thickness is fairly precise. We demonstrate the lithography patterns with relatively thicker positive-tone resists. The electron beam in CABL-UH is less forward-scattering because of the high acceleration voltage, and we can form a very deep but straight structure, which are useful for deep reactive ion etching (deep RIE) process and conventional lift-off process.
ZEP-520A positive-tone resist (Nihon ZEON) is spin-coat on a silicon wafer surface, followed by pre-baking. The resist thickness is 1.4 micrometers. The larger acceleration voltage of 130 kV is promising for making deep and straight gaps (fig.4a). The current is 200 pA. The beam diameter would be broadened by a Coulomb effect, but still the gap at the bottom is measured as 80 nm. The deep RIE process can form deep micro-fin structures.
The tee-shaped gate (T-gate) pattern is a good application of CABL-UH. The T-gate is useful for high-power and high-speed transistors. A large massive metal sits on a wire. Both are electrically connected each other. The cutting cross section looks a tee shape. The massive metal transmits a high frequency signal while the wire depletes electron in a high electron mobility transistor (HEMT). This pattern usually needs a several steps of exposure but we offer a single step of the lithography method with CABL-UH.
A ZEP-520A resist covers a bare surface of a wafer. Buffer layer of a Polymethylglutarimide (PMGI) resist separates two ZEP-520A layers. Using different development method for each ZEP-520A layer, we can form a step structure by changing the dose times. We expose a stronger dose at the center of a line, and the wafer surface shows up. The electron beam is restricted as 20 pA, which allow us to fabricate so narrow gate width as 50 nm. The top layer of a ZEP-520A resist is developed by a commercial liquid developer. The bottom layer of a ZEP-520A resist can be influenced with the same liquid but is still protected by a buffer layer resist of PMGI. After developing the buffer layer, the critical layer of ZEP-520A is finally removed (fig.4b). After proper etching treatment of surface cleaning, we can deposit gate metals.
Summary
Crestec developed an EBL system, CABL-UH series with high resolution, high stability and high reliability. Writing results for both negative- and positive-tone resists show that CABL-UH has potential in a wide range of application fields such as FET devices, MOS devices, photonic crystals, optical modulators, nano-imprint molds and HEMT.
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