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Nanofabrication of air-spaced field-plate gates with ultra-short footprint
Microelectronic Engineering 143 (2015) 11–14
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanofabrication of air-spaced field-plate gates with ultra-short footprint Jinhai Shao a, Jianpeng Liu a, Junjie Li a, Sichao Zhang a, Bing-Rui Lu a, W. Lu b, Yifang Chen a,⇑ a b
State Key Lab of ASIC and System, School of Information Science and Engineering, Fudan University, Shanghai 200433, China Department of Electrical and Computer Engineering, Ohio State University, Columbus, OH 43210, USA
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 25 December 2014 Accepted 19 January 2015 Available online 28 January 2015 Keyword: Air-spaced field-plate gate Electron beam lithography Reactive ion etch SiNx
a b s t r a c t This paper reports our recent work on nanofabrication of air-spaced field-plate (ASFP) gates with ultrashort foot-print using one step electron beam lithography (EBL) combined with reactive ion etch (RIE) for which are suitable for GaN based high electron mobility transistors (HEMTs). Electron beam lithography on a tri-layer stack with PMMA350K/PMMA100K/Al/UVIII is carried out to form the required profile in resist. A low damage RIE dry-etch process is developed to create two different kinds of shapes in the 100 nm SiNx layer which serves as a footprint definition and mechanical support to the field-plate. The process enables us to fabricate air spaced field-plates with 60 nm foot-print and 50 nm air gap. Its advantages over traditional field-plates as well as T shape gate are discussed. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction High electron mobility transistors (HEMTs) are the key components in communication and imaging applications in both microwave and terahertz [1]. AlGaN/GaN heterojunctions based on broad band GaN have unique advantages of high carrier density, high mobility and deep confining well, being widely applied in HEMTs with high power. However, the uneven charge accumulated in the heterojunction causes a static field peak near the drain, leading to low breakdown voltage. To suppress the peak field, a broad field-plate gate is usually implemented directly on the SiNx passivation layer at the cost of scarifying high frequency performance because of the induced parasitic capacitance [2]. To solve this problem, we have developed an air-spaced field-plate gate (ASFP gate) with significantly reduced contact area, which appears like a broad head T shape gate as conceptually shown in Fig. 1c. In this configuration, the air space underneath the field-gate greatly reduces the parasitic capacitance [3]; meanwhile, the short space still enables the field-plate to suppress the peak field near the drain [4]. It is therefore expected that with such an ASFP gate, both high frequency and high power can be achieved in GaN based HEMTs. To the best of our knowledge, air-spaced field plates particularly on GaN-HEMTs have not been reported so far. It is realized that when the aspect ratio (headwidth/footprint) is increased, its mechanical reliability becomes unstable [5–8]. The main focus of this paper is to design the layer structure in resists based on the processing study of EBL with the mechanical reliabil⇑ Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.mee.2015.01.019 0167-9317/Ó 2015 Elsevier B.V. All rights reserved.
ity taking into consideration, and fabricate the ASFP gates using multilayer resists stacks and SiNx layer by combing EBL with RIE technique. The advantages of ASFP gates lie in the reduced parasitic capacitance, reduced gate resistance and increased breakdown voltage comparing with traditional field plates. It is believed that the developed fabrication technique is hopefully applicable for high frequency and high power GaN based HEMTs. 2. Experiments Fig. 1 shows the process flow for the fabrication of the ASFP gate. First, a 100 nm thick SiNx is deposited on the substrate using low pressure chemical vapor deposition (LPCVD). The SiNx layer is used to improve the definition of the foot and also acts as a mechanical supporting layer. Then, a 60 nm thick PMMA (350K) and 150 nm PMMA (100K) are spin coated onto the substrate, followed by an oven bake at 180 °C for one hour for each layer. After that, a 10 nm thick aluminum is deposited using a thermal heating evaporator. In the end, a 500 nm thick UVIII, which is chemically amplified resist (CAR) delivered by Shipley Ltd., is spin coated and subsequently baked on a hot plate at 130 °C for 1 min. As a positive tone CAR, UVIII is widely used for electron beam lithography due to its high sensitive capability and nanoscale capability [9]. The thin aluminum layer between the PMMA bilayer and UVIII layer facilitates the removal of the UVIII scum caused by heavy dose of e-beam exposure [10]. Electron beam exposure is carried out by a JEOL 6300FS at 100 keV with a typical beam current of 500 pA and an 8 nm beam-spot size. A dose between 40 and 50 lC/cm2 is used to pattern the head of the broad field-plate gate. There are at least two folds of advantages with the UVIII being used
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J. Shao et al. / Microelectronic Engineering 143 (2015) 11–14
for the plate. One is that the writing time is shortened by a factor of 6 comparing with when a copolymer layer is applied for the fieldplate. The other is the sensitivity difference between the UVIII and PMMA is large (13), causing negligible influence on the foot layer PMMA. Dose tests for the footprint are undertaken with a dose range from 900 lC/cm2 to 1500 lC/cm2 for the PMMA (350K) and from 400 lC/cm2 to 500 lC/cm2 for the PMMA (100K), respectively. As shown in Fig. 1a, after e-beam exposure, post exposure bake (PEB) is immediately done on a hot plate at 130 °C for 90 s, followed by a two-step developing process. Development of the exposed UVIII in Shipley CD 26 developer is first carried out for 60 s. In this step, the 10 nm Al layer is also removed by the alkali developer, creating a cleared area for the field-plate. In the second step, the bilayer of PMMA (350K)/PMMA (100K) is developed in O-xylene for 60 s, leaving a wide V-groove like profile in the foot part due to the difference of sensitivities between the two PMMA layers. Since O-xylene is not dissolved in water, after developing, it is cleared away by a stream of flowing de-ionized water. In both steps, a fixed developing temperature of 23 °C is used. Reactive ion etching is undertaken to transfer the foot profile from the bilayer of PMMA into the SiNx layer, as shown in Fig. 1b. Metallized gates are formed by thermal heating evaporation of chromium (15 nm)/gold (300 nm) and the lift-off process is done in warm acetone (Fig. 1c). The inspection of the resist profiles is carried out by a scanning electron microscope, (Zeiss Sigma-HD SEM). The lithography property of PMMA and UVIII, such as contrast and sensitivity is characterized by contrast curves using a Bruker stepper. The dissolution rates of PMMA and UVIII are also derived from the contrast curves. The RIE process is carried out by a Samco RIE-10NR etcher. The thermal heated evaporator, Kurt J. Lesker Nano 36 is used for the metallization process. 3. Results and discussion 3.1. Characterization of PMMA350K/PMMA100K/UVIII trilayer resists To evaluate the trilayer resist, both contrasts and sensitivities are measured by contrast curves for UVIII, PMMA (MW 350K) and PMMA (MW 100K), which are presented in Fig. 2 for comparison, where the lines are just a guide to the eye. Table 1 concludes the contrasts and sensitivities of UVIII, PMMA (MW 350K) and PMMA (MW 100K) derived from the contrast curves in Fig. 2. Each
Fig. 2. Contrast curves of UVIII, PMMA100K and PMMA350K, respectively, measured by a Bruker stepper. The exposed UVIII is developed in alkali CD26 developer. Both PMMA (100K) and PMMA (350K) are developed by O-xylene solution.
Table 1 Characteristics of UVIII, PMMA100K, and PMMA350K resist, which are derived by the contrast curves. Resist
Contrast
Sensitivity (lC/cm2)
Actual dose (lC/cm2)
CD26 PMMA100K PMMA350K
10.6 3.56 3.43
22.8 260 340
40–50 400–500 900–1500
contrast is calculated by the average slope of the curve between two heights at 25% and 75% of the resist thickness, respectively. The sensitivity, also known as the clearing dose, is defined by the intercept of the straight line passing through the two points (25% and 75%) with the dose-axis. In this work, the contrast curves, which carry the information of sensitivity and contrast of the resists, are used as a guide in designing the layer structure for achieving a large field-plate with a narrow foot. On one hand, the bilayer of PMMA, the top PMMA (100K) is slightly more sensitive than the bottom PMMA (350K), creating a wide V-groove like profile after lithography as demonstrated in Fig. 3. On the other hand, UVIII has much higher sensitivity than both PMMA350K and PMMA100K, it generates a broad open area for the field-plate as presented in Fig. 3 by a dose of 40–50 lC/ cm2. Such a small dose has negligible effect on the PMMA layer
Fig. 1. Schematic diagrams for the process flow and the SiNx/PMMA350K/PMMA100K/UVIII layer stack for air spaced field-plate (ASFP) gates on substrate. (a) T shape profile after electron beam exposure and a two-step development process; (b1) tapering shape profile in the resist/SiNx layer after RIE etch; (b2) curvy shape profile in the resist/ SiNx layer after RIE etch; (c1) tapering shape foot of ASFP gates after metallization and lift off process; (c2) curvy shape foot of ASFP gates after metallization and lift off process. The air-space underneath the field-plate should be kept very small for suppressing the peak-field existing in the conducting channel.
J. Shao et al. / Microelectronic Engineering 143 (2015) 11–14
Fig. 3. A SEM micrograph for the resist profile in PMMA (100K)/PMMA (350K)/UVIII trilayer stack on 100 nm SiNx/Si substrate. The 100 nm SiNx layer is marked by a white line.
13
not be sufficient. If it is too thick, it is not only difficult to achieve narrow trenches in SiNx but also it requires much thicker PMMA acting as an etch mask. An optimized thickness of 100 nm is worked out in this work. Fig. 3 shows the well-defined ASFP profile in the tri-layer resist stack after EBL. The footprint in PMMA bilayer is then transferred to the SiNx layer by dry etching in a plasma with CHF3/O2 mixture, using a parallel plate reactor (Samco 10-NR RIE System). In our initial experiments, only CHF3 is used, large roughness on etched PMMA is observed, as shown in Fig. 4. The addition of oxygen gas to the CHF3 plasma can not only reduce the surface roughness but also enhance the etch rate on SiNx, thus improve the selectivity of the SiNx/PMMA. Also, it is found in our work that the roughness on etched PMMA surface is clearly reduced. After a series of etch tests, the optimum etching condition is found to be: the etch forming gases are the mixture of O2 (12 sccm) and CHF3 (50 sccm), the RF power of 300 W and the pressure of 9 Pa. Using this recipe, the etch rate is carefully measured by a Bruker AFM to be 2 nm/s. It is important to point out that the final etched shape in SiNx can also be influenced by the replicated profile of PMMA. Fig. 5 shows two kinds of etched shapes in SiNx, one is curvy shape (Fig. 5a) and the other is tapering one (Fig. 5b), depending on the profile in the PMMA layer, either square trench or curvy one, which can be imagined into the SiNx layer. Both of the shapes are beneficial in reducing the footprint and achieving gates with different geometries. Especially they are able to enhance the mechanical strength to improve the reliability of HEMTs. For example, in our experiment, the footprint in PMMA with curvy shape is 200 nm, after the RIE, the foot size in SiNx is shrunk down to 60 nm. 3.3. Metallization of ASFP gates
Fig. 4. The roughness on etched PMMA resist surface after using PMMA as mask to etch SiNx.
underneath, which enables us to generate a broad field plate with a narrow footprint. For the same reason, the thickness of UVIII can be as much as 1 lm for the ease of lift-off process. 3.2. Dry etching of 100 nm SiNx for the footprint It is found that the thickness of the SiNx layer is critical. If it is too thin, its mechanical support to the metalized field-gate may
After the EBL with two-step development process and SiNx etch for the foot trench, a well-defined ASFP profile is achieved. The cross sectional images of the ASFP profile by SEM are shown in Fig. 4. Two kinds of foot trench shapes in SiNx layer, one is curvy shape and tapering shape, are succeeded. It is understood that the curvy shaped foot should provide enhanced mechanical support to the big field-plate, and the tapering shaped foot should offer ultra-short footprint. With these profiles as templates, 15 nm Cr and 300 nm Au are consecutively deposited by thermal evaporation and the unwanted metallic area are removed by a lift-off process in warm acetone to realize the metalized ASFP gates. Fig. 6 shows the fabricated air spaced field-plates with an ultra-small footprint as a T shape configuration. The success rate is as high as over 90%, indicating the mechanical strength capability of this process. The similar process can also be found in Ref. [7], in which a 30 nm thick SiNx layer is used to ankle the foot-stem for good mechanical support. Further work by applying the ASFP gates onto GaN based HEMTs are under way in our laboratory and results will be published elsewhere.
Fig. 5. SEM images for two kinds of ASFP profiles after the RIE process. (a) The curvy shape in the PMMA/SiNx layer; (b) the tapering shape.
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J. Shao et al. / Microelectronic Engineering 143 (2015) 11–14
Fig. 6. SEM images with cross-sectional views for completed ASFP gates. The footprint is completely encapsulated in SiNx. (a) Overview of the gate with a 54 nm footprint and a 50 nm air gap. (b) Close up view for the shape of the 54 nm footprint, indicating a very strong mechanical contact with the substrate. (c) The completed ASFP gate with 60 nm footprint in the tapering shape.
4. Conclusions
References
By summary, we have developed a novel process for the airspaced field plate with ultra-short foot print, which can be applied on GaN based HEMTs. Such a novel gate configuration is hopefully able to achieve high frequency with raised break voltage. 60 nm footprint of the ASFP gate has been achieved in this process. The bilayer of PMMA (350K)/PMMA (100K) with a small gradient in the sensitivity helps to form a crooked foot instead of a steep one, which is important for improving the mechanical stability. Thanks to the high sensitivity ratio of UVIII over PMMA, ultra short footprint is still feasible with a large area field-plate on the top.
[1] I. Kallfass, J. Antes, T. Schneider, F. Kurz, D. Lopez-Diaz, S. Diebold, H. Massler, A. Leuther, A. Tessmann, IEEE Trans. Terahertz Sci. Technol. 1 (2011) 477–487. [2] A. Chini, D. Buttari, R. Coffie, L. Shen, S. Heikman, A. Chakraborty, S. Keller, U. Mishra, IEEE Electron Dev. Lett. 25 (2004) 229–231. [3] T. Palacios, E. Snow, Y. Pei, A. Chakraborty, S. Keller, S. DenBaars, U. Mishra, in: IEEE International Electron Devices Meeting, IEDM Technical Digest, IEEE, 2005, pp. 3 and 789. [4] S. Vitanov, V. Palankovski, S. Maroldt, R. Quay, S. Murad, T. Rodle, S. Selberherr, IEEE Trans. Electron Dev. 59 (2012) 685–693. [5] S. Bentley, X. Li, D.A.J. Moran, I.G. Thayne, Microelectron. Eng. 85 (2008) 1375– 1378. [6] S. Bentley, X. Li, D.A.J. Moran, I.G. Thayne, Microelectron. Eng. 86 (2009) 1067– 1070. [7] Y. Chen, D. Edgar, X. Li, D. Macintyre, S. Thoms, J. Vac. Sci. Technol., B 18 (2000) 3521–3524. [8] S.-J. Yeon, M. Park, K.-S. Seo, IEEE (2007). [9] J. Shao, S. Zhang, J. Liu, Y. Ma, B.-R. Lu, Y. Sun, W. Lu, N. Taksatorn, Y. Chen, Microelectron. Eng. (2014) [10] Y. Chen, D. Macintyre, S. Thoms, Electron. Lett. 35 (1999) 338–339.
Acknowledgements This work is partially supported by the National Natural Science Foundation of China (No. 61205148). The authors are grateful to the Technical Service Team and the state-of-the-art cleanroom facility of the School of Microelectronics at Fudan University.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Nanofabrication of air-spaced field-plate gates with ultra-short footprint Jinhai Shao a, Jianpeng Liu a, Junjie Li a, Sichao Zhang a, Bing-Rui Lu a, W. Lu b, Yifang Chen a,⇑ a b
State Key Lab of ASIC and System, School of Information Science and Engineering, Fudan University, Shanghai 200433, China Department of Electrical and Computer Engineering, Ohio State University, Columbus, OH 43210, USA
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 25 December 2014 Accepted 19 January 2015 Available online 28 January 2015 Keyword: Air-spaced field-plate gate Electron beam lithography Reactive ion etch SiNx
a b s t r a c t This paper reports our recent work on nanofabrication of air-spaced field-plate (ASFP) gates with ultrashort foot-print using one step electron beam lithography (EBL) combined with reactive ion etch (RIE) for which are suitable for GaN based high electron mobility transistors (HEMTs). Electron beam lithography on a tri-layer stack with PMMA350K/PMMA100K/Al/UVIII is carried out to form the required profile in resist. A low damage RIE dry-etch process is developed to create two different kinds of shapes in the 100 nm SiNx layer which serves as a footprint definition and mechanical support to the field-plate. The process enables us to fabricate air spaced field-plates with 60 nm foot-print and 50 nm air gap. Its advantages over traditional field-plates as well as T shape gate are discussed. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction High electron mobility transistors (HEMTs) are the key components in communication and imaging applications in both microwave and terahertz [1]. AlGaN/GaN heterojunctions based on broad band GaN have unique advantages of high carrier density, high mobility and deep confining well, being widely applied in HEMTs with high power. However, the uneven charge accumulated in the heterojunction causes a static field peak near the drain, leading to low breakdown voltage. To suppress the peak field, a broad field-plate gate is usually implemented directly on the SiNx passivation layer at the cost of scarifying high frequency performance because of the induced parasitic capacitance [2]. To solve this problem, we have developed an air-spaced field-plate gate (ASFP gate) with significantly reduced contact area, which appears like a broad head T shape gate as conceptually shown in Fig. 1c. In this configuration, the air space underneath the field-gate greatly reduces the parasitic capacitance [3]; meanwhile, the short space still enables the field-plate to suppress the peak field near the drain [4]. It is therefore expected that with such an ASFP gate, both high frequency and high power can be achieved in GaN based HEMTs. To the best of our knowledge, air-spaced field plates particularly on GaN-HEMTs have not been reported so far. It is realized that when the aspect ratio (headwidth/footprint) is increased, its mechanical reliability becomes unstable [5–8]. The main focus of this paper is to design the layer structure in resists based on the processing study of EBL with the mechanical reliabil⇑ Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.mee.2015.01.019 0167-9317/Ó 2015 Elsevier B.V. All rights reserved.
ity taking into consideration, and fabricate the ASFP gates using multilayer resists stacks and SiNx layer by combing EBL with RIE technique. The advantages of ASFP gates lie in the reduced parasitic capacitance, reduced gate resistance and increased breakdown voltage comparing with traditional field plates. It is believed that the developed fabrication technique is hopefully applicable for high frequency and high power GaN based HEMTs. 2. Experiments Fig. 1 shows the process flow for the fabrication of the ASFP gate. First, a 100 nm thick SiNx is deposited on the substrate using low pressure chemical vapor deposition (LPCVD). The SiNx layer is used to improve the definition of the foot and also acts as a mechanical supporting layer. Then, a 60 nm thick PMMA (350K) and 150 nm PMMA (100K) are spin coated onto the substrate, followed by an oven bake at 180 °C for one hour for each layer. After that, a 10 nm thick aluminum is deposited using a thermal heating evaporator. In the end, a 500 nm thick UVIII, which is chemically amplified resist (CAR) delivered by Shipley Ltd., is spin coated and subsequently baked on a hot plate at 130 °C for 1 min. As a positive tone CAR, UVIII is widely used for electron beam lithography due to its high sensitive capability and nanoscale capability [9]. The thin aluminum layer between the PMMA bilayer and UVIII layer facilitates the removal of the UVIII scum caused by heavy dose of e-beam exposure [10]. Electron beam exposure is carried out by a JEOL 6300FS at 100 keV with a typical beam current of 500 pA and an 8 nm beam-spot size. A dose between 40 and 50 lC/cm2 is used to pattern the head of the broad field-plate gate. There are at least two folds of advantages with the UVIII being used
12
J. Shao et al. / Microelectronic Engineering 143 (2015) 11–14
for the plate. One is that the writing time is shortened by a factor of 6 comparing with when a copolymer layer is applied for the fieldplate. The other is the sensitivity difference between the UVIII and PMMA is large (13), causing negligible influence on the foot layer PMMA. Dose tests for the footprint are undertaken with a dose range from 900 lC/cm2 to 1500 lC/cm2 for the PMMA (350K) and from 400 lC/cm2 to 500 lC/cm2 for the PMMA (100K), respectively. As shown in Fig. 1a, after e-beam exposure, post exposure bake (PEB) is immediately done on a hot plate at 130 °C for 90 s, followed by a two-step developing process. Development of the exposed UVIII in Shipley CD 26 developer is first carried out for 60 s. In this step, the 10 nm Al layer is also removed by the alkali developer, creating a cleared area for the field-plate. In the second step, the bilayer of PMMA (350K)/PMMA (100K) is developed in O-xylene for 60 s, leaving a wide V-groove like profile in the foot part due to the difference of sensitivities between the two PMMA layers. Since O-xylene is not dissolved in water, after developing, it is cleared away by a stream of flowing de-ionized water. In both steps, a fixed developing temperature of 23 °C is used. Reactive ion etching is undertaken to transfer the foot profile from the bilayer of PMMA into the SiNx layer, as shown in Fig. 1b. Metallized gates are formed by thermal heating evaporation of chromium (15 nm)/gold (300 nm) and the lift-off process is done in warm acetone (Fig. 1c). The inspection of the resist profiles is carried out by a scanning electron microscope, (Zeiss Sigma-HD SEM). The lithography property of PMMA and UVIII, such as contrast and sensitivity is characterized by contrast curves using a Bruker stepper. The dissolution rates of PMMA and UVIII are also derived from the contrast curves. The RIE process is carried out by a Samco RIE-10NR etcher. The thermal heated evaporator, Kurt J. Lesker Nano 36 is used for the metallization process. 3. Results and discussion 3.1. Characterization of PMMA350K/PMMA100K/UVIII trilayer resists To evaluate the trilayer resist, both contrasts and sensitivities are measured by contrast curves for UVIII, PMMA (MW 350K) and PMMA (MW 100K), which are presented in Fig. 2 for comparison, where the lines are just a guide to the eye. Table 1 concludes the contrasts and sensitivities of UVIII, PMMA (MW 350K) and PMMA (MW 100K) derived from the contrast curves in Fig. 2. Each
Fig. 2. Contrast curves of UVIII, PMMA100K and PMMA350K, respectively, measured by a Bruker stepper. The exposed UVIII is developed in alkali CD26 developer. Both PMMA (100K) and PMMA (350K) are developed by O-xylene solution.
Table 1 Characteristics of UVIII, PMMA100K, and PMMA350K resist, which are derived by the contrast curves. Resist
Contrast
Sensitivity (lC/cm2)
Actual dose (lC/cm2)
CD26 PMMA100K PMMA350K
10.6 3.56 3.43
22.8 260 340
40–50 400–500 900–1500
contrast is calculated by the average slope of the curve between two heights at 25% and 75% of the resist thickness, respectively. The sensitivity, also known as the clearing dose, is defined by the intercept of the straight line passing through the two points (25% and 75%) with the dose-axis. In this work, the contrast curves, which carry the information of sensitivity and contrast of the resists, are used as a guide in designing the layer structure for achieving a large field-plate with a narrow foot. On one hand, the bilayer of PMMA, the top PMMA (100K) is slightly more sensitive than the bottom PMMA (350K), creating a wide V-groove like profile after lithography as demonstrated in Fig. 3. On the other hand, UVIII has much higher sensitivity than both PMMA350K and PMMA100K, it generates a broad open area for the field-plate as presented in Fig. 3 by a dose of 40–50 lC/ cm2. Such a small dose has negligible effect on the PMMA layer
Fig. 1. Schematic diagrams for the process flow and the SiNx/PMMA350K/PMMA100K/UVIII layer stack for air spaced field-plate (ASFP) gates on substrate. (a) T shape profile after electron beam exposure and a two-step development process; (b1) tapering shape profile in the resist/SiNx layer after RIE etch; (b2) curvy shape profile in the resist/ SiNx layer after RIE etch; (c1) tapering shape foot of ASFP gates after metallization and lift off process; (c2) curvy shape foot of ASFP gates after metallization and lift off process. The air-space underneath the field-plate should be kept very small for suppressing the peak-field existing in the conducting channel.
J. Shao et al. / Microelectronic Engineering 143 (2015) 11–14
Fig. 3. A SEM micrograph for the resist profile in PMMA (100K)/PMMA (350K)/UVIII trilayer stack on 100 nm SiNx/Si substrate. The 100 nm SiNx layer is marked by a white line.
13
not be sufficient. If it is too thick, it is not only difficult to achieve narrow trenches in SiNx but also it requires much thicker PMMA acting as an etch mask. An optimized thickness of 100 nm is worked out in this work. Fig. 3 shows the well-defined ASFP profile in the tri-layer resist stack after EBL. The footprint in PMMA bilayer is then transferred to the SiNx layer by dry etching in a plasma with CHF3/O2 mixture, using a parallel plate reactor (Samco 10-NR RIE System). In our initial experiments, only CHF3 is used, large roughness on etched PMMA is observed, as shown in Fig. 4. The addition of oxygen gas to the CHF3 plasma can not only reduce the surface roughness but also enhance the etch rate on SiNx, thus improve the selectivity of the SiNx/PMMA. Also, it is found in our work that the roughness on etched PMMA surface is clearly reduced. After a series of etch tests, the optimum etching condition is found to be: the etch forming gases are the mixture of O2 (12 sccm) and CHF3 (50 sccm), the RF power of 300 W and the pressure of 9 Pa. Using this recipe, the etch rate is carefully measured by a Bruker AFM to be 2 nm/s. It is important to point out that the final etched shape in SiNx can also be influenced by the replicated profile of PMMA. Fig. 5 shows two kinds of etched shapes in SiNx, one is curvy shape (Fig. 5a) and the other is tapering one (Fig. 5b), depending on the profile in the PMMA layer, either square trench or curvy one, which can be imagined into the SiNx layer. Both of the shapes are beneficial in reducing the footprint and achieving gates with different geometries. Especially they are able to enhance the mechanical strength to improve the reliability of HEMTs. For example, in our experiment, the footprint in PMMA with curvy shape is 200 nm, after the RIE, the foot size in SiNx is shrunk down to 60 nm. 3.3. Metallization of ASFP gates
Fig. 4. The roughness on etched PMMA resist surface after using PMMA as mask to etch SiNx.
underneath, which enables us to generate a broad field plate with a narrow footprint. For the same reason, the thickness of UVIII can be as much as 1 lm for the ease of lift-off process. 3.2. Dry etching of 100 nm SiNx for the footprint It is found that the thickness of the SiNx layer is critical. If it is too thin, its mechanical support to the metalized field-gate may
After the EBL with two-step development process and SiNx etch for the foot trench, a well-defined ASFP profile is achieved. The cross sectional images of the ASFP profile by SEM are shown in Fig. 4. Two kinds of foot trench shapes in SiNx layer, one is curvy shape and tapering shape, are succeeded. It is understood that the curvy shaped foot should provide enhanced mechanical support to the big field-plate, and the tapering shaped foot should offer ultra-short footprint. With these profiles as templates, 15 nm Cr and 300 nm Au are consecutively deposited by thermal evaporation and the unwanted metallic area are removed by a lift-off process in warm acetone to realize the metalized ASFP gates. Fig. 6 shows the fabricated air spaced field-plates with an ultra-small footprint as a T shape configuration. The success rate is as high as over 90%, indicating the mechanical strength capability of this process. The similar process can also be found in Ref. [7], in which a 30 nm thick SiNx layer is used to ankle the foot-stem for good mechanical support. Further work by applying the ASFP gates onto GaN based HEMTs are under way in our laboratory and results will be published elsewhere.
Fig. 5. SEM images for two kinds of ASFP profiles after the RIE process. (a) The curvy shape in the PMMA/SiNx layer; (b) the tapering shape.
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J. Shao et al. / Microelectronic Engineering 143 (2015) 11–14
Fig. 6. SEM images with cross-sectional views for completed ASFP gates. The footprint is completely encapsulated in SiNx. (a) Overview of the gate with a 54 nm footprint and a 50 nm air gap. (b) Close up view for the shape of the 54 nm footprint, indicating a very strong mechanical contact with the substrate. (c) The completed ASFP gate with 60 nm footprint in the tapering shape.
4. Conclusions
References
By summary, we have developed a novel process for the airspaced field plate with ultra-short foot print, which can be applied on GaN based HEMTs. Such a novel gate configuration is hopefully able to achieve high frequency with raised break voltage. 60 nm footprint of the ASFP gate has been achieved in this process. The bilayer of PMMA (350K)/PMMA (100K) with a small gradient in the sensitivity helps to form a crooked foot instead of a steep one, which is important for improving the mechanical stability. Thanks to the high sensitivity ratio of UVIII over PMMA, ultra short footprint is still feasible with a large area field-plate on the top.
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Acknowledgements This work is partially supported by the National Natural Science Foundation of China (No. 61205148). The authors are grateful to the Technical Service Team and the state-of-the-art cleanroom facility of the School of Microelectronics at Fudan University.