Sub-30 nm InAs Quantum-Well MOSFETs with Self-Aligned Metal Contacts and Sub-1 nm EOT HfO2 Insulator

Sub-30 nm III-V planar Quantum-Well (QW) n-type MOSFETs are fabricated through a self-aligned CMOS compatible front-end process. Good performance and short-channel effect mitigation are obtained through the use of a QW-channel that incorporates a thin pure InAs subchannel and extremely scaled HfO2 gate dielectric on a very thin InP barrier (total barrier EOT<1 nm). The devices also feature self-aligned metal contacts that are 20-30 nm away from the edge of the gate. At Lg=30 nm, transconductance of 1420 μS/μm and subthreshold swing of 114 mV/dec at 0.5 V are obtained. 22-nm gate-length devices are demonstrated through this process. Long-channel devices exhibit nearly ideal subthreshold swing of 69 mV/dec, and high channel mobility of 4650 cmVs at Ns=4x10 cm. Introduction InAs and InGaAs are considered promising candidates for channel material in future CMOS applications [1,2]. Impressive device prototypes have recently been demonstrated [3-8]. However, a III-V MOSFET structure that combines high performance, ability to harmoniously scale down to sub-30 nm gate length dimensions and CMOS-type manufacturability is still to be realized. In this work, we demonstrate a novel QW-MOSFET that addresses these challenges. For this, we use an extremely-scaled HfO2 gate insulator. The fabrication closely follows CMOS requirements, particularly self-alignment of the refractory metal gate and metal contacts, very low thermal budget, gate-last process that uses RIE extensively and an entirely lift-off free process in the frontend. MOSFETs with gate length dimensions in the 20-30 nm range and outstanding electrical characteristics are obtained. Device Fabrication The process flow is shown in Fig. 1, a device cross-section schematic is illustrated in Fig. 2 and TEM pictures of finished devices are shown in Fig. 3. The starting heterostructure is grown by MBE and it features a 2 nm InAs sub-channel clad by 3 and 5 nm of In0.7Ga0.3As and a novel n-InP/n-InAlAs/i-InP ledge design to reduce access resistance. Low-ρ Mo is first sputtered as metal contact (Rsh=5 Ω/□) followed by CVD SiO2 deposition. After mesa isolation, the gate pattern is defined by E-beam lithography. The SiO2 and Mo layers are then patterned by RIE. A thermal annealing step at 350C in N2 is performed to remove RIE damage [9]. The top n InGaAs cap is wet-etched in a well-controlled manner that yields an undercut < 20 nm, as shown in Fig. 3 (a). The top n InP spacer is etched through highly-directional Ar etch, stopping at n InAlAs. Subsequently, the Mo at the S/D regions is pulled back by isotropic RIE [10]. We use a self-limiting digital etch technique (plasma oxidation + diluted H2SO4) to remove the n-InAlAs and i-InP in a well-controlled manner. One cycle of this etch removes about 0.9 nm of InAlAs or InP. Through careful calibration, we can obtain about 1 nm InP barrier remaining above the channel. Immediately after the last digital etch cycle, the sample is loaded into ALD for gate dielectric deposition. 2 nm HfO2 is used as gate dielectric. From the dielectric constant extracted by a separate calibration experiment, the EOT for the HfO2 alone is ~ 0.4 to 0.5 nm. The total EOT including the InP barrier is ~0.8 nm. Evaporated Mo is used as gate metal and patterned by RIE. The device is finished by pad formation. This is the only lift-off step in the backend of the process. The final gate length is defined by the recess opening in the SiO2. This can reach below 20 nm. The ALD gate dielectric is conformal and passivates the access region between the edges of the gate and the n InGaAs cap. This distance is Lside~ 20-30 nm. HR-TEM shows the high quality device interface in Fig. 3 (c). The highest temperature step in the entire process is the 350C damage annealing. There is no high temperature step after the ALD film is deposited. Results and discussion The output characteristic of a device with Lg= 30 nm are shown in Fig. 4. Its Ron is 475 Ω.μm. Rsd~455 Ω.μm is obtained by Lg-extrapolation method. In our self-alignment design, ohmic metal Mo with Rsh=5 Ω/□ is placed in very close proximity to the gate edge. This represents an order of magnitude reduction in the access region Rsh with respect to n-InP or n-InGaAs, as used in other schemes where the metal contacts are still far apart [4,5]. Previous work also shows that Mo offers a very low contact resistance to n InGaAs [11]. Our relatively high Rsd mainly arises from the un-capped Lside regions. This can be solved through an improved n-InP S/D ledge design. 2 The subthreshold characteristics of this device are shown in Fig. 5. Steep subthreshold swings (103 mV/dec at 50 mV, 114 mV/dec at 0.5 V) over a wide range of Vgs values are obtained. DIBL is 236 mV/V. We believe this relatively high value is associated with the heterostructure buffer and is not related to the fabrication process. Despite using a pure HfO2 gate dielectric of 2 nm, the maximum Ig over the voltage range of Fig. 5 is below 1x10 A/μm. The transfer and gm characteristics of the same Lg= 30 nm device are shown in Fig. 6. A peak gm of 1420 μS/μm is obtained at Vds=0.5 V. Very small hysteresis (< 10 mV) in Id is shown. A maximum gm,pk=1530 μS/μm is obtained at Lg=60 nm. A fully operational Lg= 22 nm MOSFET has been obtained (XTEM in Fig. 3b). The output and transfer characteristics are shown in Figs. 7 and 8. At Vds = 0.5 V, the device delivers a transconductance of 1050 μS/μm. Fig. 9 shows device subthreshold characteristics from Lg=150 nm to 22 nm at 0.5 V. Fig. 10 shows S at 0.5 V vs. physical gate length Lg for our transistors, recent III-V MOSFETs, and InGaAs HEMTs [2-8] down to 30 nm. Our ultra-scaled barrier MOSFETs exhibit a subthreshold swing that is superior to any other planar III-V MOSFET and that matches the best Tri-gate III-V devices [3]. In addition, our devices feature outstanding transport properties. As shown in Fig. 11 among transistors with Lg≤60 nm, for a given S, the transconductance is superior to any other planar or Tri-gate III-V MOSFET. Fig. 12 shows Ion for an Ioff=100 nA/μm at Vdd=0.5 V. This FOM combines short-channel effects and drive current. For Lg>70 nm, our devices are among the best reported to date. At Lg=60 nm, our device is superior to a Tri-gate III-V MOSFET [3]. A further manifestation of the scalability of this technology is shown in Fig. 13 that graphs saturation Vt roll-off. Good scalability is demonstrated down to 30 nm with 2 nm HfO2 gate dielectric. For comparison, a device run made with a 2 nm thick Al2O3 gate dielectric shows relatively poor Vt roll-off as a result of poor electrostatic control. S at 0.5 V vs. Id for Lg= 150 nm devices is plotted for three different gate dielectrics in Fig. 14. For the sample with pure Al2O3, S is high as a consequence of its higher EOT. With pure HfO2, S stays almost constant across 3 orders of magnitude in Id (10 to 10 A/μm). With Al2O3 at the interface and HfO2 on top, the minimum value of S is comparable to that with pure HfO2, but as Id decreases it rises at higher values of Id just as with pure Al2O3. In both samples with Al2O3 at the InP interface, the rise of S happens while Ig«Id. This suggests that the shape of S is due to the different distribution of Dit across the InP bandgap. With HfO2, a lower and more uniform Dit distribution is present close to midgap. Closer to the conduction band edge, Al2O3 has a slightly lower Dit. These results are consistent with recent findings [12]. As part of our process development, we fabricated devices on a different heterostructure with a 15 nm thick In0.53Ga0.47As channel and an Al2O3/HfO2 (0.4/2 nm) composite gate dielectric. The InP barrier is scaled down by a time-controlled low-energy Ar dry etch [10]. The final InP thickness is 1 nm. Long-channel MOSFETs (Lg=300 μm) with nearly ideal S= 69 mV/dec at Vds=50 mV were obtained (Fig. 15). This result is among the best ever reported for III-V MOSFETs. This is seen in Fig. 16 that graphs S vs. dielectric EOT for several long-channel III-V FETs at low Vdd. This graph illustrates the importance of Dit and dielectric scaling. Our gate-last process, in spite of its multiple RIE steps delivers good interfacial quality with a very thin dielectric. Fig. 17 shows split C-V measurements of long-channel MOSFETs with HfO2 and Al2O3 gate dielectrics. A noticeable Dit bump is observed in the case of Al2O3 in weak inversion at low frequencies, indicating a significant amount of slow traps close to mid-gap. This is absent when using a pure HfO2 dielectric. Fig. 18 shows electron mobilities extracted from 20-μm long MOSFETs. Mobility of 4650 cmVs at Ns of 4x10 cm is obtained. This is the one of the highest mobility values at this Ns published in InGaAs MOSFETs to date [4, 13]. Gate current densities (Jg) for devices with different dielectrics on the two heterostructures are shown in Fig. 19. Due to a larger CB discontinuity, Al2O3 results in reduced Jg for the same physical thickness. A 2 nm HfO2 gate dielectric delivers Jg <1 A/cm across the entire 0.5-V forward Vg span. Conclusion In this work, we have demonstrated the shortest functional III-V MOSFETs to date. Our InAs QW-MOSFETs feature an ultra-scaled pure HfO2 gate dielectric and are fabricated through a self-aligned CMOS compatible process with Lside=20-30 nm. Fully operational devices as short as Lg=22 nm have been achieved. Outstanding performance and short-channel effects in devices down to Lg=30 nm has been demonstrated. A long-channel subthreshold swing of 69 mV/dec and an electron channel mobility of 4650 cmVs at high Ns=4x10 cm have also been demonstrated. Acknowledgements: This work is sponsored by FCRP-MSD Center and Intel Corp. Device fabrication was carried out at the Microsystems Technology Laboratories of MIT. MBE growth is provided by Intelliepi.