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Article

Area-Scalable 109-Cycle-High-Endurance FeFET of Strontium Bismuth Tantalate Using a Dummy-Gate Process

by
Mitsue Takahashi
* and
Shigeki Sakai
National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
*
Author to whom correspondence should be addressed.
Nanomaterials 2021, 11(1), 101; https://0-doi-org.brum.beds.ac.uk/10.3390/nano11010101
Submission received: 14 December 2020 / Revised: 28 December 2020 / Accepted: 29 December 2020 / Published: 4 January 2021
(This article belongs to the Special Issue Electronic Nanodevices)
Browse Figures
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Abstract

:
Strontium bismuth tantalate (SBT) ferroelectric-gate field-effect transistors (FeFETs) with channel lengths of 85 nm were fabricated by a replacement-gate process. They had metal/ferroelectric/insulator/semiconductor stacked-gate structures of Ir/SBT/HfO2/Si. In the fabrication process, we prepared dummy-gate transistor patterns and then replaced the dummy substances with an SBT precursor. After forming Ir gate electrodes on the SBT, the whole gate stacks were annealed for SBT crystallization. Nonvolatility was confirmed by long stable data retention measured for 105 s. High erase-and-program endurance of the FeFETs was demonstrated for up to 109 cycles. By the new process proposed in this work, SBT-FeFETs acquire good channel-area scalability in geometry along with lithography ability.

1. Introduction

Ferroelectric-gate field-effect transistors (FeFETs) comprising SrBi2Ta2O9 (SBT) or CaxSr1-xBi2Ta2O9 (CSBT) ferroelectrics have unique characteristics of high endurance against at least 108 cycles of program and erase operations [1,2,3,4,5,6,7,8,9,10,11,12]. CSBT is a kind of SBT family which was derived from original SBT by Sr-site substitution with Ca. The material natures of SBT [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32] and CSBT [33,34,35,36] have been intensively studied previously. FeFETs using CSBT with about x = 0.2 showed larger memory windows than those with SBT [5]. The invention of long-retention FeFET was first reported in 2002 and consisted of a metal/ferroelectric/insulator/semiconductor (MFIS) stacked-gate structure of Pt/SBT/(HfO2)0.75(Al2O3)0.25(HAO)/Si [37]. Since then, we have investigated characteristics of (C)SBT-FeFETs [1,2,3,38,39,40,41,42,43,44], improved the device performance [4,5,6,7,8,45,46], and developed FeFET-integrated circuits [9,10,11,12,47,48,49,50,51,52]. For improving the single FeFET performance, we succeeded in reducing gate voltage (Vg) from the initial 6~8 [1] to 3.3 V [8]. Another progress was in shrinking gate-metal length (Lm) from the initial 10 μm [1] to 100 nm [7].
The conventional (C)SBT-FeFETs were formed by etching the gate stacks. By decreasing the FeFET gate length, SBT etching-damage problems [29,30,31,32] on the gate-stack sidewalls became significant. Since we recognized that Lm = 100 nm was approaching the shortest limit by the conventional method based on etching, we changed the fabrication strategy to shape the gate stacks from etching-down to filling-up. The new (C)SBT-FeFET process is outlined as follows: Dummy-gate transistor patterns with self-aligned source- and drain regions are prepared in advance. The dummy substance is selectively removed to leave grooves which are later filled up with SBT precursor. Gate electrodes are formed. Finally, whole gate stacks of Ir/SBT/HfO2/Si are annealed for SBT crystallization. In the new FeFET process, the (C)SBT sidewall of the gate stack is not exposed to etching plasma. The sidewall is thus free from etching damage problem [6]. Consequently, the ferroelectric becomes more controllable in terms of quality and more scalable in terms of geometry than by the etching. The new FeFET dimensions follow good lithography progress with an adequate height of (C)SBT to show large memory windows increasing with the ferroelectric thickness [3,43]. In this work, SBT-FeFETs with gate channel lengths Lch = 85 nm were first reported by adopting the proposed process. Excellent characteristics were demonstrated such as 109 cycle erase-program endurance and long stable retention for 105 s. The endurance and retention were as good as those of the conventional (C)SBT-FeFETs formed by the gate-stack etching [1,2,3,4,5,6,7,8,9,10,11,12].

2. Materials and Methods

2.1. Device Fabrication Process

The fabrication process (schematic drawings shown in Figure 1) in this work is as follows:
  • Step 1: Si substrate preparation.
    A p-type Si substrate patterned with FET active areas was prepared. Local-oxidation-of-silicon (LOCOS) process was used in the patterning for device isolation. The LOCOS patterns with various channel widths (W) were designed in a sample chip. Areas for source-, drain- and substrate-contact holes on the Si were heavily ion-doped. Sacrificial SiO2 on Si was removed with buffered hydrogen fluoride.
  • Step 2: Insulator deposition.
    A 5 nm thick HfO2 was deposited on the Si substrate by a large-area pulsed-laser deposition system (Vacuum Products Corporation, Kodaira, Tokyo, Japan) [53]. A KrF laser was irradiated on a ceramic HfO2 target in 15.3 Pa N2 ambient [54]. The substrate temperature was 220 °C.
  • Step 3: Lithography.
    Electron-beam (EB) lithography was performed by spin-coating an organic resist, exposing 130 kV EB, and developing. Resist patterns 550 nm tall were left on the HfO2/Si. They were later used as ion-implantation mask in Step 4 and as dummy gates in Step 7.
  • Step 4: Ion implantation.
    HfO2 uncovered with resist was etched out by inductively-coupled-plasma reactive-ion etching (ICP-RIE). On the exposed Si, As+ ions were implanted for source and drain. The energy and dose conditions were 4 keV and 5.0 × 1012/cm2.
  • Step 5: SiO2 deposition.
    An 830 nm thick SiO2 was deposited to cover the resist patterns on the substrate by 300 W rf sputtering in 0.1 Pa Ar.
  • Step 6: Flattening SiO2.
    The SiO2 was etched back and flattened by ICP-RIE with 1.0 Pa Ar-CF4 mixed gas until tops of the resists or dummy gates were exposed.
  • Step 7: Leaving grooves on gates.
    The dummy-gate substances were selectively removed by O2 plasma ashing. There remained grooves in a 410 nm tall SiO2 isolation. The grooves were located on the HfO2 with self-aligned source and drain regions prepared in Step 4. The whole chip was rapidly annealed at 800 °C in ambient N2.
  • Step 8: Ferroelectric deposition.
    SBT precursor film was deposited to fill up the grooves by a metal-organic-chemical-vapor deposition (MOCVD) system (WACOM R&D, Nihonbashi, Tokyo, Japan). Sources of Bi(C5H11O2)3, Sr[Ta(OC2H5)5(OC2H4OCH3)]2 and Ta(OCH2CH3)5 (Tri Chemical Laboratories Inc., Uenohara, Yamanashi, Japan) were used [6]. As-deposited precursor-film thickness was estimated as 80 nm on a flat place of the substrate.
  • Step 9: Metal deposition.
    Ir was deposited by rf sputtering on the SBT precursor layer. Resist mask was patterned for gate electrodes by EB lithography.
  • Step 10: Forming gate electrodes.
    Ir uncovered with resist was etched out by Ar+ ion milling. Then, the resist mask was removed by O2 plasma ashing.
  • Step 11: FeFET completed.
    SBT precursor was deposited again by MOCVD to cover the substrate [6]. The whole substrate was annealed for crystallization of the SBT to show ferroelectricity. The annealing condition was at 780 °C in an O2-N2 mixed gas we investigated before [8]. Finally, contact holes for gate, source, drain and substrate were formed by ultraviolet g-line lithography and Ar+ ion milling.

2.2. Reason for Using SBT in FeFET

The gate stack of MFIS should be regarded as MFI(IL)S, as shown in in Figure 2a, where F, I, IL, S are connected in series. The IL is an interfacial layer between I and S which is formed during the ferroelectric crystallization annealing process of FeFETs [8,39,55,56,57]. The main component of IL is silicon dioxide with an electric permittivity (εIL) of εIL = 3.9. In the MFI(IL)S, |PF| ≈ ε0·εI·|EI| = ε0·εIL·|EIL| = |QS| is satisfied in any time. The PF is ferroelectric polarization. EI and EIL are electric fields in the I and the IL. The QS is charge area density in the semiconductor surface. The εI is a relative permittivity of the I. The ε0 is the vacuum dielectric constant of ε0 = 8.85 × 10−12 F/m. For a simplified explanation, we assumed a virtual equivalent circuit of series capacitance as drawn in Figure 2a which is expressed by |PF| ≈ |QI| = |QIL| = |QS| with virtual charges QI and QIL on I and IL, respectively. In MFI(IL)S, the IL suffers from a stress of field |EIL| ≈ |PF|/(ε0·εIL) = 8.7 MV/cm even at a small |PF| = 3 μC/cm2. For example, real IL thickness is 2.6 nm [8] or about 1 nm [55,56,57]. Electric-field-assisted tunnel current through such a thin SiO2 [58,59] brings charge injection into the gate stack from S across IL. In erase-and-program operations, a large EIL derived from a large PF swing induces significant trapped-charge accumulation which accelerates endurance degradations [2,52]. According to our experience [43,52,60], |PF| should normally be less than 2.5 μC/cm2 all the time and should not exceed 2.0 μC/cm2 for further high-endurance requirements of the FeFET.
Ferroelectric materials show PF versus EF hysteresis loops as illustrated in Figure 2b. The EF is the electric field across the F. We defined Emax as the positive maximum EF and Pmax as the PF at EF = Emax. Similarly, Emin and Pmin are the negative minimum EF and the PF at EF = Emin. The loop is called “major” loop when the Emax and |Emin| are strong enough to force PF saturated, whereas it is called “minor” loop when PF is unsaturated by moderate EF swing. In SBT-FeFETs, restrictions of Pmax ≤ 2.5 μC/cm2 corresponding to the minor loops are used during all operations as we emphasized in early works [39,43,52,60].
Regarding a ferroelectric hidden in MFI(IL)S, an exact symmetric swing maximum, i.e., Pmax = |Pmin| or Emax = |Emin|, is difficult because |QS| versus ΦS is very asymmetric [61,62]. The QS is the charge area density of the semiconductor surface and ΦS is the surface potential. Presence of the flat-band voltage Vfb makes the symmetric swing further difficult. However, to simplify the physical explanation, Pmax = |Pmin| and Emax = |Emin| are assumed as shown in Figure 2b with Vfb = 0V. In every PF-EF loop, the EF width at PF = 0 is defined as Ew being related with a voltage memory window (Vw) by an approximate expression Ew = 2Ec = Vw/dF, where the Ec is a coercive field and dF is ferroelectric thickness. According to a method we proposed before [43], an important characteristic Emax of the ferroelectric can be evaluated which has not been measurable by direct probing on a FeFET. If Pmax is provided, a gate voltage Vg to achieve a target memory window Vw = Ew·dF can be estimated as a sum of Emax·dF, EI·dI, EIL·dIL and ΦS at QS = Pmax. An exact discussion can be found in the paper [43].
For instance, Pt/SBT/HAO/Si FeFETs showed Ew = 18 kV/cm at Pmax = 2.0 μC/cm2 and Emax = 25 kV/cm [43]. By adopting an advanced process [8], Ir/CSBT/HfO2/Si FeFETs had the best improved values of Ew = 65 kV/cm at Pmax = 2.0 μC/cm2 and Emax = 140 kV/cm [3,43]. A good reason for using (C)SBT in Si-based FeFETs is the (C)SBT ferroelectric nature of a convenient minor PF-EF loop [14,17,20] which has Ew available and is controllable in a restricted PF range of Pmax ≤ 2 μC/cm2 with Emax ≤ 140 kV/cm.
There are some other ferroelectric materials also intensively studied for applications in Si-based MFIS FeFETs. Regarding Pb5Ge3O11 (PGO), attempts to develop replacement-gate-type Pt/PGO/ZrO2/Si FeFETs were reported [63] but the erase-program-test results of the FeFETs were not found although the ferroelectric itself showed a good potential Pmax-Emax and Ew − Emax judging from hysteresis loops of the PGO metal/ferroelectric/metal capacitors [64]. Regarding another candidate, the ferroelectric HfO2 family [55,56,57,65,66,67,68,69,70], the intrinsic material nature may not be suitable for applying to Si-based FeFETs. Informative minor hysteresis loops were reported on Y-doped HfO2 in which Ew seemed nearly equal to 0 V/cm at Pmax = 2.0 μC/cm2, although it was as large as about 1 MV/cm at Pmax = 10 μC/cm2 [66]. Operation of the FeFETs under the restriction of Pmax ≤ 2 μC/cm2 may be difficult. Some reports suggested that HfO2-FeFETs cannot help using a large Pmax (>>2 μC/cm2) [52,55]. The large Pmax may induce significant charge injection into the gate stack. As far as we know, fair works on HfO2-FeFETs have not cleared 108 cycles endurance in spite of using sophisticated production facilities [56,67,68,69,70].

3. Results and Discussion

3.1. Device Dimensions

A cross-sectional scanning-electron-microscope photograph of an Ir/SBT/HfO2/Si FeFET fabricated by the new proposed process is shown in Figure 3a. Figure 3b shows the same picture added with support lines to clarify the material boundaries. The schematic drawing of the FeFET was assigned with four terminals of gate, drain, source and substrate (Figure 3c). The gate-channel length (Lch) was Lch = 85 nm. The gate-channel width was W = 100 μm depending on the initial LOCOS pattern designed in Step 1 in Section 2.1. The metal-gate length Lm was 150 nm which could be shorter but was not the focus in this work. The SBT precursor film thickness was about 80 nm measured on a flat place. By filling gate grooves with SBT precursor (Step 8 in Section 2.1.), the effective SBT height (H) was finally about 450 nm which was a distance between Ir and HfO2. Area scalability of the new FeFET was equivalent to that of the dummy gates which are organic resist patterns made by lithography. From the viewpoint of Si transistor technology, Lch = 10 nm is expected to be the critical limit [71]. A significant Curie-temperature decrease in SBT started when particle were sizes of around 20 nm [25]. Thus, the prospective shortest limit of Lch by our proposed FeFET process may be around 20 nm.

3.2. Electrical Characterizations

In this study, memory windows, endurance and retention of FeFETs were investigated at room temperature. A semiconductor parameter analyzer (4156C, Keysight Technologies, Santa Rosa, CA, USA) was used for measuring static drain current versus gate voltage (Id–Vg) curves of the FeFETs. A pulse generator (81110A, Keysight Technologies, Santa Rosa, CA, USA) was used to apply Vg pulses. The instruments were computer-controlled using programs written by the language of LabVIEW (ver. 10, National Instruments, Austin, TX, USA).

3.2.1. Memory Windows

As an elementary test of the FeFETs, Id–Vg hysteresis loops were investigated (Figure 4). The Id was measured by Vg increments and decrements with 0.1 V steps. The Vg sweeping ranges were Vg = 1 ± 4 V, 1 ± 5 V and 1 ± 6 V. Drain voltage (Vd), source voltage (Vs) and substrate voltage (Vsub) were fixed to Vd = 0.1 V and Vs = Vsub = 0 V during the measurements. The Id–Vg showed hysteresis loops drawn in counter-clockwise directions because the FeFET was an n-channel-type one. In an Id–Vg curve, threshold voltage (Vth) was defined as a Vg value at Id/W = 1 × 10−7 A/cm. Two Vth values were extracted from the left- and right-side curves in an Id–Vg hysteresis loop. A memory window was defined as the Vth difference. In this work, we call this a static memory window (Vw) because Vg sweep by 4156C is slow. The static Vw was, for instance, 1.0 V by sweeping Vg from −5 to 7 V then back to −5 V, or at Vg = 1 ± 6 V as expressed in Figure 4. During the measurement of a wide-range Id from 10−12 to 10−4 A as indicated in Figure 4, Vg sweep speed depends on the current range. Therefore, an Id–Vg hysteresis curve only gives reference information that is not suitable for accurate discussion.
For an accurate understanding, the FeFET performance, a pulsed Vg with a controlled time width, was applied to the FeFETs for the erase (Ers) or program (Prg) operation. The Vg pulse heights with the time widths were (VE, tE) for Ers, and (VP, tP) for Prg, respectively. For the n-channel-type FeFET, the VE was negative (VE < 0 V) and VP was positive (VP > 0 V) [9]. The pulse time widths tE and tP were the same with each other in this work (tE = tP = tEP). After, Ers and Prg, Id–Vg curves were individually measured with a small common Vg range for Read. Two Vth values were defined in the Id–Vg curves as the Vg at Id/W = 1 × 10−7 A/cm. They were expressed as VthE after Ers and VthP after Prg. The VthE was larger than the VthP [9]. The Vth difference of ΔVth = VthE − VthP was defined as a memory window obtained by read operation after erase-and-program pulse applications. The memory window ΔVth is normally smaller than the above-mentioned static Vw, because slow switching components in a ferroelectric do not respond to short pulses [27,72,73]. The VthE and VthP were investigated by repeating a series of operations: Ers, Read, Prg, Read, in this order (Figure 5a). In Ers, a pulsed Vg of (VE, tEP) was applied with keeping Vd = Vs = Vsub = 0 V. In Read after Ers, a VthE was extracted from an Id-Vg curve drawn by narrow-range varying Vg from 0 to 1.1 V at Vd = 0.1 V and Vs = Vsub = 0 V. In Prg, a pulsed Vg of (VP, tEP) was applied, keeping Vd = Vs = Vsub = 0 V. In Read after Prg, a VthP was extracted from an Id–Vg curve drawn under exactly the same conditions as those in Read after Ers.
Figure 5b shows VthE and VthP by Read after Ers and Prg for three sets of (VE, tEP) and (VP, tEP) of |VE| = VP = 6, 7 and 8 V. Every marker corresponds to the measured VthE and VthP. Memory windows, ΔVth = VthE-VthP, as a function of pulse height |VE| = |VP| (Figure 5c) and width tEP (Figure 5d) can be seen in Figure 5b, where the VthE and VthP results (not shown in Figure 5b) of other VP (=|VE|) conditions were also used. Short Vg pulses of tEP = 50 ns were available for Ers and Prg of the FeFET. Memory windows of ΔVth > 0.7 V were obtained using 8 and 8.5 V pulses.
Figure 5c,d show a clear monotonic ΔVth increases when raising either the pulse height or width. Good analog VthE and VthP controllability was suggested by smooth and linear ΔVth growths with raising log(tEP) as shown in Figure 5d. The similar tendencies of ΔVth and tEP have already been reported in our previous works [3,5,7,9,52]. In the prior FeFETs, poly-crystalized ferroelectrics were visualized by electron backscatter diffraction (EBSD) [44]. The EBSD indicated that the (C)SBT consisted of multi-grains with various crystal orientations in the FeFETs. The poly-crystalized ferroelectrics may bring the analog VthE and VthP controllability to the FeFETs. In the present FeFET, there must be numerous grains in channel-width direction with W = 100 μm whereas a single grain or a few were expected in channel-length with Lch = 85 nm which was smaller than average diameters of SBT grains freely grown in-plane [44].
In a preferable geometry of the replacement-gate FeFET in the future, only the channel area Lch × W will be intensively scaled down with remaining the height H. The H is decided by the gate-groove depth in Step 7 in Section 2.1 and Figure 1. The ΔVth in this report was not yet at its best ability considering the ferroelectric height H = 450 nm. In the vertical direction of FeFET, a gate stack by filling SBT should be essentially the same as a large Lch conventional one by etching SBT. Therefore, potential ΔVth will become the same as that of conventional FeFETs by improving the details in the fabrication process in Section 2.1. An immediate target for the present FeFET will be realizing ΔVth = 0.7 V by Ers of (−6V, 10 μs) and Prg of (6V, 10 μs) for H = 190 nm as demonstrated before using Pt/CSBT/HfO2/Si FeFETs [7].

3.2.2. Retention

Retention of a FeFET was measured by the procedures as shown in Figure 6a,b. After program (Prg), Retain and Read were repeated during the scheduled time. In Prg, a Vg pulse of (VP, tEP) was applied with Vd = Vs = Vsub = 0 V. In Retain, all the terminals were kept at zero as Vg = Vd = Vs = Vsub = 0 V. In Read at a certain time t, an Id–Vg curve was drawn by varying Vg in a narrow range from 0 to 1.0 V at Vd = 0.1 V and Vs = Vsub = 0 V. A VthP was extracted from the Id–Vg and plotted with a marker at t as shown in Figure 6c. After completing the VthP-t, VthE-t started to be measured. In erase (Ers), a Vg pulse of (VE, tEP) was applied with Vd = Vs = Vsub = 0 V. After Ers, Retain and Read were repeated during the scheduled time. The Retain and Read conditions for VthE-t were the same as those for VthP-t. In the Read at a certain time t, an extracted VthE was plotted with a marker at t as shown in Figure 6c. In this work, VP = 8 V, VE = −8 V and tEP = 10 μs. The retention was measured for 105 s in each of VthP–t and VthE–t. At t = 105 s, they were still distinguishable with a difference ΔVth = 0.26 V. When t > 103 s, as shown in Figure 6c, the gradient of the VthP-log(t) and VthE–log(t) curves appeared to be nearly zero. A possible ten-year retention was suggested by extrapolation lines drawn on the last three markers in each branch. The present Lch = 85 nm FeFET showed a good retention to the same extent as those of the conventional (C)SBT FeFETs [1,2,3,4,5,6,7,8,9,11,12,37,38,39,40,42,45,46,52].

3.2.3. Endurance

Endurance of a FeFET was measured by the procedure shown in Figure 7a. After imposing endurance cycles on FeFETs, pairs of VthE and VthP were obtained. The endurance cycles consisted of periodic bipolar Vg pulses for an alternate Ers of (VE, tEP) and Prg of (VP, tEP) with Vd = Vs = Vsub = 0 V. The endurance-cycle application was interrupted at certain scheduled cycle numbers (N). After the N cycle application, VthE and VthP were read as follows: a series operation of Ers, Read, Prg, and Read, in this order was performed. In Ers, a single Vg pulse of (VE, tEP) was applied with Vd = Vs = Vsub = 0 V. In Read after Ers, an Id-Vg was measured by varying Vg in a narrow range from 0 to 1.5 V at Vd = 0.1 V and Vs = Vsub = 0 V. A VthE was extracted from the Id–Vg and plotted with a marker at N as shown in Figure 7b. In Prg, a single Vg pulse of (VP, tEP) was applied with Vd = Vs = Vsub = 0 V. In Read after Prg, an Id–Vg was measured under the same conditions with Read after Ers. The obtained VthP was plotted with a marker at N as shown in Figure 7b.
As shown in Figure 7b, the Ers of (−7.5 V, 10 μs) and Prg of (7.5 V, 10 μs) were first applied for an endurance up to N = 108 cycles. Next, a stronger input of (−8 V, 10 μs) and (8 V, 10 μs) was applied to the same FeFET up to N = 109 cycles. No significant sifts of VthE and VthP were observed throughout the measurements. By taking the minimum of the VthE and the maximum of the VthP in the endurance test, ΔVth = 0.40 V for |VE| = VP = 7.5 V and ΔVth = 0.57 V for |VE| = VP = 8 V were obtained. These were margins for distinguishing VthE from VthP as indicated in Figure 7b. In spite of using the rather complicated dummy-gate process, the Lch = 85 nm FeFET fabricated showed high endurance up to 108~109 cycles. This is the same as the endurance level that (C)SBT-FeFETs inherently have [1,2,3,4,5,6,7,8,9,10,11,12].

4. Summary

A new fabrication process of a FeFET was proposed and demonstrated. Dummy-gate patterns with self-aligned sources and drains were prepared on a Si substrate. HfO2 with a thickness of 5 nm was inserted in advance between the dummy-gate substance and the Si substrate. The dummy substance was selectively removed to form a self-aligned groove on the gate. A thin SBT precursor film was deposited to fill up the groove. After forming the Ir gate electrode on the SBT, the whole gate stack was annealed for the SBT crystallization. The finished FeFET of Ir/SBT/HfO2/Si had a channel length Lch = 85 nm. The FeFET exhibited a 109 cycle-high endurance and long stable retentions measured for 105 s. By adopting the replacement-gate process, area-scalable SBT-FeFETs with the high endurance and long retention were successfully produced.

Author Contributions

Conceptualization followed by numerous improvements with respect to the device structure and processing, M.T.; the anneal and MOCVD processes, S.S.; SEM observation, M.T.; creation of PC-controlled measurement programs, S.S.; electrical measurement of the devices, S.S.; data analysis and discussion, S.S. and M.T.; writing—original draft preparation, M.T.; writing—review and editing, M.T. and S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This work was partially supported by WACOM R&D Corporation. We used the EB lithography system in the NPF of AIST, supported by MEXT, Japan.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. New fabrication process of Strontium bismuth tantalate (SBT)-ferroelectric-gate field-effect transistors (FeFETs) demonstrated in this work.
Figure 1. New fabrication process of Strontium bismuth tantalate (SBT)-ferroelectric-gate field-effect transistors (FeFETs) demonstrated in this work.
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Figure 2. (a) Schematic cross-section of a FeFET with an equivalent circuit of MFI(IL)S gate stack. For convenience of explanation, the circuit is represented using virtual capacitances instead of a strict physical explanation by the electric flux density continuity, D. (b) Schematic drawings of PF versus EF. All PF-EF loops are drawn in counter-clockwise directions. The inner loop (red solid) is a minor loop corresponding to unsaturated PF discussed in Section 2.2. Outer loop (blue broken) is a major loop for saturated PF added as a reference. Every loop has its Pmax at Emax and Pmin at Emin.
Figure 2. (a) Schematic cross-section of a FeFET with an equivalent circuit of MFI(IL)S gate stack. For convenience of explanation, the circuit is represented using virtual capacitances instead of a strict physical explanation by the electric flux density continuity, D. (b) Schematic drawings of PF versus EF. All PF-EF loops are drawn in counter-clockwise directions. The inner loop (red solid) is a minor loop corresponding to unsaturated PF discussed in Section 2.2. Outer loop (blue broken) is a major loop for saturated PF added as a reference. Every loop has its Pmax at Emax and Pmin at Emin.
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Figure 3. Cross-section of a FeFET with Lch = 85 nm fabricated in this work. (a) Original photo by SEM observation and (b) the photo with supporting lines added to clarify material boundaries. (c) Schematic drawing assigned with gate, drain, source and substrate terminals for electrical characterizations.
Figure 3. Cross-section of a FeFET with Lch = 85 nm fabricated in this work. (a) Original photo by SEM observation and (b) the photo with supporting lines added to clarify material boundaries. (c) Schematic drawing assigned with gate, drain, source and substrate terminals for electrical characterizations.
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Figure 4. Static static drain current versus gate voltage (Id–Vg) curves of a FeFET with Lch = 85 nm. The channel width was W = 150 μm. Vg ranges were Vg = 1 ± 4 V, 1 ± 5 V and 1 ± 6 V.
Figure 4. Static static drain current versus gate voltage (Id–Vg) curves of a FeFET with Lch = 85 nm. The channel width was W = 150 μm. Vg ranges were Vg = 1 ± 4 V, 1 ± 5 V and 1 ± 6 V.
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Figure 5. Investigation of VthE and VthP by applying Vg pulses to a FeFET with Lch = 85 nm. The channel width was W = 100 μm. (a) The measurement procedure; (b) measured original VthE and VthP; (c) pulse-height dependence of ΔVth = VthE − VthP and (d) pulse width dependence of ΔVth.
Figure 5. Investigation of VthE and VthP by applying Vg pulses to a FeFET with Lch = 85 nm. The channel width was W = 100 μm. (a) The measurement procedure; (b) measured original VthE and VthP; (c) pulse-height dependence of ΔVth = VthE − VthP and (d) pulse width dependence of ΔVth.
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Figure 6. Retention investigation after applying Vg pulses to a FeFET with Lch = 85 nm. The channel width was W = 100 μm. The measurement procedures for the retentions of (a) VthP after Prg of (VP, tEP) and (b) VthE after Ers of (VE, tEP). (c) The measured retentions for 105 s each. Dashed lines are extrapolations of VthP–log(t) and VthE-log(t) for estimating VthP and VthE after ten years.
Figure 6. Retention investigation after applying Vg pulses to a FeFET with Lch = 85 nm. The channel width was W = 100 μm. The measurement procedures for the retentions of (a) VthP after Prg of (VP, tEP) and (b) VthE after Ers of (VE, tEP). (c) The measured retentions for 105 s each. Dashed lines are extrapolations of VthP–log(t) and VthE-log(t) for estimating VthP and VthE after ten years.
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Figure 7. Endurance of a FeFET with Lch = 85 nm. The channel width was W = 80 μm. (a) The measurement procedures of applying endurance cycles and reading VthE and VthP. (b) Endurances were measured up to N = 108 cycles for 7.5 V Vg pulse heights and N = 109 cycles for 8 V.
Figure 7. Endurance of a FeFET with Lch = 85 nm. The channel width was W = 80 μm. (a) The measurement procedures of applying endurance cycles and reading VthE and VthP. (b) Endurances were measured up to N = 108 cycles for 7.5 V Vg pulse heights and N = 109 cycles for 8 V.
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Takahashi, M.; Sakai, S. Area-Scalable 109-Cycle-High-Endurance FeFET of Strontium Bismuth Tantalate Using a Dummy-Gate Process. Nanomaterials 2021, 11, 101. https://0-doi-org.brum.beds.ac.uk/10.3390/nano11010101

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Takahashi M, Sakai S. Area-Scalable 109-Cycle-High-Endurance FeFET of Strontium Bismuth Tantalate Using a Dummy-Gate Process. Nanomaterials. 2021; 11(1):101. https://0-doi-org.brum.beds.ac.uk/10.3390/nano11010101

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Takahashi, Mitsue, and Shigeki Sakai. 2021. "Area-Scalable 109-Cycle-High-Endurance FeFET of Strontium Bismuth Tantalate Using a Dummy-Gate Process" Nanomaterials 11, no. 1: 101. https://0-doi-org.brum.beds.ac.uk/10.3390/nano11010101

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