Logic Gates and Ring Oscillators Based on Ambipolar Nanocrystalline-Silicon TFTs
Nanocrystalline silicon (nc-Si) thin film transistors (TFTs) are well suited for circuit applications that require moderate device performance and low-temperature CMOS-compatible processing below 250°C. Basic logic gate circuits fabricated using ambipolar nc-Si TFTs alone are presented and shown to operate with correct outputs at frequencies of up to 100 kHz. Ring oscillators consisting of nc-Si TFT-based inverters are also shown to operate at above 20 kHz with a supply voltage of 5 V, corresponding to a propagation delay of <10 μs/stage. These are the fastest circuits formed out of nanocrystalline silicon TFTs to date. The effect of bias stress degradation of TFTs on oscillation frequency is also explored, and relatively stable operation is shown with supply voltages >5 V for several hours.
Nanocrystalline silicon (nc-Si) has attracted much interest for use in circuit applications that require low temperature, large-area fabrication, and alternative substrates. Examples of these applications may include neuromorphic architectures for vision sensing [1, 2], flexible and flat-panel displays [3–5], radio frequency identification (RFID) tags , solar cells , and many more. Many of these applications do not require high performance devices, but rather the viability of a simple and inexpensive fabrication process. Neuromorphic circuits are also defect tolerant to a certain extent, allowing low-yield devices to be employed. Nc-Si thin film transistors (TFTs) can be fabricated using a CMOS-compatible fabrication process [8, 9], which also facilitates three-dimensional integration with high-performance CMOS structures [5, 10, 11]. In such a case, TFT circuitry such as biosensor arrays or neural networks could be fabricated around a CMOS core to improve the overall functionality.
The limited use of noncrystalline silicon devices in circuits was historically due to low carrier mobility , device degradation under bias stress [13, 14], and the lack of p-channel operation . Use of nc-Si instead of amorphous silicon (a-Si) improves the electron mobility and device degradation to a certain extent. The above drawbacks have been further addressed in the recent years by high purity nc-Si deposition which reduces the incorporation of impurity oxygen in the channel layer and ensures ambipolar operation of the devices [8, 15]. In this context, the term “ambipolar” refers to either n- or p-channel operation depending on the bias conditions. The maximum mobility observed in nc-Si TFTs is 150 cm2/V·s for electrons and 25 cm2/V·s in case of holes . The performance of these devices has also been considerably improved by utilizing thin high-κ gate dielectrics that reduce the subthreshold swing and threshold voltages () of these devices . For instance, the switching power consumption of an ambipolar nc-Si TFT-based inverter operating at 100 Hz reduced from 5 pJ per switch to 350 fJ per switch when the gate dielectric was changed from 200 nm SiO2 to 20 nm HfO2. Fabrication of nanoscale TFTs has been demonstrated using electron beam lithography (EBL) . Submicron device dimensions are essential for use in high-density architectures such as neuromorphic systems . In addition, nc-Si TFTs can be fabricated at temperatures of 250°C and below, allowing the incorporation of alternative substrates including polyimide  and steel foil . The feasibility of fabricating circuits on such flexible substrates provides TFT technology with a distinct advantage over conventional CMOS technology.
The ambipolar operation of nc-Si TFTs also allows a single device type to be used to form a complementary inverter . Such an arrangement ensures better inverter output range and a higher voltage gain than a source-follower inverter formed using an n-channel device. In contrast, many other low-temperature fabrication devices such as amorphous oxide TFTs operate solely in the n-channel mode. Therefore, inverters formed from such devices require either the integration of other devices such as resistors or use of a source follower configuration that prevents rail-to-rail output [20–22]. Both n- and p-channel organic TFTs have been recently developed and are the only other materials capable of producing complementary TFT inverters at low temperatures  for many of the applications discussed above. However, such devices typically have low mobilities, and therefore the circuits operate at very low switching speeds [24, 25]. Furthermore, rapid atmospheric degradation is observed in organic semiconductors , limiting their use.
In this paper, we present logic gates and ring oscillators that operate at low supply voltages formed using ambipolar nc-Si TFTs alone. The fabrication of these circuits is described in detail, followed by their electrical characterization. The operation of ring oscillators at different supply voltages is analyzed. The performance of these circuits is compared with that of similar circuits formed using alternate low-temperature materials. Finally, the circuits are electrically stressed for several hours to examine the effect of device degradation on their operation.
2. TFT Fabrication
The fabrication of nc-Si TFT-based circuits employs a four-level lithography process. The circuits contain 100 µm square pads at input and output nodes and were laid out for ease of electrical probing rather than the minimization of circuit area. A p-type Si wafer with moderate doping (1017 cm−3) is used as the substrate on which 300 nm SiO2 was grown with thermal oxidation. Initially, 80 nm Cr metal is deposited and patterned. This layer acts as source/drain of the individual TFTs as well as the bottom level of metal lines. All metal lines are 6 µm wide. Next, an 80 nm thick nc-Si layer is deposited using plasma-enhanced chemical vapor deposition (PECVD) at 13.56 MHz, 125 W RF power, 0.9 Torr, and 250°C deposition temperature in a 1 : 100 mixture of SiH4 and H2. A detailed description of the deposition condition is contained in . The channel layer is then patterned with Cl chemistry-based plasma etching.
Following this, the HfO2 gate dielectric with equivalent oxide thicknesses (EOTs) ranging from 7 nm to 2.5 nm was deposited using atomic layer deposition (ALD). EOTs were calculated assuming dielectric constant of 17 for HfO2. This number is lower than the standard accepted value of 24 because the process temperature is 200°C. The third level of lithography was then used to pattern contact via holes through the gate dielectric, and 100 nm Au was used as contact metal. The final metal layer was Al (thickness 100 nm), used as gate metal as well as the top level of metal lines. A 2-hour long, 250°C anneal in forming gas completed the process. Individual transistors, inverters, logic gates, and the ring oscillators were fabricated using the same mask set. Figure 1 shows a schematic of the nc-Si TFT.
3. Electrical Characteristics of TFT and Inverter
Current-voltage (I-V) characteristics (both transfer and output) when a 20 µm × 10 µm nc-Si TFT with 20 nm HfO2 gate dielectric is operated with top-gate bias (grounding the substrate) are shown in Figures 2(a) and 2(b). The device exhibits ambipolar operation, that is, it operates in both the electron and hole regimes. The direction of current flow depends solely on the drain voltage. The applied gate voltage determines which charge carriers dominate in the channel: electrons constitute the charge flow when the gate voltage is positive and holes at negative bias.
Low hysteresis of less than 50 mV/dec. was observed in the I-V measurements (also Figure 2(a)), and the ratio when operated at a drain voltage of |1 V| is >105. At higher drain voltage values, the on/off ratio reduces because both hole and electron conduction occur for a larger range of gate voltages. Mobility values were extracted from the I-V characteristics. The maximum electron and hole mobilities observed are 20 cm2/V·s and 5 cm2/V·s, respectively. The corresponding subthreshold swing values are 0.6 V/decade and 0.65 V/decade.
Figure 3(a) shows the voltage transfer characteristics of a nc-Si TFT-based inverter () at different operating voltages. The static CMOS topology was selected for the fabricated inverter since it offers the best output voltage range and voltage gain for ambipolar TFTs. Since the TFTs are ambipolar, no need exists for separate n- and p-channel devices in the inverter. The inverter exhibits close to peak-to-peak operation and high voltage gain (in the range 12–15). Slight deviation from peak-to-peak exists due to the ambipolar nature of the TFTs: the drive TFT does not completely turn off at low input and vice versa. The high gain can be attributed to the good saturation characteristics of the component devices. The dynamic characteristics of the inverter were also investigated by applying square voltage pulses at different frequencies. The inverter was able to respond closely to the input beyond 2 MHz , as shown in Figure 3(b). The switching speed could be minimized further by reducing the gate-to-source/drain overlap capacitance, which was not optimized in the current process.
4. Logic Gates
Logic gate circuits including NOR, NAND, OR, and AND were fabricated. OR and AND gates were realized by adding an inverter at the output of the NOR and NAND gates, respectively, and were composed of six ambipolar TFTs each (Figure 4). A single device type is sufficient for each gate, but TFTs were sized for minimum delay according to the circuit configuration, as shown in the figure.
Figure 5 shows the operation of the different logic gates at . The input waveforms A and B have amplitude 5 V and frequency 100 kHz. The transition between input states occurs with rise and fall times of 10 ns. All possible combinations of inputs are shown. The outputs respond with no perceptible delay to each input change. The logic output values are correct in all cases, although the outputs deviated from the supply voltage or ground in some cases by up to 10% of due to the ambipolar nature of the TFTs. The nc-Si TFT-based logic gates operate at a similar frequency to high-quality organic transistor-based circuits at a much lower supply voltage .
5. Ring Oscillators
Three-, five-, or seven-stage ring oscillators were fabricated by connecting nc-Si TFT inverters in series, with the output of the last stage feeding back to the first. Figure 6 shows such a fabricated seven-stage ring oscillator. Each inverter was similar to the one in Section 3 and had a geometry ratio of four. To start the oscillations, the output of one of the stages was pulsed with a short square pulse. This set off the oscillations which stabilized at a set frequency depending on the number of stages and the supply voltage . Figure 7 shows the buildup of oscillations in three- and five-stage ring oscillators. At a supply voltage of 5 V, the oscillation frequencies are 21 kHz and 12 kHz, respectively. The total number of ring oscillators tested for each number of stages was approximately 15; the observed frequency variation was below 10%. The yield of operating circuits was >97%, which is reasonable for a first demonstration. A five-stage ring oscillator operating at 20 kHz () consumes approximately 0.5 µW.
The supply voltage was varied while testing the oscillators. An increase in voltage swing and a reduction in the propagation delay were observed with increase in . For instance, the circuit starts oscillating with as low as 3 V with propagation delay of ~14 µs/stage. At , the frequency increases to 38 kHz, corresponding to a delay of 4 µs/stage. This trend is illustrated in Figure 8. The propagation delays compare well with other ring oscillator circuits fabricated using low-temperature technology [20, 22–28]. This is also the first instance of noncrystalline silicon TFT-based ring oscillators operating at low-(<5 V)-supply voltages. The operating frequency of these circuits would also benefit from reduced parasitic capacitance in the devices.
In addition to varying the supply voltage, ring oscillators with different channel lengths were also tested (retaining the same channel widths). Figure 9 shows the relationship between channel length and propagation delay for three different values of . The delay is minimum for and increases with channel length, following a similar trend as observed in . This trend can be attributed to higher drive currents in TFTs with higher W/L values.
It is known that nc-Si TFTs suffer from threshold voltage degradation upon bias stressing [9, 13, 14, 29]. The dominant mechanism for this is believed to be defect state creation in the channel layer [5, 13]. To quantify the effect of device degradation on ring oscillator operation, a three-stage ring oscillator was operated at different supply voltages for increasing time periods. The oscillation frequency was measured after each stress period. The results (Figure 10) show that after long periods of continuous operation the oscillation frequency decreased for all supply voltages. The maximum reduction occurred for in which case the frequency reduced by 43% after a stress period of 104 s. Similar trends were observed for each value. Although any device degradation is adverse to the circuit, the observed frequency reduction is a large improvement over other low-temperature fabricated ring oscillators . This can be further improved by performing nc-Si depositions at slightly higher temperatures or potentially by further optimization of the deposition conditions.
In this work, logic gates and ring oscillators fabricated using ambipolar nc-Si TFTs have been demonstrated. Use of nc-Si as channel material enables low-cost large-area fabrication of circuits using these TFTs. The fabrication process employed uses low temperature (maximum of 250°C) and is CMOS compatible. The devices do not degrade upon exposure to the atmosphere and are robust. The logic gates were shown to operate with correct outputs at frequencies up to 100 kHz and rapid switching between states. Ring oscillators consisting of up to seven stages were fabricated and characterized. The oscillation frequencies varied from 8 kHz to 55 kHz depending on the supply voltage and channel length, corresponding to delays of approximately 3 µs to 20 µs per stage. The yield and reproducibility of either type of circuit were >97%, which is reasonable for a first demonstration. Finally, the ring oscillator circuits were electrically stressed by continuous operation beyond 104 s to examine the effect of bias stress degradation of nc-Si TFTs. The oscillation frequencies reduced upon long periods of stress, but this reduction was lower than that observed in other circuits with low-temperature fabrication processes.
Although the circuits were fabricated as large area for ease of probing, nanoscale circuits using nc-Si TFTs are certainly feasible. Submicron nc-Si TFTs fabricated using electron beam lithography have been demonstrated elsewhere, and the same process could be extended to produce compact circuits.
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