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RF performance and intertier coupling of CMOS processed in 3-D sequential integration are investigated. pMOS transistor fabricated with a 500 °C thermal budget features good RF figures of merit with ${f}_{t} =105$ GHz and ${f}_{\text {max}} =175$ GHz for a gate length of 45 nm and ${V}_{\text {DD}} = -1$ V. Moreover, we demonstrate that the low- ${k}$ SiCO oxide spacer and low polysilicon gate resistance obtained with the low temperature process contribute to ${f}_{\text {max}}$ results that are better than high temperature process (above 1000°). Finally, we illustrate the crosstalk effects and the influence of the low-tier transistor gate voltage ${V}_{\text {G}}$ on the top-tier threshold voltage ${V}_{T}$ . We also show that a polysilicon ground shield integrated under the top-tier transistor substantially attenuates the intertier RF field coupling effects.

The impact of interface charges under the gate spacer on FDSOI devices integrated in low temperature process are explored. A great number of traps (~1013/cm2) are identified on the interface between the spacer oxide and the silicon film using Terman's method for interface states characterization. Thanks to electrical characterization and TCAD simulations, it is shown that the trapped charges induce the formation of a depleted region in the vicinities of the spacer. Moreover, a strong degradation of performances on underlap channels is observed. The spacer charges influence on reliability measurements is finally explored.

We fabricated junction less and inversion-mode monocrystalline nanowire nMOSFETs down to L=18nm gate length and W=20nm width. We demonstrate record performance of nanowire junction less transistors for analog applications: $A_{VT}=1.4mV \cdot \mu$ m matching, $A_{v0}=62dB$ gain (L=200nm), $f_{T}=126GHz$ cut-off frequency and $f_{MAX}=182GHz$ maximum operating frequency (L=35nm). Junction Less transistor performances even exceed those of inversion-mode ones in terms of back-bias capability, low-frequency noise, hotcarrier degradation and fMAX. This is explained by junction less physics: channel length modulation, bulk conduction and high channel-depth sensitivity to back bias.

International audience; This work presents statistical measurements on the effects of the electrostatic coupling on the on-current, off-current and low frequency noise characteristics of individual top-tier devices, due to bottom-tier devices being biased. No inter-tier coupling impact was observed on device low-frequency noise regardless the transistor area. While for analog applications the coupling-induced ΔVt, ΔIoff and ΔIon might reach high values, it is demonstrated that regarding digital applications, the coupling-induced fluctuations are well below the mismatch effects. TCAD and SPICE simulations were used to fully understand the phenomenon, to predict the effects at SRAM bitcell level and to propose guidelines to contain the inter-tier electrostatic coupling: the coupling effect can be limited either by increasing the Inter-Layer Dielectric (ILD) thickness or through a top/bottom transistor misalignment.

To take fully advantage of Junctionless transistor (JLT) low-cost and low-temperature features we investigate a 475 degC process to create onto a wafer a thin poly-Si layer on insulator. We fabricated a 13nm doped (Phosphorous, 1E19 at/cm3) poly-silicon film featuring excellent roughness values (Rmax= 1.6nm and RMS=0.2nm). Guidelines for grain size optimization using nanosecond (ns) laser annealing are given.

For the first time, we propose a 3D-monolithic SRAM architecture with a local back-plane for top-tier transistors enabling local back-bias assist techniques without area penalty as well as the capability to route two additional row-wise signals on individual back-planes. Experimental data are extracted from a 14nm planar Fully-Depleted-Silicon-on-Insulator (FDSOI) $0.078\mu \mathrm{m}^{2}$ SRAM in order to properly model 3D top-tier cells. Simulations show this technique yields a 7% bitline capacitance reduction, a 12%/16% read/write access time improvement at $V_{DD}=0.8\mathrm{V}$ and a reduction of minimum operating voltage $V_{min}$ by 60mV at $6\sigma$ w.r.t. planar SRAMs.

International audience; Variability is a challenge for future scaling as process dimensions reduce. The emerging 3-D sequential stacking technology is more than Moore’s scaling alternative. The 3-D design flow requires the partitioning of the netlist between the tiers. This paper presents the variability analysis of circuits partitioned into different levels. A comparison among local and global variations effects on ring oscillators (ROs) and SRAM is demonstrated. The across-chip variations and correlation range are shown as a critical point for the 3-D very large-scale integrated circuits, where the local variability is dominant. The correlations between devices due to the distances or the allocation into different tiers are directly taken into account in the SPICE model due to a statically unified model applied to 3-D circuits based on Monte Carlo simulations. Design wise, the 3-D integration can further decrease the circuit variability as shown in RO output frequency and SRAM static noise margin.

For the first time, we propose a 3D-monolithic SRAM architecture with a local back-plane for top-tier transistors enabling local back-bias assist techniques without area penalty, as well as the capability to route two additional row-wise signals on individual back planes. Experimental data are extracted from a 14 nm planar Fully-Depleted-Silicon-On-Insulator (FDSOI) 0.078 µm2 SRAM cell in order to properly model 3D top-tier cells. BTI measurements are done to ensure that the proposed assist do not provide additional stress. Simulations show this technique yields a 7% bitline capacitance reduction, a 12%/50% read/write access time improvement at VDD = 0.8 V and a reduction of minimum operating voltage Vmin by 60 mV (up to 92 mV with speed penalty) at 6σ w.r.t. planar SRAMs.

session T7: Process and Material Technology; International audience; We investigate in detail, for the first time, both performance and reliability of a 3D sequential integration process. It is clearly demonstrated that the top level transistor can be successfully processed at 630°C with almost no impact on the performance and reliability of the bottom level. It is also highlighted that top level devices meet the P&NBTI reliability requirements. Finally an example of successful and robust 3D logic integration is proposed based on a 3D inverter combining a top-level PMOS with a bottom-level NMOS.

In this paper, we review the main opportunities brought by 3D-monolithic integration for CMOS device and digital circuit. Simulation results show that 3D monolithic integration can provide up to 30% power reduction at iso-performance and 30% manufacturing cost saving for digital circuits, compared to planar technology, making it an attractive alternative to the straightforward technology scaling. We benchmark the transistor-level and cell-level 3D-monolithic integrations, where the inter-tier vias (3D-contacts) are used intra-cell or inter-cell, respectively. On the one hand, transistor-level 3D-integration can be seen as a full-custom approach, both in terms of technology and circuit design. It promises more performance but at the expense of strong Design/Technology Co-Optimizations. On the other hand, the cell-level 3D-integration ensures a 50% area reduction and a re-use of the technology/design platform. The main technology challenge relative to this integration is the thermal budget constraint of the top-level process integration and the intermediate Back-End-Of-Line (iBEOL) stability. Finally, the total isolation of the top-level transistors integrated in 3D-monolithic raises new opportunities and offers new functionalities like for example an efficient dynamic back-biasing capability.

session 7: Characterization, Reliability, and Yield: Reliability of Advanced Devices (7.6); International audience; We present for the first time an experimental study of thermal effects in 3D sequential integration, including Self-Heating Effect (SHE) and thermal coupling between the two levels of ultra-thin body FDSOI transistors. We extracted a large set of experimental data using different thermometry techniques, and different heater-sensor configurations allowed by this specific stacked integration. We described SHE in top and bottom transistor levels, as well as the influence of a transistor in ON state on a transistor stacked above or below. At the same time, we provide for the first time an experimental validation that the temperature increase given by gate resistance thermometry technique is equal to the temperature in the channel given by the subthreshold slope. Finally, this work can be also used to manage thermal effects for logic or analog applications, and help further optimization of 3D sequential integrated circuits through both technology and design solutions.

We have fabricated 3D-monolithic transistors on two tiers. We experimentally evidence the asymmetric double-gate (DG) behavior of a top-tier transistor, resulting in a better ON-state current (Ion) / OFF-state current (I off ) tradeoff than in single-gate (SG) mode. Moreover, a 3D-shared contact between a top and bottom electrode is experimentally demonstrated; paving the way for a local back gate, possibly connected with the top gate by a 3D-shared contact. Assuming such a construct, we have performed extensive layout and spice simulations of standard cells and SRAMs. We evidence that the back-gate overlap on the source and drain must be minimized to mitigate the parasitic capacitances. The best layout configurations of a loaded 1-finger inverter yields a 24% frequency gain at a given static power and V dd = 0.6V supply voltage, compared to SG, or to a static power divided by 5, compared to SG under Forward Body Bias (FBB). These performance boosts may be obtained without any area penalty and assuming a 20nm-thin BOX. Similarly, a 29% improvement of the read and write currents of 6T SRAMs is contemplated at V dd = 0.8V. Such new functionality provided by 3D-monolithic even enables making 4T SRAMs that are fully functional at V dd = 0.8V by improving their retention and, in turn, the maximum number of bitcells per column from 50 (SG) to 300 with a dynamic back-bias.

To set up specification for 3D monolithic integration, for the first time, the thermal stability of state-of-the-art FDSOI (Fully Depleted SOI) transistors electrical performance is quantified. Post fabrication annealings are performed on FDSOI transistors to mimic the thermal budget associated to top layer processing. Degradation of the silicide for thermal treatments beyond 400 °C is identified as the main responsible for performance degradation for PMOS devices. For the NMOS transistors, arsenic (As) and phosphorus (P) dopants deactivation adds up to this effect. By optimizing both the n-type extension implantations and the bottom silicide process, thermal stability of FDSOI can be extended to allow relaxing upwards the thermal budget authorized for top transistors processing.

session: Modeling and Characterization; International audience; We present an experimental study of thermal effects in thin film FDSOI MOSFETs, with a focus on the impact of self-heating effect (SHE) on drain current. We have performed thermal resistance extraction using the gate thermometry method, and calculated the resulting cold drain current (Id0), i.e. without SHE. We demonstrate that SHE is more pronounced in shorter and narrower devices without essential differences between nMOS and pMOS transistors. Our experiments show that although the temperature increases significantly in the channel due to SHE, its effect on the ION performances could be limited at operating voltage.

This study focuses on temperature deviation during operation of transistors inside a monolithic 3D standard cell built on two tiers. Early assessment of this topic is crucial to manage circuit design and requires both steady-state and transient thermal analysis at transistor level. A representative 3D standard cell in 14nm FDSOI technology is considered, using intermediate Back-End-Of-Line (iBEOL) and top tier BEOL. Steady-state and transient power dissipations in NMOS and PMOS are extracted from SPICE simulations with variables such as output load capacitance and operating temperature. 3D thermal simulations are then performed to assess the impact of design parameters such as routing densities, thicknesses of iBEOL and BEOL, their number of metal layers and packaging techniques. Steady-state and transient thermal simulations enable precise analysis to correlate temperature deviation of transistors with these parameters. Design guidelines are provided to limit the temperature deviation between top and bottom tier which can reach 7°Celsius during circuit operations in the worst case

In this paper, the energy process window of nanosecond (ns) laser annealing for junctions activation has been determined for several dopants (As, P, BF2). The different recrystallization states observed when tuning laser energy density are explained by numerical simulations. Within these conditions, the laser impact on the thermal stability of ULK/copper inter-tiers interconnections has been evaluated for a 28nm node backend metal 1 design rules technology both from morphological and electrical perspectives. This study highlights the interest of ns laser anneal for CoolCube™ 3D integration.

3D VLSI with a CoolCube™ monolithic integration flow allows vertically stacking several layers of devices with a unique connecting via density above tens of million/mm2. This results in increased devices density and gains in power and performance thanks to wire-length reduction without the extra cost associated to transistor scaling. In addition to power saving, this true 3D integration opens perspectives in terms of heterogeneous integration. We will review the opportunities brought by CoolCube™ and will present the most advanced technological demonstration of 3D CMOS over CMOS CoolCube™ integration.

While the 3D sequential process is still under development, the electrical influence of specific process for the bottom tier needs to be studied. As another MOS transistor layer is fabricated on top of the bottom one, contamination risk and thermal stability issues appear, thus requiring adaptation of conductors/dielectrics for intermediate Back-End Of Line (iBEOL) processing. As materials differ from usual copper/low-k, it is necessary to study how standard cells electrical characteristics will be affected. We modeled different descriptions of iBEOL in 14nm FDSOI process and simulated standard cells characteristics. The average power consumption is almost the same while large cells with high drive timing degradation can be up to 20% in the worst case. This sensitivity analysis allowed us to identify which parameters (permittivity, resistivity) have the greatest impact depending on standard cell type and provide technology and design guidelines. Our goal here was to limit the performance degradation to around 5% maximum for the bottom tier standard cells.

3D sequential integrations such as CoolCubeTM appear to be an alternative to planar scaling for the next generation nodes by stacking CMOS over CMOS and even transistors above transistors. This is possible thanks to high alignment accuracy depending only on the lithography stepper and not on the bonder performance, as in packaging technologies. Performance gain can be obtained either by optimizing each level independently or by the reduction in interconnect delay and thus, optimization of device partitioning and global routing. In this paper, data on the bottom transistors stability and some key enablers to realize a top transistor level at low temperature will be presented.

Advanced gate stack, with the introduction of high permittivity oxides (mostly Hf-based material) and a metallic gate are widely studied for the future generations of CMOS devices. In particular the integration of new materials as metal gate allows to adjust the threshold voltage of nMOS and pMOS, by changing the material (through work function modulation), the deposition process or the layer thickness [1]. However, such advanced gate stacks have also an impact on the other electrical characteristics, like the channel mobility or the reliability [2]. Correlation between mobility degradation and NBTI has thus been evidenced in advanced highk/metal gate transistors due to nitrogen incorporation originating from the nitrided metal gate or the nitridation of the high-k oxide [2,3]. In this work we have investigated the effect of the metal gate on the electrical properties of MOSFETs, with specific attention to carrier transport. We have brought new experimental results, including low temperature measurements and interface trap characterization, to understand the physical mechanisms which affect the transport.

session 5: 3D Systems and Packaging; International audience; 3D VLSI with a CoolCube™ integration allows vertically stacking several layers of devices with a unique connecting via density above a million/mm 2 . This results in increased density with no extra cost associated to transistor scaling, while benefiting from gains in power and performance thanks to wire-length reduction. CoolCube™ technology leads to high performance top transistors with Thermal Budgets (TB) compatible with bottom MOSFET integrity. Key enablers are the dopant activation by Solid Phase Epitaxy (SPE) or nanosecond laser anneal, low temperature epitaxy, low k spacers and direct bonding. New data on the maximal TB bottom MOSFET can withstand (with high temperatures but short durations) offer new opportunities for top MOSFET process optimization.

For the first time the maximum thermal budget of in-situ doped source/drain State Of The Art (SOTA) FDSOI bottom MOSFET transistors is quantified to ensure transistors stability in Sequential 3D (CoolCube™) integration. We highlight no degradation of Ion/Ioff trade-off up to 550°C. Thanks to both metal gate work-function stability especially on short devices and silicide stability improvement, the top MOSFET temperature could be relaxed up to 500°C. Laser anneal is then considered as a promising candidate for junctions activation. Based on in-depth morphological and electrical characterizations it demonstrates very promising results for high performance Sequential 3D integration.

We demonstrate for the first time that FDSOI technology can be turned into an integrated light-sensitive device technology capable of detecting or interacting with light. By designing a dedicated diode below the BOX, light absorption induced V T shifts as large as 100mV and saturation drain current modulation of 70% are measured for the transistors above the BOX. Those experimental results are supported by TCAD simulations and pave the way to More than Moore applications for FDSOI technology.

Monolithic or sequential 3D Integration is a powerful technological enabler for actual 3D IC design as the stacked layers can be connected at the transistor scale. This paper reviews the opportunities brought by M3DI and highlights the applications benefiting from this small 3D contact pitch. It also presents the technological challenges of this concept and offers a general overview of the potential solutions to obtain a high performance low temperature top transistor while keeping bottom MOSFET integrity.

To set up specification for 3D monolithic integration, for the first time, the thermal stability of state-of-the-art FDSOI (Fully Depleted SOI) transistors electrical performance is quantified. Post fabrication annealings are performed on FDSOI transistors to mimic the thermal budget associated to top layer processing. Degradation of the silicide for thermal treatments beyond 400 °C is identified as the main responsible for performance degradation for PMOS devices. For the NMOS transistors, arsenic (As) and phosphorus (P) dopants deactivation adds up to this effect. By optimizing both the n-type extension implantations and the bottom silicide process, thermal stability of FDSOI can be extended to allow relaxing upwards the thermal budget authorized for top transistors processing.

Compared with TSV-based 3D ICs, monolithic or sequential 3D ICs presnts “true” benefits of going to the vertical dimension as the stacked layers can be connected at the transistor scale. The high versatility of this technology is evidenced via several examples requiring small 3D contact pitch. Monolithic 3D is shown to enable substantial gain in area and performance as compared to planar technology without scaling the transistor technology node. This paper summarizes the technological challenges of this concept: it offers a general overview of the potential solutions to obtain a high performance low temperature top transistor while keeping bottom MOSFET integrity.

International audience; For the first time, low temperature (LT) anneal at 625°C has been demonstrated for dopants activation enabling similar ION/IOFF trade-off as standard spike anneal (>;1000°C), down to 30nm gate length (LG) for both n&p FETs. Similar short channel effect control has been achieved in LT n&p FETs as its high temperature (HT) counterparts. Influence of dopant implant tilt on LT device performance is analyzed and guidelines for device performance optimization are proposed. This demonstration paves the way to 3D sequential integration with equal performance for stacked transistors and for bottom transistors.

We evidence a unique feature of junctionless Fully-Depleted Silicon-On-Insulator (JL FDSOI) transistors: the presence of both bulk and accumulation conduction renders standard VT-variability studies incomplete. For JL transistors, we rather propose an original analysis of the (local and global) variability in all-operation-regimes, from subthreshold to accumulation. We evidence that the current variability around VT is highly sensitive to back-bias VB and film thickness $(\mathrm{t}_{\mathrm{si}})$ uniformity. We demonstrate experimentally for the first time up to 70% lower drain current (ID) local variability for Junctionless Accumulation Mode (JAM) vs. inversion mode (IM) at 1V gate voltage (V G), $\mathrm{L}=18$ nm gate length and $\mathrm{W}=20\mathrm{nm}$ width. This is attributed to the impurity screening, lowering the impact of Random Dopant Fluctuations variability.