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Plasma Etch
Published in Robert Doering, Yoshio Nishi, Handbook of Semiconductor Manufacturing Technology, 2017
Peter L. G. Ventzek, Shahid Rauf, Terry Sparks
As device scaling has meant the increase in device speeds, the BEOL has not been a significant limitation to circuit performance until recently. Significant performance effects have been seen at 90 and 65 nm high performance circuits due to the BEOL resistivity and capacitance increasing line capacitance coupling. At 45 nm some have suggested that this may actually be a more significant limitation than the transistor performance. The trend in the industry to compensate this effect has been to introduce low-κ dielectric materials. But this is proving to be more difficult than expected and the copper line resistivity has been shown to also be a critical issue. The copper line resistivity is increasing more than predicted for a given scaling factor. There are several effects such as barrier metal thickness, as well as significant grain boundary and surface effects at these dimensions. So the BEOL metallization may become the new limitation for continuing on Moore’s Law scaling roadmap.125
Doping, Surface Modifications and Metal Contacts
Published in Andrew Sarangan, Nanofabrication, 2016
In a complementary metal oxide semiconductor (CMOS) circuit fabrication process, high temperatures are required for creating the doped areas of the transistors. This phase of the manufacturing process is referred to as the front-end-of-line (FEOL), and all the materials used have to be compatible with high temperatures. Since metals tend to form eutectic alloys and compounds with silicon at relatively low temperatures, they are not used during the FEOL process. The common materials used during FEOL are dielectrics and polysilicon. The second phase of the process, known as the back-end-of-line (BEOL), utilizes metals for interconnecting the transistors to create the functional circuitry. BEOL processes are done at significant lower temperatures to prevent the device junctions from diffusing as well as the metals from contaminating the devices. The thermal budget of the manufacturing process will also place a limit on the maximum temperatures that the finished chip can be exposed to. This includes brief exposures to high temperatures during soldering as well as degradation due to elevated temperatures during prolonged operation.
The Exploitation of the Spin-Transfer Torque Effect for CMOS Compatible Beyond Von Neumann Computing
Published in Krzysztof Iniewski, Santosh K. Kurinec, Sumeet Walia, Energy Efficient Computing & Electronics, 2019
Thomas Windbacher, Alexander Makarov, Siegfried Selberherr, Hiwa Mahmoudi, B. Gunnar Malm, Mattias Ekström, Mikael Östling
In integrated circuits the back end of line (BEOL) process refers to the fabrication of metal interconnects and the intermetal dielectrics (IMD) layers. Using successive deposition of metal (Cu), patterning of metal lines, IMD deposition, and planarization of the IMD layers, more than 10 layers of interconnecting Cu-lines can be realized. This is sufficient for the routing of signal and power supply lines in very complex circuits, with 100 millions of integrated transistors. All BEOL process steps are performed at low temperature, typically in the range 350°C–400°C. Therefore, the integration of spintronic memory and logic based on multilayer ferromagnetic metallic stacks with thin metal-oxide tunneling barriers is feasible. The MTJ stacks will not suffer from interdiffusion and the integrity of the tunneling barrier can be maintained [2,44]. Specifically the MgO barrier must be annealed under controlled conditions to obtain a proper crystallographic reorientation epitaxially along the (001) direction. More importantly, annealing is also necessary to induce the interfacial perpendicular magnetic anisotropy (PMA) effect for stacks based on CoFeB, which is intrinsically an in-plane material [70]. A comprehensive review of the PMA and its applications in [71]. The PMA can also be strengthened by using, for example, multilayer Co/Pt with inherent PMA or synthetic antiferromagnetic (SAF) layers in the MTJ stack [72]. Capping layers (e.g., silicon nitride) are used to protect the MTJ from unintentional reoxidation during later stages of processing. The MTJs are typically inserted close to the top metal layers. The MTJ bottom electrode is connected to an already available Cu-line in, for example, metal level 5 (M5), [1]. Subsequent MTJ layers are deposited without breaking the vacuum, patterned by lithography and etching and then embedded in the subsequent IMD layer. The IMD thickness depends on the layer and is chosen to minimize the interconnect capacitances. The MTJ stack total thickness is less than the IMD thickness so that the MTJ becomes fully embedded. For an illustration of production near embedded MRAM, see Figure 4.8.
Timing Closure Problem: Review of Challenges at Advanced Process Nodes and Solutions
Published in IETE Technical Review, 2019
Sneh Saurabh, Hitarth Shah, Shivendra Singh
Front end of line (FEOL) process variations: The variations in FEOL processes impact electrical properties of transistors and are due to line edge roughness (LER), random dopant fluctuations (RDFs), metal-gate work function variations (WFV), gate dielectric thickness () variations, effective channel length () variations, fin width variations (for FinFETs), source/drain length variations and spacer length variations [32,33]. These variation sources become dominant at advanced processes due to the small size of device structures. Furthermore, sensitivities of the electrical parameter to the process variations increase for smaller devices due to drain-induced barrier lowering (DIBL), velocity overshoot, etc. [34].Back end of line (BEOL) process variations: The variations in BEOL processes become crucial at advanced process nodes due to increased contribution of the interconnect resistance and capacitance (RC) to the delay of a circuit [31,35]. The process variations in interconnects are primarily manifested as variations in thickness ( tconn) and width (wconn) of interconnect and thickness of dielectric layer (tlayer) between consecutive layers of interconnect [36]. Therefore, BEOL variations are expected to result in variations in delay of a circuit [20,36].