Historically, ICs have followed Moore’s Law, doubling logic capacity every two years while costing half as much with each new generation of process technology. Today, Moore’s Law is suffering from mounting die costs, while no longer delivering traditional performance increases or keeping pace with the growing demand for increased bandwidth and capacity.
The latest technologies and approaches have set out to achieve ‘more than Moore’ to enable next-generation silicon scaling at the current-generation process technology. The process of making chips smaller and smaller is becoming increasingly complex, utilising the most advanced capabilities of physics, chemistry, electrical and mechanical engineering among other disciplines.
Every aspect of chip manufacturing latest in Chip fabrication technology is continually improved to manufacture smaller devices with better performance. Most semiconductor manufacturers and their equipment and materials suppliers spend a significant portion of their revenues on research and development.
New technology advancements
A ‘triple threat’ of new complex technology advancements faces the industry—three-dimensional interconnect (3D IC), extreme-ultraviolet lithography (EUV) and 450mm wafers. Despite highly-evolved industry collaboration processes such as the International Technology Roadmap for Semiconductors (ITRS), SEMI International Standards and the growth of international research consortiums, major process milestones in lithography, wafer sizes and new transistor architectures remain unclear and have made coordinated industry planning more necessary than ever before.
The most important challenges in semiconductor manufacturing are the move to larger (450mm) wafers, the commercialisation of EUV, and various ways to utilise three-dimensional (3D) device structures and packaging technologies.
Move to 450mm wafers
Periodically, the industry adopts a larger wafer size standard to improve the efficiency and affordability of manufacturing. These manufacturers claim that larger wafers are necessary to keep pace with Moore’s Law’s cost targets.
The industry-wide cost of R&D to transition to a larger wafer size (including the development of new manufacturing tools) is estimated between $16 billion and $30 billion. Some chipmakers have already committed to advancing this transition.
Intel has announced that its D1X fab in Oregon will be 450mm-compatible. The fab is expected to complete in 2013. Taiwan Semiconductor Manufacturing Company (TSMC) plans to build a 450mm pilot line by 2013-14. Interuniversity Microelectronics Centre, Belgium (IMEC) and International SEMATECH Manufacturing Initiative (ISMI) have well-established programmes focused on the challenges posed by manufacturing with 450mm wafers, and Albany, NY based Colleges of Nanoscale Science and Engineering at SUNY Polytechnic Institute is expanding facilities to encompass 450mm R&D programme.
EUV lithography rollout
EUV is a widely debated transition. Moving from deep-ultraviolet lithography to extreme ultraviolet lithography is a major shift. Industry players continue to make tremendous investments to develop the next-generation technology that will scale devices to smaller dimensions. The EUV lithography rollout is complex because it is intertwined with the 450mm planning challenge—reinforcing the need for more transparency in planning and funding.
Emerging architectures and technologies
Traditional planar CMOS transistors start to run into severe electrical current leakage issues past sub-20nm nodes. To overcome these issues, manufacturers of both logic and memory devices are looking at a variety of alternatives.
For logic devices, two alternatives have come to the forefront: Fin-type field-effect transistors or FinFETs, and ultra-thin-body silicon-on-insulator or UTB-SOIs (also referred to as fully depleted or FD-SOIs). Intel has already announced its first FinFET device. Other device manufacturers continue to look at both the alternatives.
Memory device manufacturers are looking at alternatives to today’s charge-based memories including resistive RAMs, spin-torque transfer RAMs and phase-change memories.
These new memories will increase the speed of memory devices while at the same time lowering the overall power requirements.
3D and 2.5D packaging
3D interconnects using through-silicon vias are critical for meeting the challenging performance and power target demands of wireless, networking, computing and graphics devices, not to mention the consumer demands for small portable devices.
3D packaging saves space by stacking chips and using through-silicon vias to connect the chips as opposed to traditional wire-bonding methods. The chips can be of the same type as in hybrid memory cubes, or an integrated stack of heterogeneous devices.
2011 was a breakthrough year for 3D ICs, as companies like IBM, Micron, Samsung, ST-Ericsson and Xilinx grabbed headlines for deliver-ing the industry’s first 3D ICs based on through-silicon via technology or disclosing their 3D IC R&D plans.
Specifically, the benefits of 3D IC innovation are:
1. Connectivity. 3D enables thousands of low-latency, low-power die-to-die connections that provide 100x bandwidth/watt of conventional printed circuit boards (PCBs) or multi-chip modules.
2. Capacity. No longer limited by the die size and defect density that Moore’s Law dictates, multiple smaller dies can be inter-connected, and greater capacity and higher yield achieved than with a monolithic IC.
3. Crossovers. With 3D one can mix and match different types of dies (processors, memories, field-programmable gate arrays (FPGAs) and analogue) in order to produce system-on-chips (SoCs) that were never before possible.
Another similar packaging technol-ogy which is gaining steam is 2.5D, where die slices are mounted side-by-side on a passive silicon interposer substrate that is used just to connect the active dies. 2.5D offers many of the benefits of full 3D without challenges like complex design rules, co-design of the different stacked dies, heat dissipation and integration of through-silicon vias with deep submicron active devices.
Both 3D and 2.5D packaging technologies have their own advantages and disadvantages depending on the application. For instance, while 3D offers significant benefits, it also presents new design and manufacturing challenges. The precision alignment of through-silicon vias and solder microbumps is critical during assembly. The differing thermal coefficients of silicon substrates and microbumps cause reliability challenges because of mechanical stress and cracking.
Notwithstanding, both 3D and 2.5D are likely to have a long future.
Semiconductor manufacturing is very expensive. In general, for 300mm wafer sizes using leading-edge technology, foundries cost $3-5 billion with capacities up to 40-50k per month. For memories, the cost is $8-11 billion with capacities up to 200-250k per month. Equipment account for majority of the cost. Front-end equipment are much more expensive than back-end equipment as these add more value to the final semiconductor product.
‘Fabless’ semiconductor companies (which design chips but don’t have their own manufacturing facilities) outsource the fabrication of the devices to a semiconductor foundry. The ‘foundry’ is a semiconductor fab that fabricates the designs of other companies.
Increasingly, there’s an industry shift towards the foundry-based model. Instead of designing and manufacturing chips themselves, today, more companies choose to focus on only the chip design to avoid the expensive R&D and equipment costs associated with manufacturing. While this improves efficiencies, lowers costs and increases yields, it will also lead to even more industry consolidation. Currently, only fifteen companies manufacture devices at 32nm and below (this does not include R&D and pilot lines). This compares to 28 companies, which were ‘leading edge’ in 2001 (manufacturing on 200mm wafers—150nm and 130nm). And it is highly unlikely that any new company will enter the market.
The big issue for all the semiconductor manufacturing companies is “Where is the return on investment (ROI)?” The cost for 3D with throughsilicon via (TSV) will be compared to the costs of the manufacturing facilities for the next node. Fab costs for the 22nm node are expected to approach $6 billion, so few logic or memory integrated device manufacturers or foundries will be able to afford such costs.
By mid-2013 to early 2014, a preliminary 450mm R&D production line will be established at the Colleges of Nanoscale Science and Engineering at SUNY Polytechnic Institute at Albany, New York. The facility will be the focus of the recently-announced Global 450 Consortium (G450C) efforts and is expected to contain 50 different tool types. The objective of the pilot line will be to develop data to support the purchase of production-line tools. Many believe that once 450mm enters full production, further 300mm node development will cease for tool suppliers due to limited resources and poor ROI.
Once 450mm is fully developed, spare capacity in 300mm will emerge, encouraging a migration from 200mm production, impacting the viability and competitiveness of both 200mm and 300mm fabs. In 15-20 years, even low-volume, mature technologies in micro-electromechanical systems (MEMS), power and analogue could migrate to 450mm fabs.
Advancements in 2.5D and 3D technologies will open up new market opportunities and enable a new level of system-level integration never before possible on silicon platforms. But there are still some hurdles to overcome for 3D IC design and manufacturing to become mainstream—both technical and business related. For example, the semiconductor industry must come together to set standards for design enablement, manufacturing, test and chip-to-chip interfaces.
The interposer-based active-on-pas-sive 2.5D stacked silicon interconnect (SSI) technology could be a lower-risk alternative to full 3D—especially for FPGAs—that will offer power, cost and reliability advantages for many years to come.
Heading into 2012
The industry is at an exciting crossroads. All these developments are good for consumers, who thrive on ‘smaller, faster, cheaper’ chips which power the smaller, faster portable electronic devices that power the world we live in. The semiconductor industry will continue to deliver what the consumer wants and needs!
Uma Gupta is an executive editor at EFY