A three-dimensional integrated circuit (3D IC) is a MOS (metal-oxide semiconductor) integrated circuit (IC) manufactured by stacking silicon wafers or dies and interconnecting them vertically using, for instance, through-silicon vias (TSVs) or Cu-Cu connections, so that they behave as a single device to achieve performance improvements at reduced power and smaller footprint than conventional two dimensional processes. The 3D IC is one of several 3D integration schemes that exploit the z-direction to achieve electrical performance benefits in microelectronics and nanoelectronics.
3D integrated circuits can be classified by their level of interconnect hierarchy at the global (package), intermediate (bond pad) and local (transistor) level. ICs; 3D heterogeneous integration; and 3D systems integration.In general, 3D integration is a broad term that includes such technologies as 3D wafer-level packaging (3DWLP); 2.5D and 3D interposer-based integration; 3D stacked ICs (3D-SICs); monolithic 3D
International organizations such as the Jisso Technology Roadmap Committee (JIC) and the International Technology Roadmap for Semiconductors (ITRS) have worked to classify the various 3D integration technologies to further the establishment of standards and roadmaps of 3D integration. ICs are widely used for NAND flash memory and in mobile devices.As of the 2010s, 3D
3D Packaging refers to 3D integration schemes that rely on traditional methods of interconnect such as wire bonding and flip chip to achieve vertical stacks. 3D packaging can be disseminated further into 3D system in package (3D SiP) and 3D wafer level package (3D WLP), Stacked memory die interconnected with wire bonds, and package on package (PoP) configurations interconnected with either wire bonds, or flip chips are 3D SiPs that have been in mainstream manufacturing for some time and have a well established infrastructure. PoP is used for vertically integrating disparate technologies such as 3D WLP uses wafer level processes such as redistribution layers (RDL) and wafer bumping processes to form interconnects.
2.5D interposer is also a 3D WLP that interconnects die side-side on a silicon, glass or organic interposer using TSVs and RDL. In all types of 3D Packaging, chips in the package communicate using off-chip signaling, much as if they were mounted in separate packages on a normal circuit board.
3D ICs can be divided into 3D Stacked ICs (3D SIC), which refers to stacking IC chips using TSV interconnects, and monolithic 3D ICs, which use fab processes to realize 3D interconnects at the local levels of the on-chip wiring hierarchy as set forth by the ITRS, this results in direct vertical interconnects between device layers. The first examples of a monolithic approach are seen in Samsung's 3D V-NAND devices.
As of the 2010s, 3D IC packages are widely used for NAND flash memory in mobile devices.
The digital electronics market requires a higher density semiconductor memory chip to cater to recently released CPU components, and the multiple die stacking technique has been suggested as a solution to this problem. JEDEC disclosed the upcoming DRAM technology includes the "3D SiC" die stacking plan at "Server Memory Forum", November 1–2, 2011, Santa Clara, CA. In August 2014, Samsung Electronics started producing 64 GB SDRAM modules for servers based on emerging DDR4 (double-data rate 4) memory using 3D TSV package technology. Newer proposed standards for 3D stacked DRAM include Wide I/O, Wide I/O 2, Hybrid Memory Cube, High Bandwidth Memory.
Monolithic 3D ICs are built in layers on a single semiconductor wafer, which is then diced into 3D ICs. There is only one substrate, hence no need for aligning, thinning, bonding, or through-silicon vias. Process temperature limitations are addressed by partitioning the transistor fabrication to two phases. A high temperature phase which is done before layer transfer followed by a layer transfer using ion-cut, also known as layer transfer, which has been used to produce Silicon on Insulator (SOI) wafers for the past two decades. Multiple thin (10s–100s nanometer scale) layers of virtually defect-free Silicon can be created by utilizing low temperature (<400℃) bond and cleave techniques, and placed on top of active transistor circuitry. Follow by finalizing the transistors using etch and deposition processes. This monolithic 3D IC technology has been researched at Stanford University under a DARPA-sponsored grant.
CEA-Leti is also developing monolithic 3D IC approaches, called sequential 3D IC. In 2014, the French research institute introduced its CoolCube™, a low-temperature process flow that provides a true path to 3DVLSI. At Stanford University, researchers are designing monolithic 3D ICs using carbon nanotube (CNT) structures vs. silicon using a wafer-scale low temperature CNT transfer processes that can be done at 120℃.
In general, monolithic 3D ICs are still a developing technology and are considered by most to be several years away from production.
There are several methods for 3D IC design, including recrystallization and wafer bonding methods. There are two major types of wafer bonding, Cu-Cu connections (copper-to-copper connections between stacked ICs, used in TSVs) IC stacking with TSVs. There are a number of key stacking approaches being implemented and explored. These include die-to-die, die-to-wafer, and wafer-to-wafer.and through-silicon via (TSV). As of 2014, a number of memory products such as High Bandwidth Memory (HBM) and the Hybrid Memory Cube have been launched that implement 3D
While traditional CMOS scaling processes improves signal propagation speed, scaling from current manufacturing and chip-design technologies is becoming more difficult and costly, in part because of power-density constraints, and in part because interconnects do not become faster while transistors do. ICs address the scaling challenge by stacking 2D dies and connecting them in the 3rd dimension. This promises to speed up communication between layered chips, compared to planar layout. 3D ICs promise many significant benefits, including:3D
Because this technology is new it carries new challenges, including:
Depending on partitioning granularity, different design styles can be distinguished. Gate-level integration faces multiple challenges and currently appears less practical than block-level integration.
Several years after the MOS integrated circuit (MOS IC) chip was first proposed by Mohamed Atalla at Bell Labs in 1960,the concept of a three-dimensional MOS integrated circuit was proposed by Texas Instruments researchers Robert W. Haisty, Rowland E. Johnson and Edward W. Mehal in 1964. In 1969, the concept of a three-dimensional MOS integrated circuit memory chip was proposed by NEC researchers Katsuhiro Onoda, Ryo Igarashi, Toshio Wada, Sho Nakanuma and Toru Tsujide.
3D ICs were first successfully demonstrated in 1980s Japan, where research and development (R&D) on 3D ICs was initiated in 1981 with the "Three Dimensional Circuit Element R&D Project" by the Research and Development Association for Future (New) Electron Devices. There were initially two forms of 3D IC design being investigated, recrystallization and wafer bonding, with the earliest successful demonstrations using recrystallization. In October 1983, a Fujitsu research team including S. Kawamura, Nobuo Sasaki and T. Iwai successfully fabricated a three-dimensional complementary metal-oxide-semiconductor (CMOS) integrated circuit, using laser beam recrystallization. It consisted of a structure in which one type of transistor is fabricated directly above a transistor of the opposite type, with separate gates and an insulator in between. A double-layer of silicon nitride and phosphosilicate glass (PSG) film was used as an intermediate insulating layer between the top and bottom devices. This provided the basis for realizing a multi-layered 3D device composed of vertically-stacked transistors, with separate gates and an insulating layer in between. In December 1983, the same Fujitsu research team fabricated a 3D integrated circuit with a silicon-on-insulator (SOI) CMOS structure. The following year, they fabricated a 3D gate array with vertically-stacked dual SOI/CMOS structure using beam recrystallization.
In 1986, Mitsubishi Electric researchers Yoichi Akasaka and Tadashi Nishimura laid out the basic concepts and proposed technologies for 3D ICs. The following year, a Mitsubishi research team including Nishimura, Akasaka and Osaka University graduate Yasuo Inoue fabricated an image signal processor (ISP) on a 3D IC, with an array of photosensors, CMOS A-to-D converters, arithmetic logic units (ALU) and shift registers arranged in a three-layer structure. In 1989, an NEC research team led by Yoshihiro Hayashi fabricated a 3D IC with a four-layer structure using laser beam crystallisation. In 1990, a Matsushita research team including K. Yamazaki, Y. Itoh and A. Wada fabricated a parallel image signal processor on a four-layer 3D IC, with SOI (silicon-on-insulator) layers formed by laser recrystallization, and the four layers consisting of an optical sensor, level detector, memory and ALU.
The most common form of 3D IC design is wafer bonding. ICs with more than three active layers.Wafer bonding was initially called "cumulatively bonded IC" (CUBIC), which began development in 1981 with the "Three Dimensional Circuit Element R&D Project" in Japan and was completed in 1990 by Yoshihiro Hayashi's NEC research team, who demonstrated a method where several thin-film devices are bonded cumulatively, which would allow a large number of device layers. They proposed fabrication of separate devices in separate wafers, reduction in the thickness of the wafers, providing front and back leads, and connecting the thinned die to each other. They used CUBIC technology to fabricate and test a two active layer device in a top-to-bottom fashion, having a bulk-Si NMOS FET lower layer and a thinned NMOS FET upper layer, and proposed CUBIC technology that could fabricate 3D
The first 3D IC stacked chips fabricated with a through-silicon via (TSV) process were invented in 1980s Japan. Hitachi filed a Japanese patent in 1983, followed by Fujitsu in 1984. In 1986, a Japanese patent filed by Fujitsu described a stacked chip structure using TSV. IC chips using TSV technology, called the "R&D on High Density Electronic System Integration Technology" project. The term "through-silicon via" (TSV) was coined by Tru-Si Technologies researchers Sergey Savastiouk, O. Siniaguine, and E. Korczynski, who proposed a TSV method for a 3D wafer-level packaging (WLP) solution in 2000.In 1989, Mitsumasa Koyonagi of Tohoku University pioneered the technique of wafer-to-wafer bonding with TSV, which he used to fabricate a 3D LSI chip in 1989. In 1999, the Association of Super-Advanced Electronics Technologies (ASET) in Japan began funding the development of 3D
The Koyanagi Group at Tohoku University, led by Mitsumasa Koyanagi, used TSV technology to fabricate a three-layer memory chip in 2000, a three-layer artificial retina chip in 2001, a three-layer microprocessor in 2002, and a ten-layer memory chip in 2005.The same year, a Stanford University research team consisting of Kaustav Banerjee, Shukri J. Souri, Pawan Kapur and Krishna C. Saraswat presented a novel 3D chip design that exploits the vertical dimension to alleviate the interconnect related problems and facilitates heterogeneous integration of technologies to realize a system-on-a-chip (SoC) design.
In 2001, a Toshiba research team including T. Imoto, M. Matsui and C. Takubo developed a "System Block Module" wafer bonding process for manufacturing 3D IC packages.
Fraunhofer and Siemens began research on 3D IC integration in 1987. In 1988, they fabricated 3D CMOS IC devices based on re-crystallization of poly-silicon. In 1997, the inter-chip via (ICV) method was developed by a Fraunhofer–Siemens research team including Peter Ramm, Manfred Engelhardt, Werner Pamler, Christof Landesberger and Armin Klumpp. It was a first industrial 3D IC process, based on Siemens CMOS fab wafers. A variation of that TSV process was later called TSV-SLID (solid liquid inter-diffusion) technology. It was an approach to 3D IC design based on low temperature wafer bonding and vertical integration of IC devices using inter-chip vias, which they patented.
Ramm went on to develop industry-academic consortia for production of relevant 3D integration technologies. In the German funded cooperative VIC project between Siemens and Fraunhofer, they demonstrated a complete industrial 3D IC stacking process (1993–1996). With his Siemens and Fraunhofer colleagues, Ramm published results showing the details of key processes such as 3D metallization [T. Grassl, P. Ramm, M. Engelhardt, Z. Gabric, O. Spindler, First International Dielectrics for VLSI/ULSI Interconnection Metallization Conference – DUMIC, Santa Clara, CA, 20-22 Feb, 1995] and at ECTC 1995 they presented early investigations on stacked memory in processors.
In the early 2000s, a team of Fraunhofer and Infineon Munich researchers investigated 3D TSV technologies with particular focus on die-to-substrate stacking within the German/Austrian EUREKA project VSI and initiated the European Integrating Projects e-CUBES, as a first European 3D technology platform, and e-BRAINS with a.o., Infineon, Siemens, EPFL, IMEC and Tyndall, where heterogeneous 3D integrated system demonstrators were fabricated and evaluated. A particular focus of the e-BRAINS project was the development of novel low-temperature processes for highly reliable 3D integrated sensor systems.
Copper-to-copper wafer bonding, also called Cu-Cu connections or Cu-Cu wafer bonding, was developed at MIT by a research team consisting of Andy Fan, Adnan-ur Rahman and Rafael Reif in 1999. –2002. In 2003, DARPA and the Microelectronics Center of North Carolina (MCNC) began funding R&D on 3D IC technology.Reif and Fan further investigated Cu-Cu wafer bonding with other MIT researchers including Kuan-Neng Chen, Shamik Das, Chuan Seng Tan and Nisha Checka during 2001
In 2004, Tezzaron Semiconductorbuilt working 3D devices from six different designs. The chips were built in two layers with "via-first" tungsten TSVs for vertical interconnection. Two wafers were stacked face-to-face and bonded with a copper process. The top wafer was thinned and the two-wafer stack was then diced into chips. The first chip tested was a simple memory register, but the most notable of the set was an 8051 processor/memory stack that exhibited much higher speed and lower power consumption than an analogous 2D assembly.
In 2004, Intel presented a 3D version of the Pentium 4 CPU.The chip was manufactured with two dies using face-to-face stacking, which allowed a dense via structure. Backside TSVs are used for I/O and power supply. For the 3D floorplan, designers manually arranged functional blocks in each die aiming for power reduction and performance improvement. Splitting large and high-power blocks and careful rearrangement allowed to limit thermal hotspots. The 3D design provides 15% performance improvement (due to eliminated pipeline stages) and 15% power saving (due to eliminated repeaters and reduced wiring) compared to the 2D Pentium 4.
The Teraflops Research Chip introduced in 2007 by Intel is an experimental 80-core design with stacked memory. Due to the high demand for memory bandwidth, a traditional I/O approach would consume 10 to 25 W. To improve upon that, Intel designers implemented a TSV-based memory bus. Each core is connected to one memory tile in the SRAM die with a link that provides 12 GB/s bandwidth, resulting in a total bandwidth of 1 TB/s while consuming only 2.2 W.
An academic implementation of a 3D processor was presented in 2008 at the University of Rochester by Professor Eby Friedman and his students. The chip runs at a 1.4 GHz and it was designed for optimized vertical processing between the stacked chips which gives the 3D processor abilities that the traditional one layered chip could not reach. One challenge in manufacturing of the three-dimensional chip was to make all of the layers work in harmony without any obstacles that would interfere with a piece of information traveling from one layer to another.
In ISSCC 2012, two 3D-IC-based multi-core designs using GlobalFoundries' 130 nm process and Tezzaron's FaStack technology were presented and demonstrated:
The earliest known commercial use of a 3D IC chip was in Sony's PlayStation Portable (PSP) handheld game console, released in 2004. The PSP hardware includes eDRAM (embedded DRAM) memory manufactured by Toshiba in a 3D system-in-package chip with two dies stacked vertically. Toshiba called it "semi-embedded DRAM" at the time, before later calling it a stacked "chip-on-chip" (CoC) solution.
In April 2007, Toshiba commercialized an eight-layer 3D IC, the 16 GB THGAM embedded NAND flash memory chip, which was manufactured with eight stacked 2 GB NAND flash chips. In September 2007, Hynix introduced 24-layer 3D IC technology, with a 16 GB flash memory chip that was manufactured with 24 stacked NAND flash chips using a wafer bonding process. Toshiba also used an eight-layer 3D IC for their 32 GB THGBM flash chip in 2008. In 2010, Toshiba used a 16-layer 3D IC for their 128 GB THGBM2 flash chip, which was manufactured with 16 stacked 8 GB chips. In the 2010s, 3D ICs came into widespread commercial use in the form of multi-chip package and package on package solutions for NAND flash memory in mobile devices.
Elpida Memory developed the first 8 GB DRAM chip (stacked with four DDR3 SDRAM dies) in September 2009, and released it in June 2011. TSMC announced plans for 3D IC production with TSV technology in January 2010. In 2011, SK Hynix introduced 16 GB DDR3 SDRAM (40 nm class) using TSV technology, Samsung Electronics introduced 3D-stacked 32 GB DDR3 (30 nm class) based on TSV in September, and then Samsung and Micron Technology announced TSV-based Hybrid Memory Cube (HMC) technology in October.
High Bandwidth Memory (HBM), developed by Samsung, AMD, and SK Hynix, uses stacked chips and TSVs. The first HBM memory chip was manufactured by SK Hynix in 2013. GB per stack.In January 2016, Samsung Electronics announced early mass production of HBM2, at up to 8
In 2017, Samsung Electronics combined 3D IC stacking with its 3D V-NAND technology (based on charge trap flash technology), manufacturing its 512 GB KLUFG8R1EM flash memory chip with eight stacked 64-layer V-NAND chips. In 2019, Samsung produced a 1 TB flash chip with 16 stacked V-NAND dies. As of 2018, Intel is considering the use of 3D ICs to improve performance. As of April 2019, memory devices with 96-layer chips can be bought from more than one manufacturer; with Toshiba having made 96-layer devices in 2018.
An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny MOS transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to integrated circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.
Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically the metal–oxide–semiconductor (MOS) devices used in the integrated circuit (IC) chips that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.
Very large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining millions of MOS transistors onto a single chip. VLSI began in the 1970s when MOS integrated circuit chips were widely adopted, enabling complex semiconductor and telecommunication technologies to be developed. The microprocessor and memory chips are VLSI devices. Before the introduction of VLSI technology, most ICs had a limited set of functions they could perform. An electronic circuit might consist of a CPU, ROM, RAM and other glue logic. VLSI lets IC designers add all of these into one chip.
Moore's law is the observation that the number of transistors in a dense integrated circuit (IC) doubles about every two years. Moore's law is an observation and projection of a historical trend. Rather than a law of physics, it is an empirical relationship linked to gains from experience in production.
Flash memory is an electronic (solid-state) non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory are named after the NAND and NOR logic gates. The individual flash memory cells, consisting of floating-gate MOSFETs, exhibit internal characteristics similar to those of the corresponding gates.
In electronics manufacturing, integrated circuit packaging is the final stage of semiconductor device fabrication, in which the block of semiconductor material is encapsulated in a supporting case that prevents physical damage and corrosion. The case, known as a "package", supports the electrical contacts which connect the device to a circuit board.
A hybrid integrated circuit (HIC), hybrid microcircuit, hybrid circuit or simply hybrid is a miniaturized electronic circuit constructed of individual devices, such as semiconductor devices and passive components, bonded to a substrate or printed circuit board (PCB). A PCB having components on a Printed Wiring Board (PWB) is not considered a true hybrid circuit according to the definition of MIL-PRF-38534.
The back end of line (BEOL) is the second portion of IC fabrication where the individual devices get interconnected with wiring on the wafer, the metalization layer. Common metals are copper and aluminum. BEOL generally begins when the first layer of metal is deposited on the wafer. BEOL includes contacts, insulating layers (dielectrics), metal levels, and bonding sites for chip-to-package connections.
A multi-chip module (MCM) is generically an electronic assembly where multiple integrated circuits, semiconductor dies and/or other discrete components are integrated, usually onto a unifying substrate, so that in use it can be treated as if it were a larger IC. Other terms, such as "hybrid" or "hybrid integrated circuit", also refer to MCMs. The individual ICs that make up an MCM are known as chiplets. Intel and AMD are using MCMs to improve performance and reduce costs, as splitting a large monolithic IC into smaller chiplets allows for easy performance improvements, more ICs per wafer, and improved yield, as smaller dies have a reduced risk of getting destroyed by dust particles and process variations during semiconductor fabrication. This approach also allows for chiplets to be reused in several products, reduces the amount of time required to design a module compared to designing a monolithic IC, and also reduces the number of bugs that have to be solved during design. However, communication between chiplets consumes more power and has higher latency than components in monolithic ICs. Each chiplet is physically smaller than a conventional monolithic IC die,. An example of MCMs in use for mainstream CPUs is AMD's Zen 2 design.
Integrated circuit design, or IC design, is a subset of electronics engineering, encompassing the particular logic and circuit design techniques required to design integrated circuits, or ICs. ICs consist of miniaturized electronic components built into an electrical network on a monolithic semiconductor substrate by photolithography.
A system in a package (SiP) or system-in-package is a number of integrated circuits enclosed in one or more chip carrier packages that may be stacked using package on package. The SiP performs all or most of the functions of an electronic system, and is typically used inside a mobile phone, digital music player, etc. Dies containing integrated circuits may be stacked vertically on a substrate. They are internally connected by fine wires that are bonded to the package. Alternatively, with a flip chip technology, solder bumps are used to join stacked chips together. A SiP is like a system on a chip (SoC) but less tightly integrated and not on a single semiconductor die.
The transistor count is the number of transistors in an electronic device. It typically refers to the number of MOSFETs on an integrated circuit (IC) chip, as all modern ICs use MOSFETs. It is the most common measure of IC complexity. The rate at which MOS transistor counts have increased generally follows Moore's law, which observed that the transistor count doubles approximately every two years.
A die, in the context of integrated circuits, is a small block of semiconducting material on which a given functional circuit is fabricated. Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon (EGS) or other semiconductor through processes such as photolithography. The wafer is cut (diced) into many pieces, each containing one copy of the circuit. Each of these pieces is called a die.
Package on package (PoP) is an integrated circuit packaging method to combine vertically discrete logic and memory ball grid array (BGA) packages. Two or more packages are installed atop each other, i.e. stacked, with a standard interface to route signals between them. This allows higher component density in devices, such as mobile phones, personal digital assistants (PDA), and digital cameras, at the cost of slightly higher height requirements. Stacks with more than 2 packages are uncommon, due to heat dissipation considerations.
In electronic engineering, a through-silicon via (TSV) or through-chip via is a vertical electrical connection (via) that passes completely through a silicon wafer or die. TSVs are high-performance interconnect techniques used as an alternative to wire-bond and flip chips to create 3D packages and 3D integrated circuits. Compared to alternatives such as package-on-package, the interconnect and device density is substantially higher, and the length of the connections becomes shorter.
Integrated Passive Devices (IPD's) "or Integrated Passive Components (IPC's) or Embedded Passive Components" are electronic components where resistors (R), capacitors (C), inductors(L)/coils/chokes, microstriplines, impedance matching elements, baluns or any combinations of them are integrated in the same package or on the same substrate. Sometimes integrated passives can also be called as embedded passives, and still the difference between integrated and embedded passives is technically unclear, . In both cases passives are realised in between dielectric layers or on the same substrate.
Wafer-level packaging (WLP) is the technology of packaging an integrated circuit while still part of the wafer, in contrast to the more conventional method of slicing the wafer into individual circuits (dice) and then packaging them. WLP is essentially a true chip-scale package (CSP) technology, since the resulting package is practically of the same size as the die. Wafer-level packaging allows integration of wafer fab, packaging, test, and burn-in at wafer level in order to streamline the manufacturing process undergone by a device from silicon start to customer shipment.
High Bandwidth Memory (HBM) is a high-speed computer memory interface for 3D-stacked SDRAM from Samsung, AMD and SK Hynix. It is used in conjunction with high-performance graphics accelerators, network devices and in some supercomputers. The first HBM memory chip was produced by SK Hynix in 2013, and the first devices to use HBM were the AMD Fiji GPUs in 2015.
A 2.5D integrated circuit combines multiple integrated circuit dies in a single package without stacking them into a three-dimensional integrated circuit (3D-IC) with through-silicon vias (TSVs). The term “2.5D” originated when 3D-ICs with TSVs were quite new and still horrendously difficult. Chip designers realized that many of the advantages of 3D integration could be approximated by placing bare dies side by side on an interposer instead of stacking them vertically. If the pitch is very fine and the interconnect very short, the assembly can be packaged as a single component with better size, weight, and power characteristics than a comparable 2D circuit board assembly. This half-way 3D integration was facetiously named “2.5D” and the name stuck. Since those early days, 2.5D has proven to be far more than just “half-way to 3D.” Some benefits:
Glossary of Microelectronics Manufacturing Terms