A Comparative Analysis between Standard and mm-Wave Optimized BEOL in a Nanoscale CMOS Technology

Abstract

This paper presents an extensive comparison of two 28-nm CMOS technologies, i.e., standard and mm-wave-optimized (i.e., thick metals and intermetal oxides) back-end-of-line (BEOL). The proposed comparison is carried out at both component and circuit level by means of a quantitative analysis of the actual performance improvements due to the adoption of a mm-wave-optimized BEOL. To this end, stand-alone transformer performance is first evaluated and then a complete mm-wave macroblock is investigated. A 77-GHz down-converter for frequency modulated continuous wave (FMCW) long-range/medium range (LR/MR) radar applications is exploited as a testbench. For the first time, it is demonstrated that thicker metals and intermetal oxides do not guarantee significant improvements at mm-wave frequencies and a standard (low-cost) BEOL is competitive in comparison with more complex (expensive) ones.

Interstacked Transformer Quad-Core VCOs

“…To better understand the geometry of the transformer, Figure 4b Figure 3. BEOL of the adopted 28 nm CMOS technology with eight metal layers [15,32].…”

“…A quad-core VCO to be used in a narrowband 77 GHz CMOS radar system for automotive applications [15] was designed in 28 nm fully depleted silicon on insulator (FD-SOI) CMOS technology by STMicroelectronics, exploiting the four-port interstacked transformer shown in Section 4. The technology provides low-V T transistors, which exhibit a transition frequency, f T , up to 270 GHz [32].…”

“…The technology provides low-V T transistors, which exhibit a transition frequency, f T , up to 270 GHz [32]. The process features a general purpose BEOL with eight copper layers and a top aluminum one [15], as shown in Figure 3.…”

“…Traditionally, RF/mm-wave ICs are implemented in BiCMOS or CMOS technologies [9][10][11][12][13][14]. Despite several advantages of BiCMOS over CMOS in terms of noise (i.e., lower flicker noise corners), thicker back-end-of-line (BEOL) [15], higher transistor breakdown voltage (BV), and lower transconductance, g m , at a given current level, CMOS is becoming the reference process since it is highly suitable for system-on-chip (SoC) integration, which is pursued by microelectronic industries to reduce chip cost, power consumption, and area occupation [16,17]. Unfortunately, the transition from BiCMOS to CMOS requires proper topologies and design approaches for almost all main RF/mmwave front end blocks.…”

Design of phase shifters in nanowire membrane technology and VCOs in CMOS technology at millimeter-waves for future full frontend integration.

“…Figure 31 shows two different ways a BEOL of a standard, digital CMOS technology can be adapted for mmW needs: the first is re-designing the metal and dielectric layers, increasing thicknesses and distances of the topmost layers, as can be seen by comparison of the standard 130-nm CMOS and the BiCMOS 9MW, 130nm technology, both from ST Microelectronics; the second is adding thicker metal layers on top of the BEOL, thus keeping the high-integration capabilities of the lower metals while adding the low-loss metal that mm-wave passive devices need, which can be seen by comparing the BEOL of ST Microelectronics' 65-nm CMOS process and the 55-nm BiCMOS process. In [57], a comparative study was conducted regarding the performance of W-band transformers and the pros and cons of different transformer topologies in two different BEOLs for the same technological node. The technology in question is a 28 nm FD-SOI process by ST Microelectronics using the standard, 8-layer digital BEOL and a mm-wave optimized BEOL, featuring 8 layers of thicker metals and dielectrics.…”