# A 28-nm CMOS Low-Power/Low-Voltage 60-GHz LNA for High-Speed Communication
## 3 Integration Technology and Passive Component Design
### Passive Component Design  
Integrated transformers are crucial for the design of the proposed LNA. Generally, stacked windings provide the highest magnetic coupling factor, \(k\), and optimal area efficiency. Interleaved or tapped structures suffer from larger footprints and lower \(k\), while interstacked transformers can be effectively implemented if two equal metal layers are available. The choice of the most suitable transformer topology among tapped, interleaved, stacked and interstacked ones depends on the BEOL (i.e., thickness of the metals, intermetal oxide thickness, distance from the substrate, etc.) and circuit specifications. Although ground shields are often included beneath the coils to reduce the substrate losses, their effectiveness is quite poor at mm-wave frequencies and they reduce the self-resonance frequency (_SRF_). For these reasons, no ground shields were adopted [12]. The very first design step was the definition of the transformer topology and winding geometries by means of paper-and-pencil calculations in order to meet circuit specs. Then, transformer optimization was carried out by means of electromagnetic (_EM_) simulations to maximize the magnetic coupling factor, \(k\), and the \(Q\)-factor at 60 GHz, while trading off the \(SRF\). As already explained, the input transformer, \(T_{\text{IN}}\), is responsible for simultaneous noise/input matching based on the impedance transformation _PE-SNIM_ technique, along with ESD protection. Specifically, a turn ratio, \(N\), of about 2 is required to perform impedance transformation from \(R_{\text{S}}\) to 4\(R_{\text{S}}\) (i.e., 50 \(\Omega\) to 200 \(\Omega\)). This means that the secondary coil inductance, \(L_{2}\), should be about four times higher than the primary one \(L_{1}\). Figure 3 depicts the transformer structure used to implement \(T_{\text{IN}}\). It exploits an octagonal shape and the topmost and thickest metal layers, AP and M9 for \(L_{1}\) and \(L_{2}\), respectively, in order to reduce the resistive losses and minimize the capacitive parasitics of the coils toward the substrate, therefore maximizing both the \(Q\)-factor and \(SRF\). The stacked configuration is adopted to take advantage of the high magnetic coupling factor, \(k\), by choosing different metal widths, \(w\), for primary and secondary spirals, as depicted in Figure 3a,b. Indeed, this configuration allows us to use one-turn and two-turn coils for primary and secondary windings, respectively, and thus to implement the turn ratio required for input impedance transformation while preserving the stacked configuration, as shown in Figure 3c. Specifically, the primary coil adopts an alucap width of 8 \(\upmu\)m that helps to improve the \(Q\)-factor without affecting the \(SRF\). A lower width of 3 \(\upmu\)m is instead used for the secondary coil that is built in metal 9. As far as ESD protection is concerned, the adopted 28-nm CMOS technology defines a minimum AP width of 1.44 \(\upmu\)m tolerating an ESD current of 1.3 A for the 2-kV human body model (HBM), which fully complies with the width of the primary coil of \(T_{\text{IN}}\). Moreover, the LNA input is connected to a standard model for the HBM test consisting of a series resistor and a capacitor with the values of 1.5 k\(\Omega\) (average skin resistance) and 100 pF (average body capacitance), respectively [3; 4]. A 2-kV ESD protection is achieved, guaranteeing that the gate voltages of the input pair, \(M_{1,2}\), are well below the breakdown voltage of 1 V.  
Based on 2D EM simulations with ADS MomentumTM, the primary, \(L_{1}\), and secondary, \(L_{2}\), inductance values of transformer \(T_{\text{IN}}\) are 47 pH and 143 pH at 60 GHz, respectively, while the magnetic coupling factor, \(k\), is 0.59. It is well known that minimizing the input matching network loss requires equal resonance frequencies for both primary and secondary windings.