# 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
In this case, regardless of the value of \(k\), the impedance is transferred by the square of the turn ratio, \(N^{2}=L_{2}/L_{1}\), which is about 3 for the adopted \(T_{\text{IN}}\). Therefore, to  
Figure 3: Transformer, \(T_{\text{IN}}\): primary coil, \(L_{1}\) (**a**); secondary coil, \(L_{2}\) (**b**); stacked transformer (**c**). achieve the required impedance transformation, the primary-winding resonance frequency is set a little bit higher than the secondary one. Indeed, in this case, at the expense of slightly higher loss in the network, the impedance is theoretically transferred by the factor of \(\left(\frac{N}{R}\right)^{2}\)[13]. The latter should be higher than 4 to compensate for non-idealities of the transformer and guarantee 1:4 impedance transformation. The first tuning of the input matching network is performed, while fine-tuning is carried out by using final post-layout EM simulations. Both interstage and output transformers, \(T_{\text{INT}}\) and \(T_{\text{OUT}}\), adopt a one-turn octagonal stacked topology by using M9 and AP layers for the primary and secondary spirals, respectively. The stacked configuration guarantees the highest values of the transformer characteristic resistance (\(TCR\)) thanks to a magnetic coupling factor as high as \(k\) values of 0.7, thus enabling better gain values for both stages. It is worth mentioning that due to the different thicknesses of AP and M9, there is a small difference in terms of the inductance and \(Q\)-factor between the primary and the secondary coils. For this reason, the primary coils, \(L_{3}\) and \(L_{5}\), of \(T_{\text{INT}}\) and \(T_{\text{OUT}}\), respectively, are implemented on M9. Finally, the 50-ohm output matching and differential-to-single-ended conversion are achieved by means of the integrated transformer, \(T_{\text{BUF}}\). The latter adopts the same geometry of the input transformer \(T_{\text{IN}}\). A summary of geometrical and EM-extracted parameters of the three integrated transformers is reported in Table 1.  
The performance of the LNA is highly affected by the losses of the LC resonant networks formed by the above-described integrated transformers and capacitors (i.e., \(C_{2}\), \(C_{\text{L2}}\), \(C_{\text{L2}}\) and \(C_{\text{L3}}\)). In the adopted 28-nm CMOS technology, available standard MOM capacitors have very poor \(Q\)-factors at 60 GHz (i.e., about 15) along with very high process variations that impact both gain and frequency bandwidth performance. For this reason, customized differential metal-insulator-metal (MIM) capacitors have been designed to be used instead of MOM capacitors. These MIM capacitors are implemented by means of two top metal layers of AP and M9, as shown in Figure 4. Though not standard, MIM capacitors have been designed to be fully compatible with the TSMC 28-nm process, minimizing reliability risks. The EM-simulated \(Q\)-factor is as high as 100 at 60 GHz, which guarantees reduced losses of the resonant LC networks of the LNA. \begin{table}
\begin{tabular}{l c c c c} \hline \multicolumn{1}{c}{**Parameters**} & \(\boldsymbol{T_{\text{IN}}\) (\(T_{\text{BUF}}\))} & \(\boldsymbol{T_{\text{INT}}}\) & \(\boldsymbol{T_{\text{OUT}}}\) & **Unit** \\ \hline Transformer topology & Stacked & Stacked & Stacked & \\ \hline Primary/secondary inner diameter (d\({}_{IN}\)) & 22 & 58 & 63 & \(\mu\)m \\ \hline Primary/secondary metal width (\(w\)) & 8/3 & 3/3 & 2/2 & \(\mu\)m \\ \hline Spacing (s) & -/2 & - & - & \(\mu\)m \\ \hline Number of turns (\(n\)) & 1:2 & 1:1 & 1:1 & - \\ \hline Primary coil inductance @ 60 GHz & 47 & 151 & 173 & pH \\ \hline Secondary coil inductance @ 60 GHz & 143 & 155 & 178 & pH \\ \hline Primary coil \(Q\)-factor @ 60 GHz & 15 & 23.6 & 23.5 & - \\ \hline Secondary coil \(Q\)-factor @ 60 GHz & 16 & 19.6 & 19.7 & - \\ \hline Magnetic coupling factor (\(k\)) @ 60 GHz & 0.58 & 0.72 & 0.74 & - \\ \hline Self-resonance frequency (\(SRF\)) & 144 & 121 & 112 & GHz \\ \hline \end{tabular}
\end{table} 
Table 1: Geometrical/electrical parameters of integrated transformers of the LNA.