Structure Optimization And Realization Of RF On-chip Spiral Inductor

Recently, with the increasing operating frequency of radio frequency (RF) integrated circuits, fabricated in silicon technology, RF on-chip spiral inductors are widely used due to their low cost and ease of process integration. They are used as part of circuits such as low-noise amplifiers, matching networks, filters, and voltage controlled oscillators. The behavior of on-chip inductors is a dominant factor in overall circuit performance. So the optimization of quality factor becomes the key point of inductor research. The research in this paper is on the improvement of quality factor (Q ) of inductor by layout optimization.A group of inductors are simulated and the obtained results show that optimizing the ratio of metal strip width to space (R_ws) can improve the performance of inductor significantly. Moreover, the magnetic field intensity of a spiral inductor increasing gradually from outside to inside. The magnetically induced losses depend on the magnetic-field flux through the metal strip. And the associated magnetic field has a maximum intensity in the center of the coil. Consequently, a narrow metal strip width is expected to decrease the magnetically induced losses, especially in the center of the coil. However, for the sheet resistance of the metal strip is inversely proportional to the metal strip width, a wide metal strip width is expected to decrease the ohm losses. This gradually changed structure gives attention to both magnetically induced losses and ohm losses and gets higher Q factor.Based on all of the above, on-chip inductor with novel gradually changed layout structure has been presented in this paper. In this structure, the sum of metal strip width and space of each coil is fixed while the ratio of the metal strip width to space is gradually reduced from outside to inside. Some valuable design rules have been obtained through a large number of data analyses. Inductor with this gradually changed layout structure obtains better performance than the conventional inductor with fixed metal width and space and it is also better than the single gradually changed inductor with fixed space and gradually changed metal width.Based on these analyses, inductors on high-resistivity silicon substrate have been fabricated. The experimental results have also corroborated the validity of the proposed method. For the 7-nH gradually changed inductor Q8-3, whose R_wsi changes from 4 to 1, Q of 10 at 2.1GHz is 23.5% higher than the conventional inductor and it is also 13.6% higher than the single gradually changed inductor. Same results are also obtained in the other groups. Moreover, the proposed optimizing method is relatively easy to implement without any additional process cost and IC designers will benefit from them.