Comparison of small cross inductors and rectangular inductors designed in 0.35um CMOS technology

Abstract

This paper describes the design, and experimental characterization of cross inductors in a 0.35µm CMOS technology. The Sonnet tools were used to simulate and evaluate the performance of the new inductor structure, whose core has perpendicular crossed segments at the two highest layers available of the technology. Due to this arrangement, the positive as well as the negative mutual inductance are almost null. To make a fair comparison, the new structure and conventional rectangular inductor were designed to get similar inductance value, and with the same segment width and spacing fixed at 10µm. The proposed cross structure occupied an area of 160x140µm 2 while the square planar inductor presents an area of 180x180µm 2 . Thus, the last one occupies 45% more area than the cross inductor. Some deembedding structures were fabricated and used to subtract the effect of the test-fixture in the on-wafer measurements. The main experimental results are: for cross inductor an inductance value of 2.1nH and Q of 3.3 at 5.2GHz and for the square one an inductance of 1.9nH and Q of 5.5 at 5.5GHz. Although, for similar inductance value, the quality factor of the cross inductor is lower than for the rectangular inductor one, the new cross inductor structure shows to be useful at RF circuits where the inductors are largely used and the most area consuming components.

Analysis of the effects of coupling through substrate and the calculus of the Q factor

“…As presented in [8], the advantage of using diagonal segments in the cross inductor is to increase the inductance of the device by means of increasing the number of turns. As a consequence, the coupling effects trough the substrate increase because of the 3 metal levels employed.…”

“…As explained in many works, a bottle neck in the design of radio frequency (RF) and monolithic microwave integrated circuits (MMIC), is the inductor [1][2][3][4]. Nowadays, inductor design is based on improve the quality factor (Q) using the lower area, as possible [1,2,[5][6][7][8]. To achieve a larger Q, more turns in the design of the inductor needs to be incorporated using more than one metal level.…”

Projeto e modelagem de indutores planares para aplicações em circuitos integrados de radiofrequência.

“…O primeiro conjunto de testes comparou os resultados apresentados em [45] para um indutor espiral e um cross, ambos com terminais opostos, com seus análogos projetados através das equações de projeto, enquanto que o segundo teste envolveu o projeto de indutores espirais e cross de terminais paralelos com dimensões diferentes daquelas usadas na elaboração das equações de projeto.…”

“…Um dos objetivos desta tese é prover um conjunto de equações de projeto para o indutor cross proposto por Moreira [28] , assim, neste teste serão comparados os resultados das equações de projeto para um indutor espiral e um indutor cross com as mesmas dimensões, arquitetura e tecnologia de fabricação dos indutores apresentados em [45] e seus respectivos resultados.…”

“…De acordo com [45] , dois indutores com terminais opostos foram fabricados em tecnologia AMS 0,35 μm, um na arquitetura espiral e outro na arquitetura cross, conforme ilustra a Figura 44, a qual mostra a fotografia do chip.…”