Abstract:
A novel complimentary shielded inductor on a semiconductor is disclosed. A region of electrically floating high resistive material is deposited between the inductor and the semiconductor substrate. The high resistive shield is patterned with a number of gaps, such that a current induced in the shield by the inductor does not have a closed loop path. The high resistive floating shield compliments a grounded low resistive shield to achieve higher performance inductors. In this fashion, noise in the substrate is reduced. The novel complimentary shield does not significantly degrade the figures of merit of the inductor, such as, quality factor and resonance frequency. In one embodiment, the grounded shield is made of patterned N-well (or P-well) structures. In still another embodiment, the low resistive electrically grounded shield is made of patterned Silicide, which may be formed on portions of the substrate itself.

Description:
FIELD OF THE INVENTION 
     The present claimed invention relates to the field of semiconductor processing. More particularly, the present claimed invention relates to a novel low noise on-chip inductor with a shield comprising a grounded low resistive material and an electrically floating high resistive material. 
     BACKGROUND ART 
     Increasing demands for personal mobile communication equipment have motivated recent research activities to focus on the development of inexpensive, small size, low power consumption and low noise level systems. Silicon, with its mature technology, low fabrication cost as well as high packing density is recognized as the only material able to satisfy the needs of this rapidly growing communication market. To fulfill all the above-mentioned requirements, one of the most important and indispensable circuit components is the on-chip silicon-based spiral inductor. 
     Advances in technology are making it possible to develop radio frequency (RF) circuits on a single silicon chip. In radio frequency operation, substrate induced noise inherent in the silicon technology is one of the main factors that limits achieving high performance low noise transceivers. Low Noise Amplifiers (LNA), being the first stage in the receiving path, are an important building block of a single-chip transceiver. The primary goal for the LNA is to minimize the noise figure of the receiver. However, in addition to RF noise from transistors, the thermal noise from the substrate resistance contributes significantly to the overall noise figure of the LNA. On-chip inductors, which are essential circuit components in building LNAs, VCOs etc., often generate significant amounts of such substrate noise that will degrade the overall circuit performance tremendously. 
     Several approaches that help reduce the effect of substrate thermal noise have been reported. However, all have significant problems. One conventional approach makes some improvements by introducing a ground shielded bonding pad structure (see, e.g., A. Rofougaran, J. Y. C. Chang, M. Rofougaran and A. A. Abidi, “A 1 GHz CMOS RF Front-End IC for a Direct Conversion Wireless Receiver, “IEEE Journal of Solid State Circuits, vol.31, pp880-889, 1996). Unfortunately, a ground shield produces a large pad to ground capacitance. 
     Another conventional technique uses thick polyimide inter-level dielectric underneath on-chip inductors (see, e.g., Y. Nakahara, H. Yano, T. Hirayama, Y. Suzuki and A. Furakawa, “Impact of interconnect Capacitance Reduction on RF-Si Device Performance, “IEDM Tech. Dig., pp861-864, 1999). Though significant results may be achieved, tedious, non-conventional and expensive processing steps are required with this method. 
     Hence, the key challenges for building on-chip silicon-based inductors lie not only in achieving high quality factor but also in fabricating inductors having low noise performance. For silicon-based inductors, it has been reported that having Silicide ground shields underneath the inductor improves its noise performance (see, e.g., H. Fujii, H. Suzuki, H. Yoshida and T. Yamazaki, “A0.15 μm/0.6 dB-Nfmin RF BiCMOS Technology using Cobalt Silicide Ground Shields,” IEEE BCTM, pp98-101, 2000). 
     FIG. 1A illustrates a schematic cross section of an on-chip silicon-based inductor  110  without a ground shield. Conventional semiconductor processing uses a very conductive silicon substrate  102 . Without any ground shield, the thermal noise in the substrate  102  may be substantial. FIG. 1B shows a cross section of an inductor  110  and a plate type ground shield  120  made of CoSi 2 . FIG. 1C shows a ground shield patterned by trenches  130 . Using a ground shield, however, leads to an unavoidable increase in the parasitic capacitance, which severely degrades the inductor&#39;s  110  performance. 
     FIG. 2 shows figures of merit for the inductors  110  of FIGS. 1A-1C. FIG. 2 shows the quality factor as a function of frequency for each case. As FIG. 2 shows, the quality factor is lower with the trench ground shield  130  when compared to the unshielded inductor. The quality factor is still lower with the plate shield  120 . Furthermore, FIG. 2 shows that shielding the inductor  110  degrades the resonant frequency. 
     Referring now to FIG.  3 A and FIG. 3B, one conventional method adds a patterned electrically grounded low sheet resistance polysilicon shield  320  between the inductor  110  and the substrate  102 . Studies have been conducted to investigate inductors  110  with such patterned low sheet resistance polysilicon ground shields  320 . FIG. 3A also shows an underpass  111  connected to the inductor  110  by vias  115 . The semiconductor also comprises an inter-metal dielectric layer  322  and a field oxide layer  324 . Unfortunately, this technique may lead to an increase in the parasitic oxide capacitance (C ox ). Effectively, this technique brings a new conductive silicon substrate (e.g., the polysilicon shield  320 ) which is electrically grounded and closer to the inductor  110  than the substrate  102 . As seen, the substrate  102  has its own capacitance C si  and resistance R si . The capacitance C s  between the inductor  100  spiral and underpass  110  is also shown. 
     Additionally, this configuration results in degradation in the inductor&#39;s performance. FIG. 4 shows a plot of the quality factor and inductance of an unshielded inductor with one shielded with low sheet resistance patterned polysilicon. As the graph shows, at 5 GHz there is considerable degradation of the quality factor. Additionally, above 4 GHz the inductance begins to rise significantly due to self resonance at high frequencies. 
     Thus, although low sheet resistance polysilicon or Silicide ground shields may improve the inductor&#39;s noise performance, the quality factor and resonant frequency are degraded. 
     As yet another concern, in order to achieve widespread acceptance, and to ensure affordability, any method of forming an on-chip silicon-based inductor, which overcomes the above-listed drawbacks, should be compatible with existing semiconductor fabrication processes. 
     Thus, a need exists for a shielded on-chip silicon-based inductor having low noise characteristics. A further need exists for such an inductor that does not have a significantly degraded quality factor. A further need exists for an inductor and a method for forming the inductor wherein the inductor and the formation method meet the above needs and are compatible with existing semiconductor fabrication processes such that significant revamping of semiconductor capital equipment is not required. 
     SUMMARY OF INVENTION 
     The present invention provides a shielded on-chip silicon-based inductors having a low substrate thermal noise. The present invention provides for such an inductor that does not have a significantly degraded quality factor. The present invention provides for an inductor and a method for forming the inductor wherein the inductor and the formation method meet the above needs and are compatible with existing semiconductor fabrication processes such that significant revamping of semiconductor capital equipment is not required. 
     A novel complimentary shielded inductor on a semiconductor is disclosed. The inductor is fabricated in one or more layers of a semiconductor. A region of electrically floating high sheet resistance material is deposited between the inductor and the semiconductor substrate. The high resistive shield is patterned with a number of gaps, such that a current induced in the shield by the inductor does not have a closed loop path. The high resistive floating shield compliments a grounded low resistive shield to achieve higher performance inductors. In this fashion, noise in the substrate is reduced. The novel complimentary shield does not significantly degrade the figures of merit of the inductor, such as, quality factor and resonance frequency. 
     In one embodiment, both the high resistive shield and the low resistive shield are patterned to prevent formation of eddy current in the shield. In one embodiment, the grounded low resistive shield, which may be made of silicided polysilicon, is aligned to cover the gaps of the high resistive shield, such that the inductor may not ‘see’ the substrate. 
     In one embodiment, the grounded shield is made of patterned N-well (or P-well) structures. The N-well structures may be aligned such that they compensate for the gaps in the electrically floating high resistive shield. 
     In still another embodiment, the low resistive electrically grounded shield is made of patterned Silicide, which may be formed on portions of the substrate itself. The substrate projects into a portion of a field oxide layer between the rest of the substrate and the high resistive shield. Thus, the low resistive shield is formed to block the gaps in the high resistive shield. 
    
    
     These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrates embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
     FIG. 1A is a side sectional view illustrating a conventional silicon based inductor above a substrate. 
     FIGS. 1B-1C are side sectional views illustrating conventional shields between an inductor and a substrate. 
     FIG. 2 is a graph illustrating the quality factor versus frequency for the conventional inductors of FIGS. 1A-1C. 
     FIG. 3A is a top sectional view of a conventional square spiral inductor with a grounded low sheet resistance patterned polysilicon shield. 
     FIG. 3B is a side sectional view of the conventional square spiral inductor with a grounded low sheet resistance patterned polysilicon shield of FIG.  3 A. 
     FIG. 3C is a graph illustrating quality factor and inductance versus frequency for a conventional unshielded inductor and the conventional inductor of FIGS. 3A and 3B. 
     FIG. 4 is a graph illustrating improved quality factor for an inductor having a high sheet resistance floating shield over an unshielded inductor, in accordance with embodiments of the present claimed invention. 
     FIG. 5A is a top sectional view of a square spiral inductor with a complimentary shield that comprises both a floating high resistive patterned polysilicon material and a grounded low resistive material, in accordance with embodiments of the present claimed invention. 
     FIG. 5B is a side sectional view of the square spiral inductor of FIG. 5A, in accordance with embodiments of the present claimed invention. 
     FIG. 6A is a top sectional view of a square spiral inductor with a floating high sheet resistance patterned polysilicon shield and a grounded Silicide shield, in accordance with embodiments of the present claimed invention. 
     FIG. 6B is a side sectional view of the square spiral inductor with shield of FIG. 6A, in accordance with embodiments of the present claimed invention. 
     FIG. 7A is a top sectional view of a square spiral inductor with a floating high sheet resistance patterned polysilicon shield and a grounded N-well shield, in accordance with embodiments of the present claimed invention. 
     FIG. 7B is a side sectional view of the square spiral inductor with shields of FIG. 7A, in accordance with embodiments of the present claimed invention. 
     FIG. 8 is a flowchart illustrating the steps of a process of forming a shielded inductor, in accordance with embodiments of the present claimed invention. 
    
    
     The drawings referred to in this description should be understood as not being drawn to scale except if specifically noted. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be obvious to one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention. 
     Embodiments of the present invention provide for a complimentary shield of a both a floating high resistive shield and a grounded low resistive shield between the inductor and the substrate. However, the present invention is not limited to using both the high and low resistive shield; in one embodiment, only a floating high resistive shield is used. When a floating high sheet resistance patterned polysilicon shield is fabricated underneath the inductor, an improvement in quality factor is observed. Referring to FIG. 4, the improvement in the quality factor is seen for a case in which only a floating high resistive shield is used. In contrast to the prior art case which used only a grounded low sheet resistance shield seen in FIG. 3C, the floating high resistive shield shows an improvement in quality factor even at low frequencies. The floating high resistive shield may be used to compliment an inductor with only a grounded low resistive shield that has good noise performance. 
     Referring now to FIG.  5 A and FIG. 5B, an embodiment provides for an electrically floating high resistive shield  510 . The high resistive shield  510  is preferably non-metallic and should be an electrical insulator. In one embodiment, the high resistive shield  510  comprises lightly doped unsilicided polysilicon. Preferably, the high resistive shield  510  may have a sheet resistance of greater than 10,000 Ohms per square, although embodiments with lower resistances may also be suitable. 
     FIG. 5B is taken along line AA′ of the top sectional view of FIG.  5 A. The views show a square spiral inductor  110 . However, the present invention is not limited to such inductors. In general, embodiments may be used with any element for which shielding is desired, including, but not limited to, circular, polygonal, spiral, or stacked inductors, etc. The inductor may be made of any material, including, but not limited to aluminum and copper. 
     The inductor  110  has an underpass  111 , which may be connected to the spiral by vias  115 . Embodiments have a floating high resistive shield  510  in an inter metal dielectric  322  between the inductor  110  and the substrate  102 . There may also be a field oxide region  324  between the floating high resistive shield  510  and the substrate  102 . 
     In a preferred embodiment, an electrically grounded low resistive shield  520  resides below the floating high resistive shield  510 . This shield  520  may provide a terminating path for the inductor electric field (e.g., a short). The low resistive shield  520  may be made of highly doped silicided polysilicon, although this is not required. In one embodiment, the low resistive shield  520  may have a sheet resistance of a few Ohms per square, although a wide range is suitable. The ratio of the resistance of the high resistive shield  510  to the low resistive shield  520  may be on the order of 1000; however, other ratios are suitable. Furthermore, the present invention is not limited to using both shields. In one embodiment, only the electrically floating high resistive shield  510  is used. Furthermore, the present invention is not limited to the particular orientation of the two shields. In another embodiment, the electrically grounded low resistive shield  520  resides above the electrically floating high resistive shield  510 . 
     The patterning of the shields  510 ,  520  may be configured in a non-continuous fashion such that a current induced in the shield  510 ,  520  by the inductor  110  will not have a closed loop path. It may also be stated that the shields  510 ,  520  are patterned to prevent formation of eddy currents in the shields  510 ,  520 . Thus, in one embodiment, the pattern may be composed of a number of segments with a slot or gap  525  between them. In one embodiment, the segments have a width on the order of 10 microns and the slots or gaps  525  have a width on the order of 1 micron. However, the present invention is not limited to these widths. Furthermore, the present invention is not limited to patterned shields  510 ,  520 . Thus, embodiments with full shields without patterning may be suitable. 
     With only one shield, the inductor  102  may be able to ‘see’ through to the substrate  102 . Beneficially, an embodiment uses two shields such that where one shield has a gap  525  the other does not and vice versa. In this fashion, the inductor  110  cannot ‘see’ the substrate  102 . Thus, the electric field is substantially blocked in that it does not have an open path through a gap  525  in a shield  510 ,  520 . 
     For illustrative purposes, the graph of FIG. 4 is based on the following parameters. However, the present invention is not limited to these parameters. The inductor  110  may be a 3.5-turns conventional square spiral inductor, fabricated on Chartered Semiconductor Manufacturing Ltd&#39;s 0.25 μm logic baseline process with plug-in analog modules. The spiral may consist of the topmost metal with a thickness of 0.6 μm. For the underpass  111 , a 0.5 μm thick aluminum may be used to contact the center of the spiral. The inter-metal dielectric  322  thickness between the spiral  110  and high resistive shield  510  may be about 5.0 μm. Field oxide  324  sandwiched by the low resistive shield  520  and substrate may be 0.36 μm in thickness. The inductor  110  may have an inner diameter of 75.0 μm and its width and spacing may be 10.0 μm and 2.0 μm respectively. Polysilicon strips with a fixed width of 7.5 μm and a slot width of 1.5 μm may be used to construct the entire low resistive ground shield  520  as well as electrically floating high resistive shield  510 . The ground low resistive shield  520  may use strips of very low resistive silicided polysilicon. The electrically floating high resistive shield  510  may achieve behavior of an opened substrate by exploiting un-doped polysilicon with a high sheet resistance to mimic high resistive silicon. 
     Referring now to FIG.  6 A and FIG. 6B, one embodiment uses an electrically grounded Silicide shield  620  instead of an electrically grounded patterned shield  520 . Preferably, the electrically grounded Silicide shield  620  is low resistance to provide a terminating path to ground for the electric field of the inductor  110 . The electrically grounded Silicide shield  620  may be, for example, Platinum Silicide, Titanium Silicide, or the like. Silicide is selected because it is a convenient material to work with. However, other materials may be used here, as well. 
     In this embodiment, shallow trench isolation (STI) is used to construct projections  622  of substrate  102  upon which the electrically grounded Silicide shield  620  is placed. The Silicide shield  620  may be patterned. The substrate  102  regions within the field oxide  324  are a consequence of the semiconductor fabrication method. Advantageously, this and other embodiments are well suited to be used in existing semiconductor fabrication methods. 
     The electrically grounded low resistive shield  620  is placed below the gaps  525  of the electrically floating high resistive shield  510 . In this fashion, the electrically grounded Silicide shield  620  serves to provide a terminating path for portion of the electric field of the inductor  110  that may pass through the gaps  525  of the electrically floating high resistive shield  510 . 
     Referring to FIG.  7 A and FIG. 7B, another embodiment uses an electrically grounded N-well (or P-Well) shield  720 . The grounded N-well shield  720  may be patterned. In one embodiment, electrically grounded N-well shields  720  are formed in the p-substrate  102 . Alternatively, P-well shields may be formed in a n-substrate  102 . The well shields  720  are formed near the top surface of the substrate  102  and may be positioned such that they are aligned with the gaps  525  in the electrically floating high resistive shield  510 . By using electrically grounded N-well (or P-well) structures as the bottom shield, parasitic capacitance is minimized. A guard-ring-like structure with N+ or P+ contacts may be used to ground the N-well or P-well shield  720 . 
     Referring now to FIG. 8, an embodiment provides for a method of forming a silicon-based shielded inductor  110 . In step  810 , a region of electrically grounded low sheet resistance material is formed above a substrate  102 . The low sheet resistance material may be a patterned low resistive shield  520 , a Silicide shield  620  formed on projections  622  of substrate  102 , or an N-well shield  720 . 
     In one embodiment, step  810  comprises forming projections  622  of substrate  102  into a field oxide layer  324  above the rest of the substrate  102  by using a shallow trench isolation (STI) process. The electrically grounded low resistive shield  620  is then formed on top of the projections  622 . The shield  620  may be made of Titanium Silicide, Platinum Silicide, etc. 
     In step  820 , a region of electrically floating high resistive material  510  is formed above the electrically grounded low resistive material ( 520 ,  620 ,  720 ). The high resistive material  510  may be made of un-doped silicon, for example, or other materials discussed herein. 
     In step  830 , the high resistive material is patterned. In this fashion, induced currents do not have a closed loop path. 
     In step  840 , an inductor  110  is formed above the electrically floating high resistive shield  510 . The present invention is well-suited to a wide variety of inductors, including, but not limited to circular spiral, square spiral, stacked, etc. 
     By utilizing both a low resistive ground shield  520  as well as high sheet resistive electrically floating polysilicon shield  510 , a low noise and high performance inductor  110  with an improved quality factor can be easily achieved. More importantly, there are no complicated or additional processing steps involved, and the new inductor design can be implemented easily in all current CMOS and BiCMOS technologies. Beneficially, the structures and formation methods of the present embodiments are realized using existing semiconductor fabrication processes such that significant revamping of semiconductor capital equipment is not required. As a result, the present embodiments do not require significant costs to implement. 
     Thus, the invention provides for a shielded silicon based inductor having low substrate thermal noise. The invention provides for such an inductor that does not have a significantly degraded quality factor. The invention provides for such an inductor fabricated in a semiconductor in such as fashion as to not increase the parasitic capacitance. The invention provides for an inductor and a method for forming the inductor wherein the inductor and the formation method meet the above needs and are compatible with existing semiconductor fabrication processes such that significant revamping of semiconductor capital equipment is not required. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.