Abstract:
A stressed metal technology may fabricate high-Q, three-dimensional microelectronic inductors and transformers. The fabrication method may allow the production of inductors and transformers on high-resistivity silicon substrate and with metal deposition of Au and Cr that is fully compatible with semiconductor fabrication technologies. The produced inductors and transformers exhibit Q factors&gt;60 at frequencies of 3 to 7 GHz. High efficiency, high-Q transformers with coupling factors 0.6&lt;k&lt;0.9 may be created with very high self-resonance frequencies

Description:
PRIORITY CLAIM  
       [0001]     This application claims the benefit of priority from U.S. Provisional Application No. 60/634,713, filed Dec. 8, 2004, which is incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     This invention relates to semiconductor processing of electronic components. In particular, the invention relates to 3-D inductor and transformer elements formed by semiconductor processing.  
         [0004]     2. Related Art  
         [0005]     Nanoscale CMOS and SiGe BiCMOS processes have recently enhanced the performance of Si-based RFICs up to microwave frequencies. With f T  and f max  of the transistor approaching 200 GHz, the requirement for high performance passive elements is the bottleneck in designing state-of-the-art RF circuits. Inductors and transformers play a role in the performance achieved by RF circuits such as voltage controlled oscillators (VCOs), low-noise amplifiers (LNAs), filters, mixers, and power amplifiers (PAs). Among inductor and transformer parameters that limit the performance of an RF integrated circuits are quality factor (Q, where Q=ωL/R, ω=frequency, L=inductance of inductor, R=resistance of inductor)), self-resonance frequency (f sr ), and distributed effects. Higher Q inductors and transformers help minimize RF power loss, RF noise, phase noise and DC power consumption of RFIC circuits.  
         [0006]     Self-resonance frequency of the inductors and transformers may also affect circuit implementation at higher frequencies. Little work has been done on high frequency transformers with a respectable quality factor and resonance frequency beyond 5 GHz. Some of these approaches may use unconventional fabrication technologies that are difficult to integrate, such as the use of metals like Co.  
         [0007]     The quality factor of an inductor or a transformer may be determined by its geometry, the type of interconnect metal (Al, Au or Cu), thickness of the metallization, vertical distance between the underpass/airbridge to the inductor windings, dielectric loss of the substrate (low or high resistivity), and by the selective removal of silicon from beneath the inductor structure. Thicker metallization and removal of substrate material may provide a substantial improvement in Q. Substrate removal has the disadvantage of additional cost of post processing on the inductors and weaker mechanical strength.  
         [0008]     High-Q three-dimensional inductors based on stressed metal technology have been recently introduced. Stressed metal technology creates three-dimensional, out-of-plane structures by releasing metal structures from the pre-processed substrate and allowing the metal structures to move, under the material stresses in the metal, in a desired direction. Unlike spiral, in-plane inductors, the magnetic field in these inductors is not perpendicular to the substrate. These recently introduced inductors have shown very high quality factors up to 1 GHz operational frequency due to a reduction of losses associated with eddy currents generated in the Si substrate. At higher frequencies, the quality factor has degraded substantially, due to the magnetic field penetration into the lossy Si substrate.  
         [0009]     Therefore, a need exists for three-dimensional microelectronic inductors and transformers exhibiting high Q factors at suitable frequencies.  
       SUMMARY  
       [0010]     In the present invention, high-Q three-dimensional, out-of-plane inductors and transformers are fabricated on a Si substrate, using a stressed metal technology. The fabrication technology of the present invention is based on depositing Cr and Au, which are both compatible with integrated circuit fabrication. By adjusting the thickness of deposited Cr and Au, the correct amount of stress may be achieved (compressive in upper metal and tensile in lower metal), such that upon releasing the Cr and Au metal combination from its sacrificial layer, the two ends of each metal finger meet the neighboring fingers above the substrate to form a three-dimensional device such as an inductor or transformer.  
         [0011]     The present invention also provides a method for fabricating high-Q three-dimensional inductors and transformers on a pre-processed silicon substrate using a stressed metal technology. The steps in the fabrication process are as follows. A sacrificial layer composed of photo-resist is defined by a photolithographic process. Next, metal layers composed of a chromium (Cr) and gold (Au) combination are deposited on top of the sacrificial layer using an e-beam evaporation process.  
         [0012]     After the metal deposition step, a photo-resist mask, for defining metal fingers in the underlying metal, is deposited. Then, chromium and gold metal etch chemistry is used to remove excess metal layers to define the metal fingers which will make up the inductor or transformer structure. A wet etch process is employed to remove excess photo-resist. The wet etch process also removes the sacrificial layer underneath the metal finger areas.  
         [0013]     During this releasing step, as the metal fingers in the Cr and Au metal layer are defined, and the overlying photo-resist and sacrificial layer are etched away, the metal fingers are released from the sacrificial layer and bend upward. By adjusting the thickness of the deposited Cr and Au, the correct amount of stress in the layer can be achieved (compressive in the upper metal and tensile in the lower metal), such that upon releasing the metal fingers, the two ends of each metal finger meet the neighboring fingers above the substrate to form a three-dimensional device such as an inductor or transformer.  
         [0014]     Finally, after the processing steps are complete, an electroplating process is performed to improve the conductivity of the three-dimensional metal windings and the stiffness of the three-dimensional structure.  
         [0015]     The present invention also provides high-Q, three-dimensional microelectronic inductors and transformers on pre-processed silicon substrates. By employing the above-described fabrication process, out-of-plane metal structures in the form of fingers are joined to form circular or oval-shaped structures that function as inductors and transformers when integrated with other microelectronic structures. The inductors and transformers formed have a coil axis substantially parallel to the plane of the underlying substrate, and the three-dimensional structures have cross-sections that are substantially circular or oval-shaped. Inductors have been created with high Q factors greater than 60. In addition, high-Q transformers with 3:4 ratios have been created with coupling factors of 0.6&lt;k&lt;0.9 with very high self-resonance frequencies (8 GHz&lt;fres&lt;16 Ghz).  
         [0016]     Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views.  
         [0018]      FIG. 1  is a flow diagram depicting the fabrication steps of a three-dimensional inductive device.  
         [0019]      FIG. 2  presents a first step of a fabrication process.  
         [0020]      FIG. 3  presents a second step of a fabrication process.  
         [0021]      FIG. 4  presents a third step of the fabrication process.  
         [0022]      FIG. 5  presents a fourth step.  
         [0023]      FIG. 6  presents a fifth step of the fabrication process.  
         [0024]      FIG. 7  presents a view of a high-Q, three-dimensional 3:4 transformer with corresponding dimensional parameters.  
         [0025]      FIG. 8  presents a top-down view of a processed inductive device.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]      FIG. 1  presents a fabrication process for producing a high-Q, three-dimensional inductor or transformer. A sacrificial layer of photo-resist  210  (see  FIG. 2 ) is deposited, at block  110 , using semiconductor photolithographic processing equipment, on a pre-processed silicon substrate  240 . This pre-processed substrate may comprise high resistivity silicon  240 , silicon dioxide (SiO 2 )  220 , or high dielectric constant materials such as SU-8™  230 . The photo-resist  210  may be a negative or positive-type photo-resist, as needed to operate with the photolithography process. The photo-resist  210  may be a polymer sensitive to certain wavelengths of light, typically ultraviolet radiation. A mask plate (not shown) that embodies an image of the structures to print on the circuit may be placed above the photo-resist layer  210  and the photo-resist is exposed to an ultraviolet light. The exposed resist may be developed in a solution. The solution may be a base such as potassium hydroxide (KOH). The areas of the photo-resist that are exposed are hardened and therefore do not wash in the developer. Areas that are not exposed are removed in the developer. The desired shape from the mask is then transferred to the photoresist  210 . This sacrificial layer  210  will be removed in a later releasing process.  
         [0027]     A combination bi-metal layer  350  (see  FIG. 3 ) is deposited, at block  120 , on the sacrificial layer  320 . The bi-metal layer  350  may include chromium (Cr) and gold (Au). The bi-metal layer  350  may be deposited using an e-beam evaporation process. The bi-metal layer thickness  350  may be about 1 μm. By adjusting the thickness of the deposited Cr and Au in the combination metal layer, the correct amount of stress may be achieved. The correct amount of stress may be compressive in the upper metal portions of the layer and tensile in the lower metal portions of the layer. Upon releasing the Cr and Au metal combination from its sacrificial layer in a later process, the bi-metal combination layer will bend upward. In addition, the use of Cr and Au may be compatible with integrated circuit fabrication without requiring different process equipment.  
         [0028]     After the Cr and Au bi-metal combination layer  450  ( FIG. 4 ) is deposited at block  120 , the bi-metal layer  450  is patterned, at block  130 , to form the selected areas which will form the coils of the inductor or transformer structure at the end. After deposition of the bi-metal layer  450 , another photolithography process step is performed, also at block  130 , to deposit a layer of photo-resist  460  on top of the bi-metal layer  450 . The photolithography masking step defines regions of the metal ( 860  and  880 ) (see  FIG. 8 ) that will remain after a metal etch step. The photolithography masking step defines the finger-shaped areas ( 860  and  880 ) of the bi-metal layer  450  that will form the structure of the completed inductor or transformer. The masking process does this by depositing a photo-resist which will protect certain metal layer areas (the areas forming the metal fingers  860  and  880 ) from succeeding metal etch steps.  
         [0029]     A metal etch process employing Cr and Au metal etchant chemistries is performed, at block  140 , to remove excess metal areas and leave behind the desired finger-shaped metal areas ( 860  and  880 ). In some methods, the processed substrate with the deposited metal and photo-resist layers is dipped in an acid solution that preferentially etches away the species of metal that comprises the deposited metal layer. The metal layer areas that are not protected by photo-resist  460  are removed by the metal etch, leaving the desired metal features ( 860  and  880 ) protected underneath the photo-resist  460 .  
         [0030]     A wet etch process is employed to remove the photo-resist mask  460  and the sacrificial layer  410  underneath the metal layers  450 , during the following releasing blocks  150  and  160 . After the sacrificial layer  410  is etched away, the remaining finger-shaped metal areas ( 860  and  880 ) are released, at block  160 , from the underlying substrate. The finger-shaped metal areas  550  (see  FIG. 5 ) bend upward under the stresses of the Cr and Au combination bi-metal layer  450 —compressive stress in the upper portion of the bi-metal layer, tensile stress in the lower portion of the bi-metal layer—away from the underlying pre-processed substrate  520 .  
         [0031]     As the finger-shaped metal areas  860  bend upward, at block  160 , they will join neighboring finger-shaped metal areas  880  to form a three-dimensional, substantially circular or oval shaped structure  650 , at block  170  (see  FIG. 6 ). The joined finger-shaped metal areas ( 860  and  880 ) become the finished inductor or transformer. The central axis of the formed coil of the inductor or transformer will be substantially parallel to the plane of the underlying pre-processed substrate. After the three-dimensional structure is formed by joining the neighboring finger-shaped metal areas  860  and  880 , an electroplating process is performed, at block  180 . The electroplating process may improve the conductivity of the three-dimensional windings of the structure  650  and the stiffness of the three-dimensional structure  650 .  
         [0032]     The process illustrated in  FIG. 1  also may provide for high-Q three-dimensional inductive devices such as inductors and transformers for integration with microelectronic components.  FIG. 7  presents a representative 3:4 ratio transformer, where the primary line  720  contains 3 turns and the secondary line  710  contains 4 turns. Metal line interconnects from a microelectronic integrated circuit may be joined to the formed inductor or transformer to use the inductor or transformer as an active inductive device in the integrated circuit. These metal interconnects may run along the surface of the substrate. Generally, one interconnect line  770  will form one coil (the primary coil)  720  of the transformer and a second interconnect line  790  will form a second coil (the secondary coil) of the transformer  710 . If the interconnects to the coils  770  and  790  are changed to include the same metal interconnect line, an inductor is formed. If the interconnects  770  and  790  are different metal lines, then a transformer will be formed.  
         [0033]     The transformer may include a number of primary coil turns  720  and a number of secondary coil turns  710 . The parameters defining the device may include the diameter of the three-dimensional structure  750 , the width of the coils (formed from joined finger-shaped metal areas)  780 , and gap between successive coil turns  730 . By varying these parameters along with the number of turns of the coil, different results may be obtained. For example, 3 turn inductors with gaps  730  of 9 μm and widths  780  of 25 μm were created with Q&gt;60, and 3:4 transformers were created with coupling ratios of 0.77 (0.9 at Qmax/2) with an f sr  of 7.9 GHz. The typical diameter  750  of the formed structures is around 300 μm. The turn ratio for the transformer may be anything that is desired depending on the intended application ranging from 1:1 to any other ratio. For applications at high frequency, such as in RF applications, typically values like 1:3, 3:4, or 3:5 are implemented. For applications in power converters, turn ratios such as 4:64 may be more suitable. These examples do not limit the possible permutations of ratios that a transformer coil may embody.  
         [0034]     Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims.