Patent Publication Number: US-2002008301-A1

Title: Monolithic high-q inductance device and process for fabricating the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a process for fabricating semiconductor devices, and more particularly to a monolithic high-Q inductance device and a process for fabricating the same. The process involves forming a conducting coil on an insulating layer having a low dielectric constant, so as to decrease the parasitic capacitance effect.  
       [0003] 2. Description of the Prior Art  
       [0004] Miniaturization of electronic circuits is a goal in virtually every field, not only to achieve compactness in mechanical packaging, but also to decrease the cost of manufacture of the circuits. Many digital and analog circuits, including high-capacity memory devices, high-level microprocessors and operational amplifiers, have been successfully implemented in silicon based integrated circuits (ICs). These circuits typically include active devices such as bipolar junction transistors (BJTs) and field effect transistors (FETs), diodes of various types, and passive devices such as resistors and capacitors.  
       [0005] One area that remains a challenge to miniaturize is radio frequency (RF) circuits, such as those used in cellular telephones, wireless modems, and other types of communication equipment. The problem is the difficulty in producing a good inductor in silicon technologies that is suitable for RF applications. Attempts to integrate inductors into silicon technologies have yielded either inductors of low quality factor (hereinafter, Q value) and high loss, or required special metalization layers such as gold.  
       [0006] Ewen et al. in U.S. Pat. No. 5,446,311 has disclosed a process for manufacturing high-Q inductors without using a noble metal such as gold. The process involves forming multiple metal layers with identical spiral patterns stacked up on an insulating layer to construct an inductance device. Such multiple metal layers can decrease series resistance, thus increasing the Q value. The lump-sum equivalent circuit is as shown in FIG. 1. In FIG. 1, C d  indicates the parasitic capacitance between the metal layers, L is the inductance, R s  is the series resistance of the spiral metal levels, and C 1  and C 2  are the parasitic capacitance between the substrate and the metal layers. If the semiconductor substrate is made of a lossy material such as silicon, then R 1  and R 2  indicate the parasitic resistances connected in parallel with C 1  and C 2 , respectively. In addition, since the semiconductor substrate is usually grounded, R 1 , R 2 , C 1 , and C 2  are grounded at one end.  
       [0007] In semiconductor techniques, silicon oxide (SiO x ) is the most common insulating material, which has a relatively high dielectric constant (or relative permittivity) between 3.9 and 4.5. Since the resonant frequency is inversely proportional to C −½  and the capacitance (C) is proportional to the dielectric constant, when the dielectric constant increases, the self-resonance frequency decreases. Therefore, in U.S. Pat. No. 5,446,311, though the Q value is increased by the multiple metal layers, because of the high dielectric constant of the silicon oxide, the self-resonant frequency of the inductance device is decreased, thus limiting the application of the inductance device on high frequency.  
       [0008] Abidi et al. in U.S. Pat. No. 5,539,241 have disclosed an inductor which is formed in an oxide layer overlying a silicon substrate in which the silicon material underneath the inductor is selectively removed to form a pit so as to space the inductor from the underlying silicon substrate. In the illustrated embodiment, the silicon beneath the inductor is removed by etching, leaving the inductor suspended on the oxide layer overlying the substrate. The pit beneath the inductor is filled with an insulating medium such as air so that the parasitic capacitance of the inductor is substantially reduced and yet retains a relatively large self-resonant frequency on the order of 2 GHz or more. However, the etching of the substrate makes the whole process more complicated and incompatible with BiCMOS or CMOS standard processes.  
       SUMMARY OF THE INVENTION  
       [0009] Therefore, an object of the present invention is to solve the above-mentioned problems and to provide an inductance device with high-Q and to provide a process for fabricating the inductance device. The process involves forming a conducting coil on an insulating layer of a relatively low dielectric constant, so as to decrease the parasitic capacitance effect.  
       [0010] Another object of the present invention is to provide an inductance device with high-Q and to provide a process for fabricating the inductance device, in which the process is compatible with the BiCMOS and CMOS standard processes.  
       [0011] The above objects of the present invention can be achieved by providing a high-Q inductance device. The inductance device of the present invention is formed on a semiconductor substrate, and includes a first insulating layer, a second insulating layer, and a conducting coil. The first insulating layer and the second insulating layer are covered on different surfaces of the semiconductor substrate respectively, and the second insulating layer has a lower dielectric constant than the first insulating layer. The conducting coil is formed on the second insulating layer.  
       [0012] In addition, the present invention also provides a process for fabricating an inductance device. A first insulating layer and a second insulating layer are formed on different surfaces of a semiconductor substrate, respectively. The second insulating layer has a lower dielectric constant than the first insulating layer. Then, a conducting coil is formed on the second insulating layer.  
       [0013] According to the present invention, since the conducting coil is formed on the insulting layer with a relatively-low dielectric constant, the parasitic capacitance between the conducing coil and the substrate can be decreased. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0014] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.  
     [0015]FIG. 1 shows the lump-sum equivalent circuit of a conventional inductance device.  
     [0016]FIG. 2 is a top view of an inductance device according to the present invention.  
     [0017]FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 2 according to an embodiment.  
     [0018]FIG. 4 is a cross-sectional view taken along the line III-III of FIG. 2 according to another embodiment.  
     [0019] FIGS.  5 A- 5 C are cross-sectional views, illustrating the process flow of forming the second insulating layer according to a first preferred embodiment.  
     [0020] FIGS.  6 A- 6 C are cross-sectional views, illustrating the process flow of forming the second insulating layer according to a second preferred embodiment.  
     [0021] FIGS.  7 A- 7 C are cross-sectional views, illustrating the process flow of forming the second insulating layer according to a third preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0022] Refer to FIGS. 2 and 3, showing an inductance device according to the present invention, in which FIG. 2 is the top view, and FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 2. In the figures, a semiconductor substrate  20  is a silicon substrate, on which some semiconductor devices such as bipolar junction transistors or field effect transistors have been formed but are not shown. A first insulating layer  21  is formed over the whole surface of the semiconductor substrate  20 . The first insulating layer  21  is usually made of silicon oxide. A portion of the first insulating layer  21 , over which a spiral conducting coil  24  will be formed, is removed by photolithography and etching to expose the semiconductor substrate  20  and form a trench  22 . If the first insulating layer  21  is made of silicon oxide, such etching can be employed by reactive ion etching (RIE) or high density plasma etching (HDP), using a mixed gas including CF 4  and/or CHF 3  as the etching gas and argon as the carrier gas.  
     [0023] Then, a second insulating layer  23  is filled into the trench  22 . According to the present invention, the second insulating layer  23  has a lower dielectric constant than the first insulating layer  21 . The second insulating layer  23  can be formed by spin coating a polymer. Such a polymer can be a polyimide having a dielectric constant between 3.0 and 3.7, a polysilsequioxane having a dielectric constant between 2.7 and 3.0, an F-doped polyimide having a dielectric constant of about 2.5, an organic SOG having a dielectric constant between 2.0 and 3.0, an F-doped TEOS having a dielectric constant between 3.0 and 3.5, and other similar silicon or carbon based organic polymer films.  
     [0024] Subsequently, a spiral conducting coil  24  is formed over the second insulating layer  23 . For the purpose of example, the spiral conducting coil  24  shown in FIG. 2 has three turns. Those who are skilled in the art can adjust the coil turns according to the desired inductance value. Therefore, the turns shown in FIG. 2 are not used to limit the present invention. The spiral conducting coil  24  can be formed by physical vapor deposition (PVD), photolithography, and anisotropic etching to define the patterns of the spiral conducting coil  24 . For the convenience of measuring, pads  25  and  26  are formed to connect the probe; thus, one end of the spiral conducting coil  24  is coupled to the pad  25  through a wiring  27 , and the other end is connected to the pad  26  through another wiring  28 . The wiring  27  shown in FIG. 2 is located beneath the spiral conducting coil  24 ; therefore, it is indicated by a dash line.  
     [0025] In order to reduce the serial resistance R s  shown in FIG. 1, the spiral conducting coil  24  can be in a structure of multiple metal layers as shown in FIG. 3.  
     [0026] Referring to FIG. 3, after the second insulating layer  23  is filled into the trench  22 , etching back or chemical mechanical polishing (CMP) technology can be performed to obtain a flat surface. Then, a first conducting layer M 1  is deposited and patterned. The first conducting layer M 1  is preferably made of an aluminum-copper alloy, under which a barrier layer (not shown) made of titanium or titanium nitride is optionally formed to prevent aluminum from penetrating into the silicon substrate  20 . The first conducting layer M 1  can be formed, for example, by physical vapor deposition (PVD). Then, photolithography and anisotropic etching are performed to define the pattern of the wiring  27 . Such anisotropic etching can be performed by reactive ion etching (RIE) or by high density plasma etching (HDP) in the presence of a mixed gas including a chlorine-containing reactant.  
     [0027] Subsequently, a first inter-metal dielectric layer  30  is deposited over the whole surface and is etched to define via holes  35 . A second conducting layer M 2  is deposited and patterned to form a first spiral conducting line  24 A. The second conducting layer M 2  can be made of an aluminum-copper alloy deposited by physical vapor deposition, and is directly filled into the via holes  35 , through which the second conducting layer M 2  is electrically connected to the wiring  27 . In addition, metal plugs  36  made of tungsten can be filled into the via holes  35  before the formation of the second conducting layer M 2 . In FIG. 3, the first spiral conducting line  24 A is electrically connected to the wiring  27  through the respective metal plugs  26 .  
     [0028] Subsequently, a second inter-metal dielectric layer  31  is deposited over the whole surface and is etched to define via holes  35 . A third conducting layer M 3  is deposited and patterned to form a second spiral conducting line  24 B. The third conducting layer M 3  can be made of an aluminum-copper alloy deposited by physical vapor deposition, and is directly filled into the via holes  35 , through which the third conducting layer M 3  is electrically connected to the first spiral conducting line  24 A. Alternatively, the second spiral conducting line  24 B can be electrically connected to the first spiral conducting line  24 A through the respective metal plugs  26  as shown in FIG. 3.  
     [0029] Subsequently, according to the process mentioned above, a third inter-metal dielectric layer  32  is deposited over the whole surface and is etched to define via holes  35 . A fourth conducting layer M 4  is deposited and patterned to form a third spiral conducting line  24 C. The fourth conducting layer M 4  can be made of an aluminum-copper alloy deposited by physical vapor deposition and is directly filled into the via holes  35 , through which the fourth conducting layer M 4  is electrically connected to the second spiral conducting line  24 B. Alternatively, the third spiral conducting line  24 C can be electrically connected to the second spiral conducting line  24 B through the respective metal plugs  26  as shown in FIG. 3. Simultaneously, the fourth conducting layer M 4  is patterned to form pads  25  and  26 , and the wiring  28 . The wiring  27  is electrically connected to the pad  25  through the patterned first conducting layer M 1 , the second conducting layer M 2 , and the third conducting layer M 3 .  
     [0030] Finally, a passivation layer  24  is deposited over the whole surface, which can be made of silicon oxide or silicon nitride.  
     [0031] From FIG. 3, it is known that the first spiral conducting line  24 A, the second spiral conducting line  24 B, and the third spiral conducting line  24 C construct the spiral conducting coil  24  in FIG. 2. The adjacent spiral conducting lines, such as lines  24 A and  24 B, and lines  24 B and  24 C, are electrically connected to each other by at least one metal plug  36 . By the multiple conducting layers of the present invention, the serial resistance R s  can be substantively decreased. Since the Q value is inversely proportional to the serial resistance R s , the Q value of the inductance device can be increased accordingly. In addition, the first spiral conducting line  24 A, the second spiral conducting line  24 B, and the third spiral conducting line  24 C are all located over the second insulating layer  23 , which has a relatively low dielectric constant; therefore, the parasitic capacitance C 1  and C 2  between the substrate and the conducting layers can be decreased, thus increasing the self-resonant frequency of the inductance device.  
     [0032]FIG. 4 is a cross-sectional view taken along the line III-III of FIG. 2 according to another embodiment. FIG. 4 differs from FIG. 3 in that before the formation of the second inter-metal dielectric layer  31 , the third inter-metal dielectric layer  32 , and the passivation layer  34 , a material  38  having a relatively-low dielectric constant is filled into the space around the first spiral conducting line  24 A, the space around the second spiral conducting line  24 B, and the space around the third spiral conducting line  24 C. Thus, the parasitic capacitance C d  between the lines as shown in FIG. 1 will be decreased. The material  38  having a low dielectric constant can be obtained by spin coating a polymer. Such a polymer can be a polyimide having a dielectric constant between 3.0 and 3.7, a polysilsequioxane having a dielectric constant between 2.7 and 3.0, an F-doped polyimide having a dielectric constant of about 2.5, an organic SOG having a dielectric constant between 2.0 and 3.0, an F-doped TEOS having a dielectric constant between 3.0 and 3.5, and other similar silicon or carbon based organic polymer films.  
     [0033] For example, if the first insulating layer  21  includes an undoped TEOS layer, a BPSG layer, and a plasma-enhanced deposited TEOS layer (PETEOS), the second insulating layer  23  having a low dielectric constant can be formed by various ways, three kinds of which are described as follows.  
     [0034] FIGS.  5 A- 5 C are cross-sectional views, illustrating the process flow of forming the second insulating layer  23  according to a first preferred embodiment. Referring to FIG. 5A, an undoped TEOS layer  50  and a BPSG layer  51  are formed over the whole surface of the semiconductor substrate  20  in sequence. A portion of the BPSG layer  51 , over which a spiral conducting coil  24  will be formed, is removed by photolithography and etching to expose the undoped TEOS layer  50  and form a trench  22 . Then, a second insulating layer  23  is filled into the trench  22  and is planarized by etching back or chemical mechanical polishing to obtain a structure as shown in FIG. 5B. A PETEOS layer  52  is then formed on the second insulating layer  23  and the BPSG layer  51  to obtain a structure as shown in FIG. 5C.  
     [0035] FIGS.  6 A- 6 C are cross-sectional views illustrating the process flow of forming the second insulating layer  23  according to a second preferred embodiment. Referring to FIG. 6A, an undoped TEOS layer  60 , a BPSG layer  61 , and a PETEOS layer  62  are formed over the whole surface of the semiconductor substrate  20  in sequence. A portion of the BPSG layer  61  and a portion of the PETEOS layer  62 , over which a spiral conducting coil  24  will be formed, are removed by photolithography and etching to expose the undoped TEOS layer  60  and form a trench  22 , as shown in FIG. 6B. Then, a second insulating layer  23  is filled into the trench  22  and is planarized by etching back or chemical mechanical polishing to obtain a structure as shown in FIG. 6C.  
     [0036] FIGS.  7 A- 7 C are cross-sectional views illustrating the process flow of forming the second insulating layer  23  according to a third preferred embodiment. Referring to FIG. 7A, an undoped TEOS layer  70 , a BPSG layer  71 , and a PETEOS layer  72  are formed over the whole surface of the semiconductor substrate  20  in sequence. A portion of the undoped TEOS layer  70 , a portion of the BPSG layer  71 , and a portion of the PETEOS layer  72 , over which a spiral conducting coil  24  will be formed, are removed by photolithography and etching to penetrate part of the semiconductor substrate  20  and form a trench  22 , as shown in FIG. 7B. Then, a second insulating layer  23  is filled into the trench  22 , as shown in FIG. 7C.  
     [0037] Since the capacitance is inversely proportional to the thickness, the parasitic capacitance C 1  and C 2  of the conducting coil  24  in FIG. 6C is lower than that in FIG. 5C, and the parasitic capacitance C 1  and C 2  of the conducting coil  24  in FIG. 7C is lower than that in FIG. 6C.  
     [0038] The foregoing description of the preferred embodiments of the present invention has been provided for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to under stand the invention to practice various other embodiments and make various modifications suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.