Abstract:
The structure of high-Q inductor applied in a monolithic circuit according to the invention comprises a plurality of spiral metal lines and a plurality of dielectric layers, each dielectric layer formed between two adjacent spiral metal lines. Furthermore, via plugs are formed in each dielectric layer to electrically connect two adjacent spiral metal lines. A spiral air trench is formed along the spacing of the spiral metal lines in the dielectric layers. Therefore, 3D-structure of the inductor of the invention can greatly reduce the series resistance thereof without widening the spiral metal lines. In addition, the spiral air trench, filled with air which has a lower dielectric constant, can efficiently reduce the parasitic capacitance between the spacing of the spiral metal lines. As a result, the inductor of the invention has a higher quality factor at a proper RF operating frequency region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application Ser. No. 87113032, filed Aug. 7, 1998, the full disclosure of which in incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and a structure of manufacturing an inductor in an monolithic circuit, and more particularly to a method and a structure of manufacturing an inductor with a high-quality factor and an air trench. 
     2. Description of the Related Art 
     The continuous miniaturization of integrated circuits (ICs) is a main trend in the semiconductor industry for the purpose of not only obtaining smaller sizes and lighter weights but also reducing manufacturing costs. Today, many digital circuits and analog circuits, such as complicated microprocessors and operational amplifiers, have been successfully mass produced into ICs by very large scale integrated (VLSI) technology. In general, the above-mentioned circuits include active devices, such as bipolar junction transistors (BJTs), field effect transistors (FETs) and diodes, and passive devices, such as resistors and capacitors. 
     However, miniaturization techniques have not been completely developed yet for certain circuits applied in specific areas, including, for example, radio frequency (FR) circuits, which are applied in communication equipment, such as cellular telephones (i.e., mobile telephones), cordless telephones, wireless modems and so on. Miniaturization of the RF circuits hinges on the ability to manufacture inductors with an appropriately high quality factor. Currently, the quality factor of inductors manufactured by semiconductor technology is less than  5 , which does not meet desirable requirements. Although certain low-resistance metals, such as gold, can be used to increase the quality factor, it cannot be implemented by the current semiconductor technology. 
     It is well know that the quality factor represents the qualities of produced inductors. It can be estimated by the following formula:        Q   =     K          ω                 L       R   s                                
     wherein ω is angle frequency, L is inductance, and R S  is series resistance. Under an ideal condition, the quality factor Q of a non-loss inductor ( that is, R=0) is approximately infinite. Even though it is impossible to manufacture the ideal inductor in the real world, an inductor with a high quality factor can be definitely obtained by decreasing the energy losses thereof. 
     Referring to FIG. 1, an equivalent circuit of a real inductor is shown. It can be considered that the real inductor consists of an ideal inductor L, a resistor R S  and a capacitor C d , wherein the ideal inductor L and the resistor R S  are connected to each other in series and then are coupled to the capacitor C d  in parallel. Generally, the resistor R s  of a spiral metal line used for forming the real inductor is considered to be a main factor in reducing the quality factor thereof. One way to resolve this problem is to widen the metal line. However, this increased the area occupied by the metal line and the parasitic capacitance C d  that follows. It is obvious that the increased area is opposed to the miniaturization of the inductor. The parasitic capacitance decreases the self-resonance frequency of the inductor, which, as a result, limits the range of the operating frequency thereof. On the other hand, the quality factor Q is directly proportional to the angle frequency and is inversely proportional to the series resistor, so the metal line cannot be optionally widened. 
    
    
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the invention is to provide a method and a structure of manufacturing an inductor with a high quality factor and an air trench in a monolithic circuit. The inductor manufactured by the invention has lower series resistance and a lower parasitic capacitance. Therefore, the inductor of the invention has lower energy losses, a higher quality factor and a higher operating frequency. 
     To attain the above-stated object, an inductor in a monolithic circuit according to the invention has the following structure. A plurality of spiral metal lines formed over a substrate. A plurality of dielectric layers, each of which is formed between two adjacent spiral metal lines. A plurality of via plugs formed in the dielectric layers to connect two adjacent spiral metal lines to each other. A spiral air trench formed along the spacing of the spiral metal lines in the dielectric layers. In such a structure having a plurality of spiral metal lines stacked on each other with the via plugs therebetween, the series resistance thereof is greatly decreased without widening the inductor. Moveover, air contained in the spiral air trench with a lower dielectric constant can efficiently reduce the parasitic capacitance of the inductor. Hence, the inductor manufactured based on the structure has a higher quality factor. 
     A method of manufacturing an inductor according to the invention comprises the following steps. A plurality of spiral metal lines aligned with each other is formed over a substrate. A plurality of dielectric layers, each of which is located between two adjacent spiral metal lines, is formed over the substrate. A via plug is formed in each dielectric layer to connect two adjacent spiral metal lines. An upper dielectric layer is formed over the spiral metal lines. A spiral air trench is formed in the dielectric layers along the spacing of the of the spiral metal lines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus do not limit the present invention, and wherein: 
     FIG. 1 is a schematic circuit diagram illustrating an equivalent circuit of a real inductor; 
     FIG. 2 is top view illustrating an inductor manufactured by a preferred embodiment of the invention; 
     FIGS. 3A-3H are cross-sectional views illustrating a method of manufacturing an inductor according to the preferred embodiment of the invention; 
     FIGS. 4A-4C are cross-sectional views illustrating another method of forming a spiral air trench after the step shown in FIG. 3E; and 
     FIGS. 5A-5C are cross-sectional views illustrating a further method of forming a spiral air trench after the step shown in FIG.  3 E. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 2 is a top view of an inductor manufactured by a preferred embodiment of the invention. In FIG. 2, an inductor  20  formed on a semiconductor substrate includes a spiral conductive line  22 . One end of the spiral conductive line  22  is electrically connected to a first bonding pad  26  via a first connective line  24  while the other end thereof is electrically connected to a second bonding pad  29  via a second connective line  28 . The bonding pads  26  and  29  are used to electrically connect other circuits. A spiral air trench  23  (indicated by a dash line) is formed along the gap of the spiral conductive line  22  to reduce the parasitic capacitance thereof and increase the quality factor thereof. 
     Referring to FIGS  3 A- 3 H, a method of manufacturing an inductor according to a preferred embodiment of the invention is shown. In FIG. 3A, a lower metal line  34 , such as an aluminum line is formed by sputtering and photolithography on an insulator  32 , such as a silicon oxide layer, which is deposited on a substrate  30 , such as a silicon substrate. The lower metal line  34  serves as a first connective line. 
     Referring to FIG. 3B, a lower dielectric layer  36 , such as a silicon oxide layer, is formed on the insulator  32  and the lower metal line  34  by, for example, chemical vapor deposition (CVD). It is then planarized by, for example, etch back or chemical mechanical polishing (CMP) to facilitate subsequent photolithography. The lower dielectric layer  36  is patterned to form via holes (not shown) by, for example, photolithography and etching until portions of the surface of the lower metal line  34  are exposed. Next, a metal layer (not shown), such as a tungsten layers, is formed over the substrate  30  by, for example, chemical vapor deposition; it completely fills the via holes to electrically connect the lower metal line  34  ( which serves as the first connective line). Then, part of the metal layer above the level of the lower dielectric layer  36  is removed by planarization form first via plugs  38 , such as tungsten plugs, by, for example, chemical mechanical polishing of etch back. 
     Referring to FIG. 3C, a first spiral metal line  40   a  and a first metal line  40   b , such as a square spiral aluminum line and an aluminum line, are formed on the lower dielectric layer  36  by, for example sputtering and photolithography. As shown in FIG  3 C, the first metal line  40   b  and the inner end of the spiral metal line  40   a  are connected to the lower metal line  34  (i.e., the first connective line) via the first via plugs  38 . 
     Referring to FIG. 3D, a first dielectric layer  42 , such as a silicon oxide layer, is formed on the spiral metal line  40   a , the first metal line  40   b  and the lower dielectric layer  36  by, for example, chemical vapor deposition. It is then planarized by, for example, etch back or chemical mechanical polishing to facilitate subsequent photolithography. Next, the first dielectric layer  42  is patterned to form via holes (not shown) by, for example, photolithography and etching, until the first spiral metal line  40   a  and the first metal line  40   b  are exposed. A metal layer (not shown), such as a tungsten layer, is formed over the substrate  30  and completely fills the via holes by, for example, chemical vapor deposition. Part of the metal layer above the level of the first dielectric layer  42  is removed to form second via plugs  44  and a third via plug  44 ′, such as tungsten plugs, in the via holes by, for example, chemical mechanical polishing or etch back, thereby connecting the spiral-shaped metal line  40   a  and the first metal line  40   b , respectively. 
     Referring to FIG. 3E, the steps shown in FIGS. 3C and 3D are repeated to form a second spiral metal line  46   a  on the second via holes  44 , a second metal line  46   b  on the third via plug  44 ′, a second dielectric layer  48  on the first dielectric layer  42 , the second spiral metal line  46   a and the second metal line  46   b , fourth via plugs  50  on the second spiral metal line  46   b  and a fifth via plug  50 ′ on the second spiral metal line  46   a . Therefore, a third spiral sluminum line  52   a , such as a square spiral metal line, is formed on the fourth via plugs  50 ; a third metal line  52   b , such as an aluminum layer is formed on the fifth via plug  50 ′; and a second connective line  52   c , such as an aluminum layer, is formed on the fourth via plug  50  just above the outer end of the second spiral metal line  46   a  by, for example, sputtering, photolithography and etching. Moreover, the third metal line  52   b  electrically connects the lower metal line  34  (i.e., the first connective line) and the first bonding pad  26  as shown in FIG. 2, while the second connective line  52   c  is electrically connected to the second bonding pad  29  as shown in FIG.  2 . 
     Referring to FIG  3 F, an upper dielectric layer, consisting, for example, of a silicon oxide layer  54  and a silicon nitrite layer  56 , is formed on the third spiral metal line  52   b , the third metal line  52   b  and the second connective line  52   c  by, for example, chemical vapor deposition. Then, a positive photoresit  58  having a trench  60  just above the third metal line  52   b  is formed on the silicon nitrite layer  56  by photolithography. Parts of the silicon oxide layer  54  and the silicon nitrite layer  56  just below the trench  60  are removed to expose the third metal line  52   b  by etching for subsequently bonding. 
     Referring to FIG. 3G, the positive photresist  58  is removed. Next a positive photoresit  62 , having a spiral trench  64  aligned with the gaps of the third spiral metal line  52   a , the third metal line  52   b  and the second connective line  52   c , is formed on the silicon nitrite layer  56  and the third metal line  52   b . The spiral trench  62  keeps an appropriate distance from the third spiral metal line  52   a  by using an original mask for the formations of the spiral metal lines  40   a ,  46   a  and  52   a  and by adjusting its exposure dose to create a photo bias during development. This step can save a one-mask cost. Referring to FIG. 3H, parts of the silicon nitrite layer  56 , the silicon oxide layer  54  and the dielectric layers  48  and  42  uncovered by the positive photoresist  62  are removed to expose the lower dielectric layer  36  by etching, thereby forming a spiral air trench  66 . Thus, the inductor according to the invention is completely manufactured. 
     Although the third metal line  52   b  is first exposed, and then the spiral air trench  66  is formed, it is obvious, for those skilled in the art that the order of the above-stated two steps is exchangeable. That is, the spiral air trench  66  can be first formed before the third metal line  52   b  is exposed. Moreover, to protect the sidewalls of the spiral air trench  66 , another silicon nitrite layer (not shown) can be formed on the inner surfaces thereof. 
     FIGS. 4A-4C show another method of forming an air trench after the step shown in FIG.  3 E. Referring to FIG  4 A, an oxide layer  68  is formed on the third spiral metal line  52   a , the third metal line  52   b , the second connective line  52   c  and the second dielectric layer  48  by, for example chemical vapor deposition. Thereafter, a positive photoresist  70 , having a spiral trench  72  aligned with the spaced of the third spiral metal line  52   a , the third metal line  52   b  and the second connective line  52   c ,is formed on the oxide layer  68  by photolithography. The spiral trench  72  keeps an appropriate distance from the third spiral metal line  52   a  by using the original mask for the formations of the spiral metal line  40   a ,  46   a  and  52   a  and by adjusting its exposure does to create a photo bias during development. 
     Referring to FIG. 4B, using the positive photoresist  70  as a mask, a spiral air trench  74  is formed in the oxide layer  68  and the dielectric layers  42  and  48  by etching. Then, a silicon nitride layer  76 , serving as a passivation, is formed on the oxide layer  68  and the inner surfaces of the spiral air trench  74 . Referring to FIG. 4C, pasts of the silicon nitride layer  76  and the oxide layer  68  just above the third metal line  52   b  are removed to form a trench  78  and to expose the third metal line  52   b  for subsequent bonding, by photolithography and etching. Thus an inductor of the invention is completely manufactured. 
     FIGS. 5A-5C show a further method of forming an air trench after the step of FIG.  3 E. Referring to FIG  5 A, an upper dielectric layer, consisting, for example, of a silicon oxide layer  80  and a silicon nitride layer  82 , is formed on the third spiral metal line  52   a , the third metal line  52   b  and the second connective line  52   c  by, for example, chemical vapor deposition. Then, a positive photoresist  84 , having a spiral trench  86  aligned with the spacing of the third spiral metal line  52   a , the third metal line  52   b  and the second connective line  52   c , is formed on the silicon nitride layer  82  by photolithography. The spiral trench  86  keeps an appropriate distance from the third spiral metal line  52   a  by using the original mask for the formations of the spiral metal lines  40   a ,  46   a  and  52   a  and by adjusting its exposure does to create a photo bias during development. 
     Referring to FIG. 5B, with the photoresist  84  serving as a mask, an etching process is performed to form a spiral air trench  88 . The photoresist  84  is removed. Next, a silicon nitride layer  90 , serving as a passivation, is formed on the silicon nitride layer  82  and the inner surfaces of the spiral air trench  88 . Referring to FIG  5 C, parts of the silicon nitride layer  90 , silicon oxide layer  82  and silicon nitride layer  80  just above the third metal line  52   b  are removed to form a trench  92 , thereby exposing the third metal line  52   b  for subsequently bonding. Thus, an inductor according to the invention is completely manufactured. 
     As can been seen from FIG. 3H,  4 C or  5 C, an inductor with an air trench according to the invention at least comprises the substrate  30 ; the spiral metal lines  40   a ,  46   a  and  52   a , and the dielectric layers including the insulator  32 , the lower dielectric layer  36 , the dielectric layers  42  and  48  and the upper dielectric layer. Furthermore, a plurality of via plugs  38 ,  44 , and  50  are formed in the lower dielectric layer  36  and the dielectric layers  42  and  48  respectively, to connect the metal lines  34 ,  40   a  ,  46   a , and  52   a  to each other. The spiral air trench  66 ,  74 , or  88  is formed in the dielectric layers  42  and  48 . In addition, the inductor, which mainly includes the spiral metal line  40   a ,  46   a  and  52   a , has the first connective line  34  and the second connective line  52   c . A silicon nitride layer, serving as a passivation, is formed on the inner surfaces of the spiral air trench. Although the inductor is formed by 4 metal lines (including 3 spiral metal lines) and a plurality of via plug, wherein there are only 3 turns for each spiral metal line, it is well known by those skilled in the art that the number of metal lines of the inductor and the number of the turns for each spiral metal line are not limited by the embodiment at all. 
     Since the inductor according to the invention includes 3 spiral metal lines and a plurality of via plugs, the cross-sectional area of the inductor is increased, resulting in a decrease in the resistance thereof. Moreover, because no additional area is taken by the structure, it is much better for integration. The spiral air trench filled with air which has a lower dielectric constant (≅1) can efficiently reduce the parasitic capacitance of the inductor created. As a result, the inductor of the invention, suitable for RF circuits operating at a higher frequency, has a higher quality factor. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention in not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so at to encompass all such modifications and similar arrangements.