Abstract:
The invention proposes methods for producing integrated circuits wherein the dielectric constant between closely spaced and adjacent metal lines is approaching 1. One method of the invention uses low-melting-point dielectric to form a barrier form a void between conductive lines. Another method of the invention uses sidewall film to form a similar barrier.

Description:
This application is a continuation of U.S. application Ser. No. 09/182,858 filed on Oct. 29, 1998, U.S. Pat. No. 6,083,821; which is a continuation of U.S. application Ser. No. 08/676,537 filed on Jul. 9, 1996 and issued as U.S. Pat. No. 5,847,439 on Dec. 8, 1998; which is a continuation of U.S. application Ser. No. 08/481,051 filed on Jun. 7, 1995 and issued as U.S. Pat. No. 5,599,745 on Feb. 4, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for providing an improved dielectric property between adjacent conductive lines in a semiconductor device. More particularly, the present invention relates to a method and apparatus where the space between closely proximate conductive lines comprises an air void, e.g. a space not filled by either: (i) the dielectric material usually needed for insulation between layers of conductive lines; or 2) the passivation overcoat formed on the top layer of conductive lines. As a result, the invention provides for minimum capacitive coupling because the space between closely positioned conductive lines is free space having a unitary dielectric constant. 
     2. Description of the Related Art 
     The integrated circuit industry is constantly reducing the separation between conductive lines to achieve a smaller integrated circuit. Minimizing integrated circuit area is important because it yields many benefits including higher reliability, lower cost, and higher speed. 
     Reducing the spacing between the conductive lines in an integrated circuit, however, results in an increase in the amount of capacitive crosstalk between these conductive lines. In conventional integrated circuits, the interconnect arrangement typically consists of many adjacent conductive lines. If the capacitive crosstalk between a first conductive line and a second conductive line is high, then the voltage on the first conductive line alters or affects the voltage on the second line. This alteration in voltage could cause an integrated circuit to misinterpret logic levels and/or voltage levels and, therefore, incorrectly process binary and/or analog information. An integrated circuit that incorrectly processes any information is usually considered inoperable or faulty. 
     Illustrated in FIG. 1 is a semiconductor structure having two conductive lines. FIG. 1 depicts a conductive line  1  and a conductive line  2  adjacent to conductive line  1 . Each of conductive lines  1  and  2  has a width “w,” and conductive line  1  is separated from conductive line  2  by a separation distance “s.” A quantity known as a pitch (pitch=w+s) is conventionally used to characterize capacitive crosstalk for adjacent conductive lines used in the integrated circuit industry. 
     As noted above, a reduction of pitch is an ongoing activity in the integrated circuit industry to achieve integrated circuits which are more dense and use substrate surface area more efficiently. The capacitance between conductive lines  1  and  2  has been found to increase exponentially as pitch is reduced or as the conductive lines  1  and  2  are brought closer together. 
     This increase in capacitive coupling between conductive lines  1  and  2  is a disadvantage because of the resulting increase in the amount of capacitive crosstalk between the conductive lines. For instance, if conductive line  1  is separated from conductive line  2  by one hundred microns, the coupling capacitance will be small and there will be virtually no capacitive crosstalk between the conductive lines. If the pitch is less than approximately one micron, however, the capacitive coupling between the conductive lines will be high. Consequently, the amount of capacitive crosstalk will be high and may render the particular integrated circuit inoperable. 
     Capacitive crosstalk is a phenomenon wherein a voltage level at conductive line  1  adversely effects and/or alters the voltage level at conductive line  2  through the capacitive coupling between conductive lines  1  and  2 ; Assume that the pitch in a conductive layer of a DRAM is 2 microns and that the DRAM has a plurality of data lines and address lines. These address and data lines are typically run across the integrated circuit together or in groups. The conductive lines making up these data lines and address lines are separated by a minimum pitch to conserve surface area on the integrated circuit. One data or address line may be carrying a logic one (i.e., a five volt signal). An adjacent data or address line may be carrying a logic zero (i.e., a zero volt signal or ground potential). 
     The capacitive crosstalk due to this small pitch may cause the logic one value on the first line to effect and/or alter the logic zero value on the adjacent line and/or vice versa. Therefore, instead of properly carrying the correct logic one voltage level or logic zero voltage level, the address and data lines may carry an erroneous voltage level. This erroneous voltage level could alter the operation  6 f the DRAM rendering it faulty or inoperable. Crosstalk phenomenon may have an even greater effect on the operation of analog circuitry. 
     One prior art method for reducing capacitive coupling, and therefore for reducing capacitive crosstalk between conductive lines having a small pitch, is the use of low dielectric constant materials to replace the conventional dielectric materials which are typically used for insulation between conductive layers and the passivation overcoat layer formed on the top conductive layer. Conventional semiconductor dielectric materials have a dielectric constant of about four. Some new dielectric materials, such as Teflon may have a dielectric constant between about two and four. The use of many dielectric materials having low dielectric constants, however, is not feasible because the equipment necessary to process properly the new dielectric materials in integrated circuits is not available. Also, the chemical or physical properties of many of these dielectric materials may be difficult to integrate into conventional integrated circuit processing. 
     Another prior art method for reducing capacitive crosstalk between conductive lines having a small pitch is the use of coaxial cable type arrangements in integrated circuits. A coaxial arrangement typically consists of a first conductor which carries a signal and a second conductor surrounding the first conductor which is used to shield the first conductor from other conductors in the integrated circuit. This coaxial arrangement is very difficult to form in a integrated circuit since one conductor must be entirely surrounded by a second conductor. Furthermore, this coaxial arrangement requires two or more conductive layers. Thus, one functional layer of conductive interconnect requires several layers of conductive material. The use of several conductive layers to form one functional conductive interconnect layer is simply not a cost effective method for reducing capacitive crosstalk. between conducive lines with a small pitch. 
     Another prior art method for reducing capacitive crosstalk between conductive lines having a small pitch proposes a partial air gap in the dielectric region between these conductive lines. In this method, a plug is formed between adjacent conductive lines to provide a partial air gap between these conductive lines when the interlevel insulation or passivation overcoat is formed on top of this plug. This method, however, does not provide a material free region between these conductive lines (i.e. 100% air gap) and will not achieve a dielectric constant approaching 1. Furthermore, this method entails the use of many additional complex and costly processing steps to form the plug and thus is not a simple and cost effective method for reducing capacitive crosstalk. 
     For example, if a dielectric layer having a dielectric constant of 4.3 is used as an interlevel dielectric, then, if there is no air gap (percentage of air gap=0%), the entire region between the conductive lines depends solely upon the dielectric constant of the dielectric material used; 4.3 in this example. If a 50% air gap is formed between conductors, i.e. only 50% of the region between the conductive lines is dielectric material, the effective dielectric constant of the region between the conductive regions is reduced to K=2.75 and, thus, the capacitance is reduced. The dielectric value K=2.75 is 50% due to air and 50% due to the remaining dielectric regions. Ideally, a material free region (100% air gap) is desired wherein the capacitance is very low and the dielectric constant is reduced significantly. 
     Yet another prior art method for reducing capacitive crosstalk between conductive lines with a small pitch proposes a material free region (100% air gap) between these conductive lines. In this method three layers including a growth or seed layer in the middle must be formed on top of the conductive lines. Then the sidewall of the growth or seed layer is selectively grown to close off the opening between the conductive lines to form the material free region. This method also entails the use of many additional, complex, and costly processing steps and, therefore, is not a simple and cost effective method for reducing capacitive crosstalk. 
     Thus, to maintain a small pitch between conductive lines on an integrated circuit and therefore achieve a denser, lower costing integrated circuit, a new, simpler and more cost effective method for reducing coupling capacitance, and therefore reducing capacitive crosstalk between these conductive lines, is needed. 
     SUMMARY OF THE INVENTION 
     The invention overcomes the disadvantages and difficulties of the prior art by introducing new methods and devices wherein the area between closely spaced conductors exhibits a dielectric constant approaching 1. To that end, the present invention proposes several methods for leaving a void between closely spaced conductors (usually metal lines) in a semiconductor device. In conjunction with these methods, the invention also proposes a semiconductor device wherein there is a void between closely spaced conductors (metal lines). 
     One method of the current invention calls for creating a barrier above closely spaced conductors (typically metal lines) by using a low-melting-point glass or organic material. More specifically, after depositing the conductors, this method calls for depositing a layer of low-melting-point glass or organic material. Both the glass and the conductor are then etched using a mask and photoresist process. Furthermore, after removal of the photoresist, the wafer is subjected to a heat cycle. The heat cycle is properly adjusted to cause the glass to sag laterally. Where conductors are closely spaced, the sagging glass from adjacent lines will touch or otherwise form a barrier to subsequently applied layers. The result is a semiconductor product wherein a void exists between the closely spaced conductors. 
     Another method of the current invention creates-a barrier between closely spaced conductors by using sidewall film. In this method, a conductive layer is deposited and then etched using a mask and photoresist process. The wafer is then subjected to a heat cycle before the photoresist is removed. The heat cycle is properly adjusted to cause the photoresist to sag laterally. Where conductors are closely spaced, the sagging photoresist from adjacent lines with touch or near-touch. The sagging photo resist carries the sidewall film on its formerly vertical edge. Therefore, being carried by the sagging photoresist, the sidewall film from closely spaced conductors will arc together or near together forming a barrier to subsequently applied layers. The photoresist is then removed leaving the barrier formed by the sidewall film. The result is a semiconductor product wherein a void exists between the closely spaced conductors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
     FIG. 1 illustrates two conductive lines for the purpose of defining the width, separation distance, and pitch of adjacent conductors. 
     FIG. 2 illustrates a layered wafer section. 
     FIG. 3 illustrates three conductive lines bearing low-melting-point dielectric and photoresist. 
     FIG. 4 illustrates three conductive lines bearing low-melting-point dielectric. 
     FIG. 5 illustrates a finished circuit having a void between closely spaced conductor lines. 
     FIG. 6 illustrates a layered wafer section. 
     FIG. 7 illustrates three conductive lines bearing photoresist and sidewall film. 
     FIG. 8 illustrates three conductive lines bearing laterally expanded photoresist and sidewall film. 
     FIG. 9 illustrates a finished circuit having a void between closely spaced conductor lines. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following describes new methods and devices wherein the area between closely spaced conductors exhibits a dielectric constant approaching 1. This desirable dielectric constant is achieved through various methods for leaving a void between closely spaced metal lines in a semiconductor device. In addition to these various methods, the products that result from these various methods will also be described. 
     Referring to FIG. 2, area  201  represents all layers of a processed silicon wafer disposed prior to metal level  202 . The term “metal level” as used herein refers to a conductive level of a semiconductor die, as is usually formed of tungsten, aluminum, or other conventionally-known materials. Metal level  202  is disposed on the wafer by any conventional technique, such as, for example, chemical vapor deposition (CVD), sputtering, etc. 
     After metal level  202  is applied, a low-melting-point dielectric  203  is disposed on the wafer. The low-melting-point dielectric  203  may be a glass or organic material that will sag before it flows upon exposure to a heat cycle. Examples of satisfactory materials are polystyrene, which melts at about 240 degrees Celcius, polypropylene, which melts at about 190 degrees Celcius, and various chalcogenide glasses made of S, Se, Te, Sb, Ge, or As, which melt between about 300 and 500 degrees Celcius depending upon the composition. The manner of disposition of the material will typically be selected in response to the material and processing considerations. For the above materials, deposition may be performed by sputtering or spinning as is done for photoresist. The thickness of the deposition will be functionally related to the line pitch of the specific structure. For structures having a line pitch of approximately 1 micron, it is anticipated that dielectric layer  203  will be formed at a thickness of approximately 2 microns. 
     Following application of dielectric  203 , photoresist  207  is disposed on its surface. Generally, any conventional photoresist will suffice, such as, for example, OCG 897I manufactured by Hunt Chemical Group. 
     After applying the photoresist, the wafer is subjected to masked light  206  which is light  205  as filtered through mask  204 . This masking process will result in a pattern formation on the surface of photoresist  207 . The undesirable portions of the photoresist  207 , dielectric  203  and metal  202  may then be etched away. In one preferred embodiment each layer will be etched sequentially—photoresist  207 , then dielectric  203 , then metal  202 . However, combinations of metal  202 , dielectric  203  and photoresist may be selected in reference to etchouts to facilitate removal of two or more of the above layers in a single operation. 
     Referring now to FIG. 3, there is depicted an exemplary die structure  304 , depicted in vertical cross section, of a wafer having structures formed and etched as described above. Structure  304  comprises first metal line  303 , with low-melting-point dielectric  302  and photoresist  301  thereover. Structures  308  and  312 , including metal lines  307  and  311 , are constructed in a fashion identical to structure  304 , ie. sections  305  and  309  are photoresist, and sections  306  and  310  are low-;melt.ing-point dielectric. Note also that metal lines  303  and  307  are relatively closely spaced yet separated by void  313 . 
     In accord with the current method, photoresist sections  301 ,  305 ,  309  are removed by conventional processes, and the wafer is subjected to a heat cycle. Depending upon the nature of the dielectric sections  302 ,  306 , an optimum temperature and time may be determined at which dielectric section  302  will sag and approach or meet sagging dielectric section  306  (but neither should flow). The resulting structure will resemble the cross sectional sample shown in FIG.  4 . Due to the heat cycle, structures  304  and  308  are now capped by sagging dielectric sections  302  and  306  respectively. Furthermore, the sagging dielectric sections  302  and  306  have substantially encapsulated void  313 . Moreover, even if there is not a perfect seal between sagging dielectric  302  and  306 , a seal will be made when the metal level wafer is passivated (or insulated) with additional dielectric material. Some attention should be given to use a passivation material. that will serve to seal any openings without flowing through into the void area. The final cross section is shown in FIG. 5 after passivation  501  has been applied. 
     In an alternative method of practicing the present invention, a sidewall film may be used to form a barrier that prevents dielectric from entering the void between closely spaced metal lines. Referring now to FIG. 6, area  601  schematically represents all layers of a processed silicon wafer disposed prior to metal level  602 . Metal level  602  is again disposed on the wafer by any conventional technique. After metal  602  is deposed, an appropriate photoresist layer  603  is disposed on its surface. 
     After applying the photoresist, the wafer is subjected to masked light  606  which is light  605  as filtered through a mask  604 . This masking process will again result in a pattern formation on the surface of photoresist  603 . The undesirable portions of the photoresist  603  and metal  602  may then again be etched away. 
     One envisioned embodiment provides for both chemical and physical etchings of the wafer structure. Generally, most conventional chemical etching processes will result in a polymer film covering the wafer surface. The polymer film is generally the result of the unavoidable mix of etching chemicals with etched metal. The etching of polysilicon using echtants containing chlorine is known to produce a film of this type when the photoresist is present. Accordingly, absent a physical etch, the polymer film would cover both horizontally and vertically disposed surfaces of the wafer. The physical etch, however, generally comprises vertically directed ion bombardment. This vertical ion bombardment removes the polymer film from all horizontally disposed surfaces. Therefore, the only remaining polymer film exists on vertical surfaces—thus the name “sidewall film.” 
     Referring now to FIG. 7, therein is depicted an exemplary die structure  701 , illustrated in vertical cross section, for a wafer formed and etched as described relative to FIG.  6 . Structure  701  comprises metal line  706 , photoresist  704  and sidewall films  713  and  710 . Structures  702  and  716  are constructed in a fashion identical to structure  701 , ie. sections  708  and  705  are photoresist, sections  711 ,  712 ,  714  and  715  are sidewall films, and sections  707  and  709  are conductive (typically metal) lines. Again, metal lines  706  and  707  are relatively closely spaced yet separated by void  703 . 
     According to one method, the wafer is subjected to a heat cycle prior to removal of photoresist sections  704 ,  705 ,  708 . The-photoresist sections  704 ,  705 ,  708 , will be chosen such that a ratio of temperature and time may be determined by which photoresist  704  will sag and meet (or nearly meet) sagging photo resist  705 , without flowing. Accordingly, the process is controllable to yield die structures as depicted in the cross section sample in FIG.  8 . 
     Referring now to FIG. 8, due to the hear cycle, structures  704  and  705  now comprise sagging photoresist sections  704  and  705  respectively. Furthermore, the sagging photoresist sections  704  and  705  have altered the shape of sidewall films  710  and  712  causing those films to substantially meet and encapsulate void  703 . Thereafter, photoresist sections  704  and  705  are removed leaving sidewall films  710  and  712  to encapsulate the void  701 . This etch is preferably performed with gentle dry process such as that available in a “down stream” dry resist removal operation. Of course, virtually any etching is appropriate which does not remove the polymer sidewall film  710  and  712 . 
     Following removal of the photoresist  704 ,  705 , the upper surface of the wafer is passivated or insulated with a dielectric material  901  as shown in FIG.  9 . Therefore, even if there is not a perfect seal between sidewall films  710  and  712 , a seal will be made when the metal level wafer is passivated (or insulated) with additional dielectric material  901 . Some attention should be given to use a passivation material that will serve to seal any openings without flowing through into the void area  703 .