Abstract:
Lower reflow temperature in dielectrics is obtained by using a composite dielectric film. The composite dielectric film includes a first layer doped in the conventional range. A borophosphosilicate glass (BPSG) thick layer having concentrations of around 4.4 wt. % boron and around 5.6 wt. % phosphorus is exemplary. The composite dielectric film includes a second layer doped excessively. A BPSG thin layer having concentrations between 1-4 wt. % phosphorus and between 7-8 wt. % boron is exemplary. A capping layer of conventional dopant concentration may be additionally added to prevent outdiffusion. A composite dielectric BPSG film can be reflowed around 700° C. as compared to the typical 800-900° C. range. After reflow, etching away the second highly doped layer removes any potential adverse effects.

Description:
This is a Division, of application Ser. No. 08/136/640, filed Oct. 12, 1993 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to the field of integrated circuits and more specifically to dielectric films used in fabricating such circuits. 
     BACKGROUND OF THE INVENTION 
     Dielectric materials are used in manufacturing integrated circuits. As described in  VLSI TECHNOLOGY , second edition, by S. M. Sze, 1988, on page 234, dielectric materials have many purposes, such as insulation between conducting layers, diffusion and ion implantation masks, diffusion from doped oxides, capping doped films to prevent the loss of dopant, gettering impurities, and as passivation to protect devices from impurities, moisture and scratches. 
     Phosphorus doped silicon dioxide (PSG) is one such dielectric material. It is frequently used as an insulator between polysilicon gates and top metallization. U.S. Pat. No. 5,112,762, issued May 12, 1992, illustrates in FIG. 3 a high density dynamic random access memory, DRAM, device having a multilevel oxide (MLO) oxide layer  30  that could be formed of PSG. 
     Phosphorus in an interlevel oxide isolation between conductive layers act as an ion migration barrier, but the main reason for doping is to decrease the reflow temperature as explained in  SEMICONDUCTOR INTEGRATED CIRCUIT PROCESSING TECHNOLOGY  by Runyan and Bean, 1990, pages 143-145. Reflowing the PSG helps reduce the topography which is good for depositing metal. A problem in interlevel connections is that etching process for forming contact apparatuses through the interlevel oxide may produce openings with sharp edges. If the softening point of the interlevel oxide can be reduced enough, then it can be reflowed after the openings are made in order to round off the abrupt edges. Similarly, after an oxide is deposited over a series of leads, the abrupt shoulders can be smoothed by reflowing. PSG softens and reflows at a relatively high temperature between 950 to 1100 C. Typical dopant concentrations in PSG are about 6 to 8 wt. % phosphorus concentration. See  VLSI TECHNOLOGY  page 234 and pages 255-258. 
     Another dielectric material commonly used is borophosphosilicate glass (BPSG). Multilevel oxide (MLO) layer  30  of the above U.S. Pat. No. 5,112,762 could be formed of BPSG. BPSG is formed by adding boron to PSG. Adding boron to PSG provides an advantage of lowering the reflow temperature of PSG. Typical reflow temperatures for BPSG are between between 800 to 900 C. Typical dopant concentrations are 1 to 4 wt. % boron and 4 to 6 wt. % phosphrous. See  VLSI TECHNOLOGY  page 257. 
     It is desirable to lower the reflow temperature of dielectric films to avoid disturbing modern shallow junction transistor devices. Modern VLSI circuit devices, such as the 16M bit DRAM in the above U.S. Pat. No. 5,112,762 have shallow junction transistors in the memory array and periphery. The source/drain regions of the transistors are formed, as for example, by ion implantation before applying a dielectric material such as BPSG. Thus, the reflow is commonly referred to as a “back end” operation; that is, an operation occurring after formation of the transistors. Exposing the source/drain regions to too high a reflow temperature in the back end adversely disturbs the dopant concentration in the source/drain regions. 
     One way to achieve lower reflow temperatures is to increase the content of the dopant species. By increasing the boron and phosphorus concentration in a BPSG layer, the reflow can be lowered. However, this is undesirable because too high levels of these dopants produce a deleterious influence on the film properties. Phosphorus concentrations above 7%-8% wt. may cause corrosion of the aluminum metallization by the acid products formed from the reaction between the phosphorus in the oxide and atmospheric moisture. Boron contents above about 4% make a glass unstable in high humidity. Bubbles of volatile phosphorus oxides and crystallites of boron-rich phases can occur unless the dopant concentrations are carefully controlled. 
     It is desirable therefore, and accordingly an object of the invention, to provide a dielectric material having low temperature reflow characteristics. 
     It is an additional object of the invention to lower the reflow temperature of doped dielectric films. 
     It is a further object of the invention to provide an improved BPSG film. 
     Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification and drawings. 
     SUMMARY OF THE INVENTION 
     An inhomogeneous dielectric film provides for lower temperature reflow. A first layer having conventional dopant concentration is formed. A second abutting layer is excessively doped with respect to the first layer. The second layer initiates reflow at a lower temperature. A third abutting layer may be added to hinder out diffusion of dopant from the second layer. An exemplary composite dielectric film is a BPSG film having a first layer concentration of about 4.4 wt % boron and about 5.6 wt. % phosphorus. The second highly doped layer contains about 7 to 8 wt. % boron and about 1 to 4 wt. % phosphorus. Utilizing such a BPSG scheme reduces the anneal temperature necessary to reflow the BPSG from about 800 to 900 C. to about 700 C. or less. The second layer is a sacrificial layer that may be removed during subsequent etching and thus provides no adverse effects from its higher dopant concentration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-3 illustrate successive steps in preparing a semiconductor wafer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As the following description will reveal, a key aspect of the invention is the addition of an excessively doped film to initite a reflow at a lower temperature than is possible with a conventional film. Providing a second more highly doped film over a first doped film significantly lowers the reflow temperature characteristics of the first doped film while keeping other film properties practically unchanged. While the below description is cast in terms of BPSG as an exemplary example, the invention is not limited to BPSG and is applicable to doped films in general including, but not limited to, PSG, arsenic doped silicate glass (ASG), and boron arsenic silicate glass (BSAG). 
     FIGS. 1-3 represent a process flow of a semiconductor wafer prepared when utilizing the invention. In FIG. 1, a semiconductor wafer  10  is prepared having a doped film layer  12  in accordance with a preferred embodiment of the invention. Semiconductor wafer  10  is representative of any generic wafer and may include, for example, VLSI devices such as a DRAM of the 16M bit or 64 M bit variety having shallow junction devices thereon. In a DRAM application, devices  14   a  and  14   b  may represent word lines forming transistor gates in the memory array of the DRAM; they may additionally represent gates of transistors in the DRAM periphery. 
     In FIG. 1, doped film  12  is formed primarily of two components: a first layer  12   a  and an overlying abutting layer  12   b . A third overlying layer  12   c  may be added. First layer  12   a  overlies devices  14   a  and  14   b . First layer  12   a  may be a thick BPSG film with conventional phosphorus and boron content. As an example, it may be about 1.8 microns thick having a concentration of about 4.4 wt. % boron and about 5.6 wt. % phosphorus and may be formed by conventional deposition. 
     In FIG. 1, second layer  12   b  may be a BPSG film having higher dopant concentration. The higher dopant concentration may be phosphorus only, boron only, or both. One example is a phosphorus concentration between 1-4 wt. % in combination with a boron concentration between 7-8 wt. %. While layer  12   b  could be a thick film, it need not be so. As illustrated in FIG. 1, layer  12   b  is a thin film of about 1000 Å (angstroms). Layer  12   b  may be deposited in situ on top of layer  12   a  or deposited as a separate step. The higher dopant concentration in layer  12   b  initiates reflow of layer  12  at a much lower temperature; it is possible to reflow composite BPSG, for example, below 750° C.-700° C. (It is also possible to reflow ASG, for example, at a temperature of about 750° C.) 
     FIG. 1 also illustrates an additional overlying layer  12   c . Layer  12   c  is deposited with dopant concentration lower than layer  12   b . While layer  12   c  is not necessary to practice the invention, it does provide an added advantage in that it functions to cap layer  12   b  and hinder out diffusion. Layer  12   c , in the BPSG example, may contain conventional dopant concentrations such as 4.4 wt. % boron and 5.6 wt. % phosphorus. Layer  12   c  need not be thick layer; it may be thinner than layer  12   b  may be around 500 Å. 
     Referring now to FIG. 2, wafer  10  of FIG. 1 is illustrated undergoing reflow. Heating layer  12  causes it to planarize, reflow, or smooth out. Because layer  12   b  has a higher dopant concentration level than layer  12   a , reflow begins at a lower temperature. The exact amount of time needed to accomplish the reflow will, of course, vary depending upon the thickness of layer  12   a . In the BPSG example described above, heating layer  12  for about 20 minutes at a temperature of approximately 700° C. will be sufficient. 
     FIG. 3 illustrates the resulting wafer  10  after reflow with layers  12   b  and  12   c  being removed. Layer  12   b  (and layer  12   c ) are preferably removed after the low temperature reflow occurs. Removal may be accomplished by etching, such as utilizing a 1% hydrogen flourine etchant. The etch should be timed to remove layers  12   b  and  12   c  and to etch layer  12   a  to the desired thickness that is, of course, dictated by design rules. As an example, in manufacturing a VLSI multilevel metal device such as a 16M DRAM, the etch may be about 11,500 Å deep. Because highly doped layer  12   b  is removed, the later added metal suffers no adverse effects. Layer  12   b  is thus a sacrificial layer providing advantages of lowering reflow temperature without disadvantages of corrosion. 
     The above description provides illustrative details for practicing the invention generally. Specific exemplary examples of process flows follow below. 
     In example one, a process flow is given for forming a multilevel oxide (MLO) layer in a multilevel metal device such as a 16M DRAM. This process flow could be used to form the previously mentioned MLO layer  28  of FIG. 3 of U.S. Pat. No. 5,112,762. In that patent, the MLO layer  28  provides isolation between the silicided polysilicon bitlines a Metal- 1  interconnect. A low pressure CVD step first forms an oxide layer about 1000 Å thick on a semiconductor wafer. A plasma enhanced CVD step then forms a first BPSG film of about 1.8 microns thick having a concentration of about 4.4 wt. % boron and about 5.6 wt. % phosphorus. An in situ deposition step then forms a BPSG film of about 1000 A thick having a concentration of about 7-8 wt. % boron and between about 1-4 wt. % phosphorus over the first BPSG layer. Another in situ deposition step then forms a third BPSG film of about 500 Å thick having concentration of about 4.4 wt. % boron and about 5.6 wt. % phosphor. A low temperature anneal occurs in a furnace heated to around 700° C. which causes the BPSG film to reflow and smooth. A BPSG etch follows wherein about 11,500 Å removes the third BPSG film, the second BPSG film, and partially removes the first BPSG film. The wafer may then be patterned for contacts, etched to form contact openings in the memory array and the periphery, and forwarded to a metal module whereupon metal will be placed into the contact openings, for example by sputtering, to form the interconnect. 
     In example two, a process flow is given for forming a metal interlevel oxide (MILO) layer in a multilevel metal device such as a 64M DRAM. This process flow could also be used to form MILO layer  24  of FIG. 3 of U.S. Pat. No. 5,112,762. In that patent, the MILO layer  24  provides isolation between the metal- 1  interconnect and the metal- 2  interconnect layer. A plasma enhanced CVD step first forms an oxide layer about 2000 Å thick on a semiconductor wafer over the metal- 1  interconnect. A plasma enhanced CVD step then forms a first BPSG film of about 1.8 microns thick having a concentration of about 4.4 wt. % boron and about 5.6 wt. % phosphorus. An in situ deposition step then forms a BPSG layer of about 1000 Å thick having a concentration of about 7-8 wt. % boron and between about 1-4 wt. % phosphour over the first BPSG layer. Another in situ deposition step then forms a third BPSG layer of about 500 Å thick having concentration of about 4.4 wt. % boron and about 5.6 wt. % phosphorus. A low temperature anneal occurs in a furnace heated to around 700° C. which causes the BPSG film to reflow and smooth. A BPSG etch follows wherein about 12,500 Å removes the third BPSG film, the second BPSG film, and partially removes the first BPSG film. Then about 1000 Å of plasma enhanced CVD oxide is deposited. The wafer may then be patterned for vias, etched to form the via openings, and forwarded to the metal- 2  module whereupon metal will be placed into the vias, as for example by sputtering. 
     In summary, the addition of an excessively doped film to an existing film initites a reflow at a lower temperature than is possible with a conventional film. BPSG may be reflowed below 750° C. at approximately 700° C. A composite, or inhomogenous, film is thus provided having improved reflow characterestics. A third conventionally doped film may be added which caps the second doped film and prevents outdiffusion of the second highly doped film. Lowering the reflow temperature is particularly advantageous when manufacturing VLSI devices having shallow junction devices as it aids in avoidance of disturbing the dopant concentrations in the shallow junctions, and thereby helps in providing less defective, and thus more manufacturable, semiconductor devices. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various other embodiments of the invention will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications of the embodiments as fall within the true scope and spirit of the invention.