Abstract:
A method is provided for depositing aluminum thin film layers to form improved quality contacts in a semiconductor integrated circuit device. All or some of the deposition process occurs at relatively low deposition rates at a temperature which allows improved surface migration of the deposited aluminum atoms. Aluminum deposited under these conditions tends to fill contact vias without the formation of voids. The low temperature deposition step can be initiated by depositing aluminum while a wafer containing the integrated circuit device is being heated from cooler temperatures within the deposition chamber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to semiconductor integrated circuits, and more specifically to a method for depositing metal layers in integrated circuits so as to form an improved interlevel contact. 
     2. Description of the Prior Art 
     In semiconductor integrated circuits, formation of metal interconnect layers is important to the proper operation of these devices. Metal interconnect signal lines make contact to lower conductive layers of the integrated circuit through vias in an insulating layer. For best operation of the device, the metal used to form the interconnect layer should completely fill the via. 
     Because of its physical properties, aluminum is especially suited for fabrication of metal interconnect lines in integrated circuits. However, the sputtering process used to apply aluminum thin film layers to an integrated circuit generally results in less than ideal filling of contact vias. Large aluminum grains tend to form on the upper surface of the insulating layer. Those grains which form at the edges of the contact via tend to block it before aluminum has a chance to completely fill the via. This results in voids and uneven structures within the via. 
     This problem is especially acute as integrated circuit devices are fabricated using smaller geometries. The smaller contacts used in these devices tend to have a larger aspect ratio (height to width ratio) than larger geometry devices, which exacerbates the aluminum filling problem. 
     The uneven thickness of the aluminum layer going into the via, caused by the step coverage problem just described, has an adverse impact on device functionality. If the voids in the via are large enough, contact resistance can be significantly higher than desired. In addition, the thinner regions of the aluminum layer will be subject to the well known electromigration problem. This can cause eventual open circuits at the contacts and failure of the device. 
     Many approaches have been used to try to ensure good metal contact to lower interconnect levels. For example, refractory metal layers have been used in conjunction with the aluminum interconnect layer to improve conduction through a via. Sloped via sidewalls have been used to improve metal filling in the via. The use of sloped sidewalls is becoming less common as device sizes shrink because they consume too much area on a chip. 
     Even with these techniques, the problems of completely filling a via with aluminum are not solved. In part this is due to the fact that aluminum is deposited at a temperature which tends to encourage fairly large grain sizes. Voids and other irregularities within the contact continue to be problems with current technologies. 
     One technique which has been proposed to overcome the via filling problem is to deposit the aluminum interconnect layers at a temperature between 500° C. and 550° C. At these temperatures, the liquidity of the aluminium is increased, allowing it to flow down into the vias and fill them. This technique is described, for example, in DEVELOPMENT OF A PLANARIZED Al-Si CONTACT FILLING TECHNOLOGY H. Ono et al June 1990 VMIC Conference proceedings, pages 76-82. This reference teaches that temperatures below 500° C. and above 550° C. result in degraded metal filling of contact vias. It is believed that use of such technique still suffers from problems caused by large grain sizes. 
     It would be desirable to provide a technique for depositing aluminum thin film layers on an integrated circuit so as to improve coverage in contact vias. It is further desirable that such a technique be compatible with current standard process flows. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for forming an aluminum contact on an integrated circuit. 
     It is another object of the present invention to provide such a method in which aluminum fills the contact via while minimizing the number of voids formed therein. 
     It is a further object of the present invention to provide such a method which is compatible with current process technology. 
     Therefore, according to the present invention, a method is provided for depositing aluminum thin film layers to form improved quality contacts in a semiconductor integrated circuit device. All or some of the deposition process occurs at relatively low deposition rates at a temperature which allows improved surface migration of the deposited aluminum atoms. Aluminum deposited under these conditions tends to fill contact vias without the formation of voids. The low temperature deposition step can be initiated by depositing aluminum while a wafer containing the integrated circuit device is being heated from cooler temperatures within the deposition chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIGS. 1 and 2 illustrate the formation of a metallic contact according to the present invention; 
     FIG. 3 is a graph illustrating preferred process conditions for formation of an aluminum contact; and 
     FIGS. 4a-4d illustrate several alternative deposition rate diagrams for forming contacts according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention. 
     Referring to FIG. 1, an integrated circuit device is formed in and on a substrate 10. An insulating layer 12, such as a reflow glass or other oxide layer as known in the art, is formed over the substrate 10. Layer 12 typically has a thickness on the order of approximately 6000-12,000 angstroms. A contact via 14 is formed through the oxide layer 12 using a mask and an isotropic etching technique as known in the art. Via 14 is shown as making contact with substrate 10 in FIG. 1, but may be formed over a lower interconnect layer as known in the art. 
     A barrier metal layer 16, such as a refractory metal, refractory metal nitride, refractory metal silicide, or combination thereof, is deposited over the surface of the device as known in the art. Layer 16 is relatively thin, typically approximately 500-2000 angstroms thick, and is deposited conformally to cover the bottom and sidewalls of contact opening 14. 
     Referring to FIG. 2, an aluminum layer 18 is deposited over the surface of the device. When the aluminum layer 18 is deposited using the process conditions described below, the layer 18 actually completely fills in the via 14 as shown in FIG. 2. This occurs because the preferred process conditions enhance the surface migration of the deposited aluminum atoms, so that aluminum formation in the bottom of the via 14 occurs preferentially to formation on the oxide layer 12 near the edges of the via 14. This ensures a high quality, reproducible contact within the via 14, greatly minimizing the problems caused by incomplete filling of the via 14. 
     FIGS. 3 and 4 illustrate preferred conditions for deposition of the aluminum layer 18 in order to provide an improved contact. Graph 30 illustrates the deposition rate, in angstroms per second, as a function of the deposition temperature in degrees Celsius. The preferred region 32 lies between 400° C.-500° C., with the maximum deposition rate lying below a line extending from a rate of about 30 angstroms per second at 400° C. to 100 angstroms per second at 500° C. 
     When aluminum is deposited within this preferred region 32, its surface migration characteristics are enhanced over metal deposited under other conditions. For example, depositing aluminum at temperatures higher than 500° C. tends to form large grains, so that blocking of the contact opening occurs as described earlier. If the deposition rate is too high, the deposited aluminum is not able to migrate quickly enough into the via to completely fill it. Therefore, the region 32 depicted in FIG. 3 outlines, approximately, a preferred pairing of processing conditions under which deposited aluminum migrates into the contact via and fills it while minimizing the formation of voids and uneven regions. 
     Process conditions can be varied slightly from that shown in FIG. 3 without departing from the teachings of the present invention. For example, temperatures a little below 400° C. can be used, as long as the deposition rates are not too high. As the temperature decreases, the mobility of the deposited aluminum atoms goes down, so that incomplete filling of the via occurs if the deposition rates are too high. 
     FIG. 4 includes four graphs illustrating preferred processes by which an aluminum interconnect layer can be formed. All of these processes utilize, to a greater or lesser degree, processing which occurs within the preferred region 32. Each of the curves 40, 42, 44, 46 illustrates a variation in the aluminum deposition rate with time. Each curve 40-46 illustrates an alternative process utilizing the concepts of the present invention. 
     Each of the four processes shown in FIG. 4 preferably uses approximately the same set of initial conditions. In the prior art, it is common to deposit a very thin layer of small grain aluminum at a relatively cold temperature, typically below 350° C., and then stop the deposition process. The wafer on which the integrated circuit device is located is then preheated to the required deposition temperature, over 500° C., by bathing the wafer with a stream of preheated argon gas. Once the wafer has reached the deposition temperature, deposition of the aluminum is resumed at such elevated temperature. 
     In the present technique, aluminum is preferably deposited on the device continuously while the device is being heated. Thus, a small amount of aluminum is deposited on the device while the wafer is at or below 350° C. As the wafer gradually heats to the desired deposition temperature, aluminum deposition continues. This gives a layer of aluminum which is deposited with very small grain sizes, tending to minimize grain size growth at later stages. The deposition temperature is between 400° C. and 500° C., and is typically reached in about 40 seconds. 
     FIG. 4 shows deposition rate curves for four alternative deposition techniques. For all of the curves in FIG. 4, the initial temperature of the wafer is assumed to be approximately 350° C., with the final deposition temperature being 450° C. Heating the wafer to 450° C. takes approximately 40 seconds. It will be appreciated by those skilled in the art that different deposition temperatures may be used. Once the wafer has heated to the deposition temperature, the temperature remains constant. 
     Curve 40 in FIG. 4(a) depicts a deposition process in which the deposition rate stays constant during the entire course of depositing the aluminum layer 18. Deposition begins when heat is first applied to the wafer in the chamber, and continues while the wafer heats to 450° C. and remains there. At a rate of 40 angstroms per second, an 8000 angstrom thick aluminum layer will take approximately 200 seconds to deposit. 
     FIG. 4(b) shows an alternative deposition process in which the deposition rate is performed at 40 angstroms per second for the first 20 seconds, and 60 angstroms per second thereafter. The temperature is increasing toward the 450° C. point during the entire deposition step at 40 angstroms per second, and for the first 20 seconds at 60 angstroms per second. For an 8000 angstrom layer, the process curve 42 will result in an aluminum layer formation process which takes approximately 140 seconds. 
     Curve 44 shows a process in which the initial deposition rate is 40 angstroms per second, followed by an increase to 80 angstroms per second after 20 seconds. After approximately one-third of the entire thickness of the aluminum layer has been deposited, the deposition rate is changed to 30 angstroms per second. This rate is maintained for the deposition of approximately another one-third of the entire layer thickness, followed by an increase of the deposition rate back to 80 angstroms per second. 
     The process depicted by curve 44 will take approximately 160 seconds to deposit an 8000 angstrom layer of aluminum. This assumes that 2400 angstroms are deposited during each of the 80 angstrom per second segments, and during the 30 angstrom per second segment. The process of FIG. 4(c) provides for an initial fast deposition of aluminium, followed by a slow deposition period in which deposited aluminium is given the opportunity to migrate into the contact opening. The 30 angstrom deposition period will last for approximately 80 seconds, in order to deposit 2400 angstroms. 
     Curve 46 in FIG. 4(d) starts in the same manner as curve 44, but ends with a higher deposition rate. Processing time is saved by the faster deposition near the end of the process. By this point in the deposition process, the contact opening has been mostly filled, and the possibility of voiding in the via has been greatly decreased. Thus, there is no harm to depositing aluminum at a rate which falls outside of the preferred region 32. 
     It will be appreciated by those skilled in the art that the processes shown in FIG. 4 are illustrative and not definitive. Other variations are possible. The precise combination of deposition temperatures and deposition rates can be varied to suit the requirements and restrictions of the particular processes at hand. For example, if large contact openings only are used, faster deposition rates can be made as the voiding problem is not so critical. For processes such as those illustrated by curves 44 and 46, it is not necessary to adhere to a one-third thickness deposition at each rate. These rates and times may be varied to suit the requirements of a production process while still taking advantage of the concepts of the invention. 
     It is also possible to use the technique of depositing aluminum within the preferred area 32 without continuously depositing aluminum while the wafer temperature is ramping up to the deposition temperature. As is done on the prior art, a thin layer of aluminum can be deposited at relatively cold temperatures, preferably below 350° C. Deposition is then stopped while the wafer is brought to a temperature between 400° C. and 500° C. Deposition is then resumed at a rate within the preferred region 32, and completed using the teachings set forth above. For example, any of the curves in FIG. 4 can be used, with a difference that the initial 40 angstroms per second deposition rate is omitted. 
     Use of the continuous layer formation while the wafer is being heated, combined with deposition at rates and temperatures within the preferred region 32, results in small deposited aluminum grain size and very good filling of the via. This is caused both by the good electromigration characteristics of the deposited aluminum layer at the temperatures and deposition rates involved, and by the fact that very small initial grain sizes result in smaller final grain sizes, having less tendency to block off the via before it is completely filled. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.