Abstract:
A method for making 0.25 micron semiconductor chips includes annealing the metal interconnect lines prior to depositing an inter-layer dielectric (ILD) between the lines. During annealing, an alloy of aluminum and titanium forms, which subsequently volumetrically contracts, with the contraction being absorbed by the aluminum. Because the alloy is reacted prior to ILD deposition, however, the aluminum is not constrained by the ILD when it attempts to absorb the contraction of the alloy. Consequently, the likelihood of undesirable void formation in the interconnect lines is reduced.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to semiconductor fabrication, and more particularly to methods for improving the reliability of 0.25μ technology semiconductors by alleviating stress in interconnect lines that might otherwise result in the formation of voids in the interconnect lines.  
         BACKGROUND OF THE INVENTION  
         [0002]    Semiconductor chips or wafers are used in many applications, including as integrated circuits and as flash memory for hand held computing devices, wireless telephones, and digital cameras. Regardless of the application, it is desirable that a semiconductor chip hold as many circuits or memory cells as possible per unit area. In this way, the size, weight, and energy consumption of devices that use semiconductor chips advantageously is minimized, while nevertheless improving the memory capacity and computing power of the devices.  
           [0003]    In chips that hold integrated circuits, the individual circuit components are interconnected by conductive elements referred to as “interconnect lines”. These interconnect lines are typically arranged in a multi-layered pattern that is deposited on a semiconductive substrate such as silicon. To insulate the interconnect lines from each other, insulative material is deposited between adjacent interconnect line layers.  
           [0004]    With the above in mind, so-called 0.25 micron technology has been developed, in which the distance between adjacent layers of interconnect lines in an integrated circuit on a semiconductor chip is equal to or less than about three-eighths of a micron. With such a small spacing between interconnect lines, which have heights of about 1.1 microns, the size of the circuits on the chip can be reduced to result in the above-noted advantages.  
           [0005]    Typically, each electrically conductive interconnect line is made of a “stack” of metal layers that typically includes a layer made of aluminum or aluminum alloy, and one or more other metal layers. The aluminum is deposited as a film over the substrate and is then lithographically patterned and chemically etched to form a desired pattern for the circuit&#39;s connector lines. Then, a process referred to as high density plasma (HDP) inter-layer dielectric (ILD) formation is used to fill the gaps between adjacent metal stacks with an electrically non-conductive material. Ordinarily, the ILD deposition step is undertaken at relatively high temperature, incidentally precipitating the formation of an intermetallic structure. In current applications, titanium is commonly used as an underlayer for the aluminum, and the intermetallic structure that forms in such a device is TiAl 3 . Also, an overlayer that includes TiN anti-reflective coating (ARC) is disposed over the stacks, for lithography purposes.  
           [0006]    As understood herein, however, voids caused by hydrostatic stresses undesirably can form in the aluminum, and the voiding of the aluminum can be accelerated by the formation of the intermetallic structure. This is undesirable, because when a void forms in a thin aluminum line, the current path through the line unfortunately is diverted, thereby adversely affecting the reliability of the chip.  
           [0007]    The present invention understands that the above-mentioned hydrostatic stresses arise because the thermal expansion coefficient of the aluminum layer is different from the thermal expansion coefficient of the encapsulating ILD and the silicon substrate, both of which mechanically constrain the aluminum. Furthermore, when an intermetallic structure such as TiAl 3  is formed, the intermetallic structure can volumetrically contract (by 5.9%, in the case of TiAl 3 ), and the aluminum in the intermetallic structure, which is the most compliant metal in the stack, will consequently absorb the contraction-induced strain and thus have an even higher stress state as a result.  
           [0008]    Fortunately, the present invention recognizes that contrary to previous methods, it is possible to anneal the metal stacks prior to ILD deposition in 0.25μ semiconductors, when only the substrate, and not the ILD, constrains the aluminum in the stacks. As recognized by the present invention, the consequence is that the likelihood of void formation in the interconnect lines is reduced, thereby improving 0.25μ chip reliability.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    A method is disclosed for making a semiconductor chip having electrically conductive interconnect lines. The method includes providing at least one substrate, and establishing at least one predetermined pattern of electrically conductive interconnect lines on the substrate. In accordance with the present invention, prior to depositing a dielectric layer between the interconnect lines, the lines are annealed.  
           [0010]    In a preferred embodiment, each line establishes a stack including at least one layer having aluminum therein and at least one layer having titanium therein, and an alloy of titanium and aluminum is formed during the annealing step. Preferably, the annealing step is undertaken by heating the electrically conductive lines to a temperature of between three hundred fifty degrees Celsius and four hundred fifty degrees Celsius (350° C.-450° C.) for a period of between ten minutes and ninety minutes. Moreover, the electrically conductive lines are exposed to one or more annealing gases, including nitrogen and hydrogen, during the annealing step. In one preferred embodiment, the ILD is TEOS that is deposited by directing TEOS onto the lines while simultaneously removing excess TEOS by sputtering. A chip made according to the present method, and a computing device incorporating the chip, are also disclosed.  
           [0011]    In another aspect, a semiconductor chip includes at least one substrate and at least one predetermined pattern of aluminum lines supported by the substrate, with adjacent lines being separated by distances equal to or less than about three-eighths of a micron. An alloy of aluminum and titanium is on the lines, with the alloy (TiAl 3 ) having been reacted prior to insulating the lines from each other.  
           [0012]    In still another aspect, a method for making a semiconductor chip includes establishing plural electrically conductive lines on at least one substrate. At least first lines are separated from second lines by distances equal to or less than three-eighths of a micron. The method further includes reacting a metal alloy on the lines, and, after reacting the alloy, depositing a dielectric between at least two lines that are adjacent each other. With this method, the dielectric does not constrain the lines during the reacting step.  
           [0013]    Other features of the present invention are disclosed or apparent in the section entitled: “DETAILED DESCRIPTION OF THE INVENTION.” 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    For fuller understanding of the present invention, reference is made to the accompanying drawing in the following detailed description of the Best Mode of Carrying Out the Present Invention. In the drawings:  
         [0015]    [0015]FIG. 1 is a flow chart of the present process for making a 0.25μ technology semiconductor chip;  
         [0016]    [0016]FIG. 2 is a schematic diagram of the substrate showing the chip after conductor line stack formation;  
         [0017]    [0017]FIG. 3 is a schematic cross-sectional diagram of the substrate showing the chip after ILD deposition/etching, schematically showing a computing device that incorporates the chip; and  
         [0018]    [0018]FIG. 4 is a schematic diagram of a high density plasma gap fill deposition chamber.  
         [0019]    Reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. In the description, the terms “vertical” and “horizontal” refer to the orientations of the figures shown, for purposes of disclosure, and do not necessarily refer to the orientation of the present wafer once the wafer is embodied in a computing device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Referring initially to FIGS. 1 and 2, the process of the present invention in producing 0.25μ technology computer chips can be understood. Commencing at block  10  in FIG. 1 and as shown in FIG. 2, plural stacks  12  are formed on a substrate  14 . Each stack  12  is a metallic, electrically-conductive stack that defines a respective interconnect line for an integrated circuit chip, generally designated  16 . The chip  16  can establish an integrated circuit such as a microprocessor or a flash memory chip (e.g., an electrically programmable memory (EPROM)) for use in the computer arts. Although only two stacks  12  are shown for clarity of disclosure, it is to be understood that more than two stacks typically are formed on the substrate  14 .  
         [0021]    Preferably, the substrate  14  includes a semiconductor such as silicon. Also, the substrate  14  can include a number of devices, such as metal oxide silicon field effect transistor (MOSFET) devices, that are electrically connected to one or more of the stacks  12  via connector plugs.  
         [0022]    In the preferred embodiment, each stack  12  includes a respective lower titanium layer  18  abutting the substrate  14 , an upper titanium layer  20  parallel to and spaced from the lower titanium layer  18 , and an aluminum layer  22  sandwiched therebetween. The titanium layers can be titanium or a titanium alloy such as titanium nitride. It is to be understood that greater or fewer layers can be provided. In a preferred embodiment, the aluminum layer  22  is made of aluminum or an aluminum alloy including aluminum and from 0.1% to about 10% by weight of one or more of copper, nickel, zinc, gold, titanium, indium, chromium, silver, palladium, and platinum.  
         [0023]    The stacks are patterned on the wafer substrate  14  in accordance with means known in the art, e.g., by depositing the various metallic layers as films, covering the films with a mask, and then directing ultraviolet light against the exposed portions of the films. After lithographic patterning, chemical etching is used to remove portions of metal not in the pattern to establish the predetermined pattern of aluminum conductive lines of the chip  16 , as shown in FIG. 2.  
         [0024]    As can be appreciated in reference to FIG. 2, the chip  16  a so-called “quarter micron chip”, in that the distance  6  between adjacent stacks  12  is about equal to or less than three-eighths of a micron (0.375μ).  
         [0025]    Moving to block  24  in FIG. 1, after forming the stacks  12 , the stacks  12  are annealed to form an intermetallic the stacks  12 , with an alloy structure  25  being formed at the interface between each layer of aluminum and a layer of titanium (only a single alloy structure  25  shown for clarity of disclosure). In the preferred embodiment, the metal alloy is an alloy of aluminum and titanium, and more specifically is TiAl 3 . Preferably, the annealing at block  24  is undertaken by heating the stacks  12  to a temperature of between three hundred fifty degrees Celsius and four hundred fifty degrees Celsius (350° C.-450° C.) for a period of between ten minutes and ninety minutes. Also, during the annealing step the stacks  12  are exposed to one or more annealing gases, preferably nitrogen at a flow rate of between eleven standard liters per minute and fourteen standard liters per minute ( 11 . 0  slm-14.0 slm) and hydrogen at a flow rate of between one standard liter per minute and two standard liters per minute (1.0 slm-2.0 slm).  
         [0026]    After annealing, the chip  16  is cooled, during which time the TiAl 3  contracts. Accordingly, as provided by the present invention the TiAl 3 , which would otherwise form during ILD deposition and contract during the subsequent cool down when the stacks  12  are constrained by the ILD, is instead formed prior to ILD deposition, thus permitting the aluminum in the stacks  12  to absorb contraction from the TiAl 3  before the stacks  12  are constrained by the ILD. This reduces the likelihood that voids will undesirably form in the interconnect lines of the chip  16 .  
         [0027]    Moving to block  26  of FIG. 1 and referring to FIG. 3, an interlayer dielectric (ILD)  28  is deposited between the stacks  12  to insulate the stacks  12 .  
         [0028]    While the present invention envisions using any appropriate high density plasma (HDP) ILD such as silane, in the preferred embodiment the ILD used is tetraethoxy silane (TEOS), to avoid the production of free hydrogen that could otherwise embrittle the aluminum.  
         [0029]    Accordingly, to understand the preferred embodiment cross-reference is made to FIGS. 3 and 4. An HDP chamber  30  (FIG. 4) is enclosed by a wall  32 . A support  34  is disposed in the chamber  30 , and the support  34  may hold an e-chuck  36 . One or more wafers or substrates  22  are in turn disposed on the e-chuck  36 .  
         [0030]    To provide for depositing ILD on the wafer  22 , a TEOS inlet  38  and an oxygen inlet  40  are associated with the chamber wall  32  to respectively direct TEOS and oxygen into the chamber  30 , and a source power lead  42  is likewise associated with the chamber  30 . In the preferred embodiment, TEOS is directed into the chamber  30  at a rate ranging from one half milliliter per minute to three milliliters per minute (0.5 mL/min-3.0 mL/min). In contrast, oxygen is directed into the chamber  30  at a rate ranging from ten standard cubic centimeters per minute to fifty standard cubic centimeters per minute (10 SCCM-50 SCCM). The pressure within the chamber  30  is maintained between one milliTorr and three Torr (1 mTorr-3 Torr), and the source power applied to the chamber  30  is between five hundred watts and five thousand watts (500 w-5000 w).  
         [0031]    To provide for simultaneous sputtering of TEOS while the TEOS is being deposited, an etchant inlet  44  is associated with the wall  32  to port a gaseous sputtering agent, preferably argon, into the chamber  30 . Also, a bias power lead  46  is connected to the e-chuck  36 , and bias power is maintained at about three thousand watts (3000 w).  
         [0032]    If desired, both the annealing step and the subsequent ILD deposition step can be undertaken in the chamber  30 . If the annealing step is to be undertaken in the chamber  30 , nitrogen and hydrogen inlets (not shown) are provided in the chamber  30 .  
         [0033]    The completion of the chip  16  may proceed in any such manner as is required by its design. The resulting chip  16  can be incorporated into a computing device  50  (FIG. 3), e.g., a computer, digital camera, wireless telephone, or hand held computer, for use by the computing device  50 .  
         [0034]    The principles of the present invention are equally applicable to a wide range of semiconductor and integrated circuit design and manufacture regimens, including but not necessarily limited to the production of non-volatile memory devices. All such implementations are specifically contemplated by the principles of the present intention.  
         [0035]    The present invention has been particularly shown and described with respect to certain preferred embodiments and features thereof. However, it should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the inventions as set forth in the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more”. The inventions illustratively disclosed herein may be practiced without any element which is not specifically disclosed herein.