Patent Publication Number: US-6710452-B1

Title: Coherent diffusion barriers for integrated circuit interconnects

Description:
TECHNICAL FIELD 
     The present invention relates generally to semiconductors and more specifically to diffusion barrier in semiconductor processing. 
     BACKGROUND ART 
     In the manufacture of integrated circuits, after the individual devices, such as the transistors, have been fabricated in the silicon substrate, they must be connected together to perform the desired circuit functions. This interconnection process is generally called “metallization”, and is performed using a number of different photolithographic and deposition techniques. 
     In one interconnection process, which is called a “single inlaid” technique, channels of conductor material are deposited in a channel dielectric layer. The process starts with the placement of a first channel dielectric layer, which is typically an oxide layer, over the semiconductor devices. A first inlaid step photoresist is then placed over the oxide layer and is photolithographically processed to form the pattern of the first channels. An anisotropic oxide etch is then used to etch out the channel oxide layer to form the first channel openings. The inlaid step photoresist is stripped and an optional thin adhesion layer is deposited to coat the walls of the first channel opening to ensure good adhesion and electrical contact of subsequent layers to the underlying semiconductor devices. A barrier layer is then deposited on the adhesion layer improve the formation of subsequently deposited conductor material and to act as a barrier material to prevent diffusion of such conductor material into the oxide layer and the semiconductor devices. A first conductor material is then deposited and subjected to a chemical-mechanical polishing process which removes the first conductor material above the first channel oxide layer and inlays the first conductor material in the first channel openings to form the first channels. 
     In another interconnection process, which is called a “dual inlaid” technique, vias and channels are formed at the same time. The via formation step of the dual inlaid process starts with the deposition of a thin stop nitride over the first channels and the first channel oxide layer. Subsequently, a separating oxide layer is deposited on the stop nitride. This is followed by deposition of a thin via nitride. Then a via step photoresist is used in a photolithographic process to designate via areas over the first channels. 
     A nitride etch is then used to etch out the via areas in the via nitride. The via step photoresist is then removed, or stripped. A second channel dielectric layer, which is typically an oxide layer, is then deposited over the via nitride and the exposed oxide in the via area of the via nitride. A second inlaid step photoresist is placed over the second channel oxide layer and is photolithographically processed to form the pattern of the second channels. An anisotropic oxide etch is then used to etch the second channel oxide layer to form the second channel openings and, during the same etching process to etch the via areas down to the thin stop nitride layer above the first channels to form the via openings. The inlaid photoresist is then removed, and a nitride etch process removes the nitride above the first channels in the via areas. An adhesion layer is often deposited to coat the via openings and the second channel openings. Next, a barrier layer is deposited on the adhesion layer, and the two layers are collectively referred to as the barrier layer. This is followed by a deposition of the second conductor material in the second channel openings and the via openings to form the second channel and the via. A second chemical mechanical polishing process leaves the two vertically separated, horizontally perpendicular channels connected by vias. 
     The use of the single and dual inlaid techniques eliminate metal etch and dielectric gap fill steps typically used in the metallization process. The elimination of metal etch steps is important as the semiconductor industry moves from aluminum to other metallization materials, such as copper, which are very difficult to etch. 
     One drawback of using copper as the conductor material is that copper diffuses rapidly through various materials. Unlike aluminum, copper also diffuses through inter layer dielectric materials, such as silicon oxide. When copper diffuses through interlayer dielectric layers, it can cause leakage to neighboring interconnect lines on the semiconductor substrate. To prevent diffusion, highly diffusion resistive barrier materials, such as tantalum, titanium, tungsten, their nitrides, and combinations thereof, are used as barrier materials. 
     After deposition of the barrier layer, a seed layer of conductor material, such as copper, is deposited by an ion metal plasma (IMP) deposition, chemical vapor deposition (CVD), or an electroless plating process. This seed layer is subsequently used as one electrode in an electroplating process which deposits the conductor material which completely fills the channels and vias. 
     One problem is that current barrier materials typically are very high in resistance, which will defeat the purpose of using high conductivity materials, such as copper, which is desirable for high speed and good interconnections. 
     Another problem with current barrier materials is that the interface between barrier and the conductor material is not very strong, and this permits electromigration (EM) of conductor material to occur which results in void formation and reliability problems. 
     A solution to these problems has been long sought but has eluded those skilled in the art. 
     DISCLOSURE OF THE INVENTION 
     The present invention provides an integrated circuit having a semiconductor substrate with a semiconductor device and a device dielectric layer formed on the semiconductor substrate. A channel dielectric layer on the device dielectric layer has a channel opening, a barrier layer lining the channel opening, and a conductor core filling the channel opening. The barrier layer has a more negative heat of formation than the channel dielectric layer whereby the barrier layer reacts with and forms a barrier to diffusion of the material of the conductor core to the channel dielectric layer. With the more negative heat of formation, the barrier layer reacts with the dielectric layer to form a compound which provides a bond which blocks surface diffusion and permits conductor core to conductor core diffusion. This also causes the barrier layer to be self-aligning in the channel opening. 
     The present invention further provides a method for manufacturing an integrated circuit having a semiconductor substrate with a semiconductor device. A device dielectric layer is formed on the semiconductor substrate and a channel dielectric layer is formed on the device dielectric layer. A channel opening is formed in the channel dielectric layer, and a barrier layer is deposited to line the channel opening. A conductor core is deposited to fill the channel opening. A barrier layer is deposited, having a more negative heat of formation than the channel dielectric layer, to cause the barrier layer to react with and form a barrier to diffusion of the material of the conductor core to the channel dielectric layer. With the more negative heat of formation, the barrier layer reacts with the dielectric layer to form a compound which provides a bond which blocks surface diffusion and permits conductor core to conductor core diffusion. This also allows the barrier layer deposition to be self-aligning in the channel opening. 
     The present invention provides an integrated circuit and method of manufacture which utilizes a material selected from a group consisting of a lanthanide series element, hafnium, zirconium, yttrium, calcium, strontium, barium, and a combination thereof, which provide self-aligned barrier layers, increase conductivity in equally sized channels over the prior art, and provide comparable diffusion resistance at thinner barrier thicknesses than the prior art. 
    
    
     The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (PRIOR ART) is a plan view of aligned channels with a connecting via; 
     FIG. 2 (PRIOR ART) is a cross-section of FIG. 1 along line  2 — 2 ; and 
     FIG. 3 is a cross-section similar to FIG. 2 (PRIOR ART) showing the channels and via according to the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to FIG. 1 (PRIOR ART), therein is shown a plan view of a semiconductor wafer  100  having as interconnects first and second channels  102  and  104  connected by a via  106 . The first and second channels  102  and  104  are respectively disposed in first and second dielectric layers  108  and  1   10 . The via  106  is an integral part of the second channel  104  and is disposed in a via dielectric layer  112 . The dielectric layers herein can be of dielectric materials, such as silicon oxide, or low dielectric constant materials, such as benzocyclobutane (BCB), hydrogen silsesquioxane (HSQ), borophospho silicate glass (BPSG), etc. 
     The term “horizontal” as used in herein is defined as a plane parallel to the conventional plane or surface of a wafer, such as the semiconductor wafer  100 , regardless of the orientation of the wafer. The term “vertical” refers to a direction perpendicular to the horizontal as just defined. Terms, such as “on”, “above”, “below”, “side” (as in “sidewall”), “higher”, “lower”, “over”, and “under”, are defined with respect to the horizontal plane. 
     Referring now to FIG. 2 (PRIOR ART), therein is shown a cross-section of FIG. 1 (PRIOR ART) along line  2 — 2 . A portion of the first channel  102  is disposed in a first channel stop layer  114  and is on a device dielectric layer  116 . Generally, metal contacts (not shown) are formed in the device dielectric layer  116  to connect to an operative semiconductor device (not shown). This is represented by the contact of the first channel  102  with a semiconductor device gate  118  embedded in the device dielectric layer  116 . The various layers above the device dielectric layer  116  are sequentially: the first channel stop layer  114 , the first channel dielectric layer  108 , a via stop layer  120 , the via dielectric layer  121 , a second channel stop layer  122 , the second channel dielectric layer  110 , and a next channel stop layer  124  (not shown in FIG.  1 ). 
     The first channel  102  includes a barrier layer  126 , which could optionally be a combined adhesion and barrier layer, and a seed layer  128  around a conductor core  130 . The second channel  104  and the via  106  include a barrier layer  132 , which could also optionally be a combined adhesion and barrier layer, and a seed layer  134  around a conductor core  136 . The barrier layers  126  and  132  are used to prevent diffusion of the conductor materials into the adjacent areas of the semiconductor device. The seed layers  128  and  134  form electrodes on which the conductor material of the conductor cores  130  and  136  are deposited. The seed layers  128  and  134  are of substantially the same conductor material of the conductor cores  130  and  136  and become part of the respective conductor cores  130  and  136  after the deposition. 
     The deposition of the barrier layer  132  is such that it fills the bottom of the via  106  at barrier layer portion  138  so as to effectively separate the conductor cores  130  and  136 . 
     In the past, for copper conductor material and seed layers, highly resistive diffusion barrier materials such as tantalum, titanium, tungsten, their nitrides, and combinations thereof are used as barrier materials to prevent diffusion. In addition to increasing the overall resistance of the semiconductor chip which contained all the semiconductor devices, the barrier region  138  would block diffusion of copper from the conductor core  130  to the conductor core  136  as electro-migration caused the movement of copper atoms out of the via  106  and allowed the formation of voids therein. Further, the interface between the barrier materials and copper does not form a strong bond and creates weak points for electro-migration to occur. 
     Thus, prior art combinations which included the various metal nitrides with copper were subject both to surface diffusion along the vias and channels and interface diffusion at the barrier region  138 . 
     Referring now to FIG. 3, therein is shown a cross-section similar to that shown in FIG. 2 (PRIOR ART) of a semiconductor wafer  200  of the present invention. The semiconductor wafer  200  has first and second channels  202  and  204  connected by a via  206 . The first and second channels  202  and  204  are respectively disposed in first and second dielectric layers  208  and  210 . The via  206  is a part of the second channel  204  and is disposed in a via dielectric layer  212 . 
     A portion of the first channel  202  is disposed in a first channel stop layer  214  and is on a device dielectric layer  216 . Generally, metal contacts (not shown) are formed in the device dielectric layer  216  to connect to an operative semiconductor device (not shown). This is represented by the contact of the first channel  202  with a semiconductor device gate  218  embedded in the device dielectric layer  216 . The various layers above the device dielectric layer  216  are sequentially: the first channel stop layer  214 , the first channel dielectric layer  208 , a via stop layer  220 , the via dielectric layer  221 , a second channel stop layer  222 , the second channel dielectric layer  210 , and a next channel stop layer  224 . 
     The first channel  202  includes a barrier layer  226  and a seed layer  228  around a conductor core  230 . The second channel  204  and the via  206  include a barrier layer  232  and a seed layer  234  around a conductor core  236 . The barrier layers  226  and  232  are used to prevent diffusion of the conductor materials into the adjacent areas of the semiconductor device. The seed layers  228  and  234  form electrodes on which the conductor material of the conductor cores  230  and  236  are deposited. The seed layers  228  and  234  are of substantially the same conductor material of the conductor cores  230  and  236  and become part of the respective conductor cores  230  and  236  after the deposition. The seed layers and conductor cores are of copper, copper-base alloys, gold, gold-base alloys, silver, silver-base alloys, and combinations thereof. 
     It will be noted the conductor cores  230  and  236  are integral in the present invention. As will hereinafter be explained, the barrier layer  232  forms a conductive intermetallic compound with the conductor material in a barrier material region  238 . The seed layer  234  also is indistinguishable between the conductor cores  230  and  236  at a seed layer region  240 . 
     In the present invention, a barrier material is used which has one or more of the following properties, without being limiting: 
     1. the heat of formation is more negative than the heat of formation of the dielectric material; 
     2. form a stable intermetallic compound with the conductor material; 
     3. coherent (or chemically similar) with both the dielectric material and the conductor material; 
     4. higher conductivity than titanium, tantalum, or tungsten; and/or 
     5. the atomic weight is larger than the atomic weight of the conductor material. 
     For the combination of oxide-based dielectric materials and copper conductor materials, materials from a group consisting of a lanthanide series element, hafnium, zirconium, yttrium, calcium, strontium, barium, and a combination thereof, without being limiting, have been been found to have a number of the desirable properties. 
     Lanthanum, for example, readily forms a metal oxide with the oxide in the oxided silicon (silicon oxide) dielectric material and also a stable intermetallic compound with copper. When lanthanum is deposited on silicon oxide, its very negative heat of formation compared to the heat of formation of silicon oxide causes it to closely bond to and reduce the silicon oxide to form lanthanum oxide (La 2 O 3 ). Lanthanum oxide is a dense metal oxide, which prevents copper diffusion into the dielectric material. This preventative function is enhanced because lanthanum has a higher atomic weight than copper to slow free movement of copper atoms through it. It further has the property of forming compounds with copper, which means it is reactive with Cu and forms a very good interface with copper because it is chemically similar. This property reduces surface diffusion. 
     Thus, when placed onto a via or a trench, lanthanum basically reacts with both the dielectric material and the subsequently deposited copper to form a “coherent” bond, or a strong chemical bond, with both materials. 
     Lanthanum also has low solid solubility in copper, which means it is rejected from the copper grain during copper deposition, so the copper grain will be of a high purity and which will result in low resistance (compared to titanium, tantalum, and tungsten which are the prior art barrier metals). 
     In the present invention, the barrier layers  226  and  232  may be formed by a number of different methods. One method involves deposition of the metal barrier material to react with the dielectric, followed by deposition of the conductor material to form the intermetalllic compound. The deposition of the metallic barrier material will be to a thickness of between 10 Å to 100 Å. 
     Another method involves deposition of the metal barrier material as a percentage of the initial deposition of conductor material. It has been discovered that up to 4% of the conductor material can be the metallic barrier material. 
     Various processes such as physical vapor deposition, chemical vapor deposition, and/or electroplating deposition may be used. 
     As would be evident to those skilled in the art, a number of advantages accrue from the present invention. 
     One advantage is that the barrier layers  226  and  232  are self-aligned in that the barrier compound of the barrier material (the metal oxide) only forms where it is in contact with the dielectric oxide. As long as a solid layer of metal is not deposited, if the metal deposition is thin, it reacts with the barrier layers and forms discontinuous intermetallic compounds and, if is deposited as an alloy, the metal can diffuse away. In either event, metallic barrier material is not formed in the barrier region  238 , so it does not block the diffusion of conductor material from the conductor core  230  to the conductor core  236 . This helps prevent the formation of voids in the via  206 . 
     Another advantage is that the barrier layers  226  and  232  can be very thin, under 100 Å, so the effective cross-sectional area of the via  206  is greater than for a conventional via which uses one of the high resistance materials. 
     Another advantage is that the coherent bonding forms strong chemical bonds which prevent surface diffusion of the conductor material. 
     As known to those skilled in the art, a higher negative heat of formation means that a reaction will occur during formation of the compound which will release energy as to a compound having a relatively lower negative value. A low energy state material is always more stable so will form before a high energy state material. The various heats of formation for reference purposes are: 
     La2O3=−539 kcal/mole 
     HfO2=−271 kcal/mole 
     ZrO2=−258 kcal/mole 
     CeO2=−235 keal/mole 
     SiO2=−202 kcal/mole 
     While the invention has been described in conjunction with a specific best mode, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the included claims. All matters hither-to-fore set forth or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.