Patent Publication Number: US-2002000657-A1

Title: Plated chrome solder dam for high power mmics

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates generally to monolithic, microwave integrated circuits (MMICs) and, more particularly, to the fabrication of flip-chip bumps therefor.  
       [0003] 2. Description of the Related Art  
       [0004] Flip-chip monolithic, microwave integrated circuits (MMICs) have been recognized as a low cost approach to fabricating microwave transmit/receive modules for active array radar. Flip-chip fabrication involves mounting the monolithic circuits face-down on a dielectric assembly substrate using a solder reflow process. MMIC devices mounted in such a fashion have exhibited superior thermal properties relative to conventional, face-up MMICs fabricated on 4-mil thick gallium arsenide substrates.  
       [0005] These superior thermal properties (e.g., reduced thermal resistance) have been made possible, in part, by electrolytically plated (i.e., “electroplated” or “plated”) metal (e.g., silver) thermal bumps that promote heat dissipation. The thermal bumps typically couple the dielectric substrate to circuit elements that generate significant amounts of heat, such as a plated gold bridge connecting the source electrodes of a multi-cell field effect transistor (FET). The lower device temperatures arising from such connections lead to improved electrical performance and circuit reliability. Flip-chip MMICs have also benefitted from the utilization of silver bumps for transmitting output signals off-chip to an output circuit disposed on the assembly substrate. In addition to promoting heat dissipation, these electrical bumps provide an efficient mechanism for establishing input/output connections to the MMIC.  
       [0006] The utilization of thermal and electrical bumps and the corresponding solder reflow approach to chip attachment has, however, raised other potential problems. For example, molten solder crawling up the side wall of a thermal bump may reach the gold bridges during the solder reflow process. To prevent solder contamination, it is necessary to have a solder dam (conventionally, a dielectric) surrounding each thermal bump to prevent the molten solder from spreading on the plated gold surface of the source electrode bridge. This potential problem is particularly acute in the absence of dielectric passivation. In addition to solder contamination, flip-chip MMICs having silver bumps may experience silver migration over the gold surface, which leads to another serious reliability concern.  
       [0007] In general, passivation layers have provided a layer of protection for gold bridges and other exposed metal layers of the MMIC. Unprotected circuit elements not only risk damage during the solder reflow process, but also during other subsequent processing steps, including back-side processing, on-wafer testing, wafer dicing, pick-and-place, die attachment, off-chip electrical interconnect, final clean, and packaging. Typical passivation layers have constituted a dielectric coating, such as silicon dioxide, silicon oxy-nitride, or a combination thereof.  
       [0008] While such dielectric layers are capable of protecting circuit elements from the environment (e.g., chemicals and dirt) and mechanical scratching, silicon dioxide and other dielectrics do not exhibit suitable bonding strengths when deposited directly on a gold surface. Thus, when current MMICs are subjected to thermal cycling or thermal shock, device failures may result from delamination at the gold/dielectric interface.  
       [0009] Gold-based metallization has been widely used in MMICs, gold being suitable for several elements, including bridges, crossovers, transmission line segments, and electrodes. Thick gold layers (ie., several microns) have been utilized in such instances to form low resistance, thermal paths that boost power handling capabilities and increase the Q-factor of the device. Gold-based MMICs have also been highly resistant to corrosion.  
       [0010] Titanium has been a conventional agent for adhering dielectric passivation layers to metal layers. A strong oxygen covalent bond between the oxidized titanium surface and the dielectric material provides the adhesive property. Techniques for depositing a thin layer (e.g., 500 Angstroms) of titanium, however, have been found to be incompatible with the electrolytic plating process. Moreover, additional masking steps are often required for using titanium with plated gold surfaces, inasmuch as titantium plating technology has been undesirable. These additional masking steps result in higher circuit fabrication costs and lower manufacturing yields.  
       [0011] Electrolytically plated nickel has also been found to improve adhesion at the dielectric/gold interface. While a thin layer of nickel (200 Angstroms) can be plated on top of the plated gold electrodes without the need for additional photolithography, nickel has introduced other processing complications. First, adhesion between nickel and the bump plating membrane (typically a titanium/gold interface layer) is less than desirable. As a result, the plated nickel film must be removed at selected areas to facilitate the silver bump formation. Further, nickel has been found to be inadequate as a solder dam, inasmuch as molten solder reacts readily with nickel.  
       [0012] For these reasons, in the non-passivation regions (e.g., at certain flip-chip and thermal bumps), the nickel layer fails to provide a solder dam for preventing molten solder from spreading over the MMIC. Past attempts to use nickel despite these drawbacks have been inadequate. These attempts, for example, have relied upon thicker dielectric layers and certain geometric design configurations to prevent the solder from reaching the nickel. Both of these approaches place undesirable design rules on the MMIC. Moreover, the extra thickness of the dielectric layer degrades the RF characteristics of the MMIC at very high frequencies (e.g., millimeter wavelengths).  
       SUMMARY OF THE INVENTION  
       [0013] In accordance with one aspect of the present invention, a microwave device comprises a circuit element, a layer comprising chromium disposed over the circuit element, and a bump disposed over the circuit element.  
       [0014] In a preferred embodiment, the layer comprising chromium may comprise a ring extending from the bump. A bump plating membrane may couple the bump to the circuit element. Alternatively, the layer comprising chromium may have a portion over which the bump is disposed and the ring may comprise a solder dam. The ring, in turn, may have a surface comprising chromium oxide, and a dielectric passivation layer may be disposed on the surface. The circuit element may have a gold plated surface, and the bump may comprise either a flip-chip bump or a thermal flip-chip bump.  
       [0015] In accordance with another aspect of the present invention, a method is useful for forming a flip-chip bump on a circuit element of a microwave device. The inventive method comprises the steps of depositing a layer comprising chromium over the circuit element, providing a bump photoresist profile, and depositing a metal layer over the circuit element to form the flip-chip bump according to the bump photoresist profile.  
       [0016] According to a preferred embodiment, the method may further include the step of removing a plating membrane and a photoresist layer prior to providing the bump photoresist profile. The plating membrane and the photoresist layer may be utilized to form the circuit element. The method may still further include the step of removing a portion of the layer comprising chromium to leave a ring-shaped portion extending from the flip-chip bump. Alternatively, the step of providing a bump photoresist profile may include depositing a bump plating membrane on the layer comprising chromium. The circuit element may have a surface comprising gold. The bump photoresist profile may cover a portion of the layer comprising chromium, while the method may still further comprise the steps of removing the bump photoresist profile and allowing a native oxide to grow on the portion of the layer comprising chromium to form a solder dam. The portion of the layer comprising chromium may comprise a ring extending from the flip-chip bump. The method may still further comprise the step of depositing a dielectric material on the portion of the layer comprising chromium to form a passivation layer.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017]FIG. 1A is a plan, schematic view of a microwave device fabricated in accordance with one embodiment of the present invention;  
     [0018]FIG. 1B is a cross-sectional, schematic view of the microwave device of FIG. 1 A taken along the lines  1 B- 1 B; and  
     [0019] FIGS.  2 A- 2 E are cross-sectional, schematic views of a semiconductor structure at various stages of a fabrication method in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0020] In general, the present invention is directed to flip-chip MMICs having electrodes with electrolytically-plated gold surfaces, and a method of making same. More particularly, flip-chip MMICs may have thermal and electrical bumps disposed over electroplated gold bridges and electrodes, respectively. In accordance with the present invention, a thin chrome film or layer accommodates these electrical and thermal bumps by serving as both a solder dam during the flip-chip attachment process and a dielectric adhesion agent in the event that the circuit is passivated. The native oxide that grows on the chrome film tends not to be wetted by conventional solder, thereby forming a solder dam during solder reflow (i.e., flip-chip attachment). When silver is utilized for the electrical and thermal bumps, the chrome solder dam also inhibits silver migration over the gold surfaces.  
     [0021] Unlike the dielectric solder dams utilized in prior MMIC devices, the chrome solder dam may be an integral part of a chrome film over which the bump is disposed. Therefore, no additional masking steps are required. Because the work function of chrome is similar to that of titanium (the material used for bump plating membranes), the resistivity of electrical bumps is not significantly increased. There also may be no need to remove the chrome film for electrical bumps because of the excellent adhesion characteristics between evaporated titanium and the chrome surface. Therefore, in one embodiment of the present invention, a continuous chrome film is left under and surrounding each plated metal bump, thereby providing a more effective solder dam and improving the reliability of high power, flip-chip MMICs without any degradations in RF performance.  
     [0022] With reference now to the drawings, FIGS. 1A and 1B show schematic views of a flip-chip monolithic microwave integrated circuit (MMIC)  10  simplified for purposes of illustration. Upon flip-chip attachment to an assembly substrate (not shown), the MMIC  10  may become part of a microwave device incorporating the present invention. The simplified, flip-chip MMIC  10  has two circuit elements, which may, for example, correspond with an output conductor or transmission line  12  and a collective source electrode  14  of a multicell transistor. Both circuit elements have been fabricated on a semiconductor substrate  18 , which may, for example, comprise gallium arsenide. The output conductor  12  includes a metal layer  20  (FIG. 1B) to provide a low resistance path from another MMIC element (not shown) disposed on (and/or in) the substrate  18  to a flip-chip electrical bump  22 , which may provide an input/output connection to the input/output circuit on the assembly substrate to which the flip-chip MMIC  10  is soldered.  
     [0023] The metal layer  20  may include a plurality of low resistivity metals or metal layers, either alone or in combination, that are known to those skilled in the art to be suitable for MMICs. For example, the metal layer  20  may include layers of titanium (for improving adhesion to a gallium arsenide substrate) and gold (for, inter alia, low resistivity). The metal layer  20  may also include platinum to prevent interdiffusion of gold into the titanium layer.  
     [0024] According to one embodiment of the present invention, the metal layer  20  comprises an electroplated gold surface  24  (FIG. 1B) over (or alternatively upon) which an electroplated chrome (i.e., chromium) layer  26  is disposed. Alternatively, the entire metal layer  20 , as well as its surface, may comprise electroplated gold. The flip-chip bump  22 , in turn, is disposed over a portion  28  (FIG. 1B) of the electroplated chrome layer  26 , leaving a ring-shaped portion  30  of the electroplated chrome layer  26  extending therefrom.  
     [0025] According to an alternative embodiment of the present invention, most (if not all) of those portions of the chrome layer  26  that would lie under the flip-chip bump  22  are removed via a commercial chrome etch or an HCl bath. As a result, the chrome layer  26  would consist essentially of the ring-shaped portion  30 , with perhaps a slight extension under the area covered by the flip-chip bump  22 . Any portions of the metal layer  20  not covered by the chrome layer  26  would, of course, be covered by the flip-chip bump  22 . Thus, regardless of whether most or all of the underlying portion of the chrome layer  26  is removed, the metal layer  20  is covered by the ring-shaped portion  30  alone or in combination with the flip-chip bump  22 .  
     [0026] The chrome layer  26  comprises a thin film having a thickness preferably in a range from about 200 Angstroms to about 2000 Angstroms, and more preferably from about 500 Angstroms to about 1500 Angstroms, and most preferably at about 1000 Angstroms. Due to its relative thinness, the chrome layer  26  amounts to a minute fraction of a wavelength at even millimeter wave frequencies. As a result, the RF performance of the MMIC  10  is not significantly affected by the portion  28  of the chrome layer  26  under the bump  22 .  
     [0027] Because the chrome layer  26  is exposed to the ambient environment, a native oxide (not shown) develops on the surface of the chrome layer  26 . Molten solder (e.g., 35/65 lead-tin or 50/50 lead-indium) commonly used for flip-chip attachment does not have the tendency to wet the native oxide on the surface of the chrome layer  26 . This characteristic of the chrome layer  26  allows the ring-shaped portion  30  of the chrome layer  26  to replace the conventional dielectric solder dam.  
     [0028] In contrast to the output conductor  12 , the collective source electrode  14  of the MMIC  10  has an electroplated thermal bump  32  disposed thereon. The structure of the solder dam for the thermal bump  32 , however, is similar to the ring-shaped portion  30  of the chrome layer  26 . More particularly, the thermal bump  32  is disposed over a circuit element having a chrome layer  34 . According to one embodiment, the chrome layer  34  may have a portion  36  (FIG. 1B) under the thermal bump  32  and a portion  38  extending from the thermal bump  32  to act as a solder dam. The portions  36  and  38  of the chrome layer  34  may be disposed over (or alternatively upon) a plated gold bridge  40  (FIG. 1B) that couples respective source regions (not shown) of a multicell transistor. The source regions may be developed in the substrate  18  beneath respective ohmic metal portions  42  (FIG. 1B).  
     [0029] Alternatively, the chrome layer  34  may constitute only a ring-shaped solder dam that extends from the thermal bump  32  similar to the portion  38 . As mentioned above, this ring-shaped solder dam may extend beneath the thermal bump  32  to a certain extent, as desired.  
     [0030] A process for fabricating a flip-chip semiconductor structure incorporating the chrome layer  26  as a solder dam and a dielectric adhesion agent will now be described in connection with FIGS.  2 A- 2 E. A semiconductor substrate  50 , which may comprise silicon, gallium arsenide, or any other suitable non-compound or compound semiconductor, has an ohmic metal layer  52  deposited thereon in accordance with conventional evaporation, sputtering, or other standard metal deposition techniques. The ohmic metal layer  52  is patterned via a prior photolithographic step and may comprise doped polysilicon or any one of the following metals, either alone or in combination: gold, germanium, and nickel (which may be in combination with a gold overlay). The thickness of the ohmic metal layer  52  is preferably in a range from about 500 Angstroms to about 2000 Angstroms, and more preferably from about 500 Angstroms to about 1000 Angstroms, and most preferably with about 750 Angstroms of gold/germanium, about 150 Angstroms of nickel, and about 1.5 microns of gold overlay.  
     [0031] At about this same time, multiple gate electrodes  53  are formed on the substrate  50  to establish multicell field-effect transistors. The structure, materials, and design of the gate electrodes are well known to those skilled in the art.  
     [0032] After the deposition of the ohmic metal portions  52  and the gate electrodes  53 , a layer of photoresist is deposited and patterned via photolithography to form a bridge post resist layer  54 . A bridge plating membrane  55  is then deposited, via evaporation, sputtering, or the like, on the bridge post resist layer  54  and the exposed portions of the ohmic metal portions  52 . The bridge plating membrane  55  may comprise titanium and/or gold to facilitate an upcoming electroplating step, and may have a thickness of about 2000 Angstroms. The bridge post resist layer  54  protects the semiconductor structure (including, for instance, the gate electrodes  53 ) from this upcoming electroplating step.  
     [0033] Referring now to FIG. 2B, a layer of photoresist  56  is then spun onto the bridge plating membrane  55  and patterned via photolithography as shown. Turning to FIG. 2C, the patterned photoresist layer  56  defines a plated gold transmission line  58  and a plated gold bridge  60 . The transmission line  58  and the bridge  60  are formed by a standard electroplating procedure to have a thickness preferably in a range from about one micron to about four microns, and more preferably in a range from about two microns to about four microns, and most preferably at about three microns. It should be noted that, unlike the transmission line  20  of FIG. 1B, the transmission line  58  is shown as coupled to a portion of the ohmic layer  52  (where, for instance, the transmission line  58  may be connected to a highly doped region of the substrate  50 ). This structure may, for example, correspond with the location where the collective source electrode for a multicell transistor is coupled to ground. Furthermore, the structure also provides a solid anchor for the bump to be formed thereon, as will be further explained below.  
     [0034] In accordance with one embodiment of the present invention, a thin film  62  of chrome is then electroplated onto the transmission line  58  and the gold bridge  60 . While a hexavalent chrome plating bath may be utilized to deposit the chrome film  62 , a trivalent chrome plating process is preferred. The high oxidation potential of hexavalent chromates may attack the masking photoresist during plating. With that exception, both types of plating baths are compatible with current flip-chip MMIC fabrication procedures. It should be noted that, unlike various prior solder dam fabrication techniques, no additional photolithographic step is required to develop the thin chrome film that will function as both a solder dam and dielectric adhesion agent.  
     [0035] As set forth above in greater detail, the chrome film  62  may have a thickness from about 200 Angstroms to about 2000 Angstroms. In accordance with the present invention, a native oxide layer (not shown) is allowed to form on the chrome film  62 . The capability of this native oxide layer to share bonds with a dielectric material provides good adhesion characteristics for a chrome-passivation layer interface.  
     [0036] After the bridge post resist layer  54 , the bridge plating membrane  55 , and the photoresist layer  56  are removed via techniques known to those skilled in the art, a first bump photoresist layer  64  is spun onto the semiconductor structure and patterned via photolithography as shown in FIG. 2D. It should be noted that the first bump photoresist layer  64  (as patterned) covers a portion of the chrome film  62 . In this manner, the flip-chip bumps to be formed later leave a small ring  65  (FIG. 2E) of the chrome film  62  exposed to act as the solder dam and adhesion agent. In accordance with one embodiment of the present invention, any exposed chrome is chemically removed at this point by a commercial chrome etch or an hydrochloric acid bath. With this chemical etch, the chrome layer  62  essentially assumes the shape of the ring-shaped portion  30  of FIGS. 1A and 1B.  
     [0037] A bump plating membrane  66  of titanium and/or gold is then deposited (via evaporation, sputtering, or another suitable deposition process) across the semiconductor structure, such that it covers both the first bump photoresist layer  64  and the chrome film  62  (in the event that it has not been etched away). The bump plating membrane  66  preferably has a thickness in a range from about 500 Angstroms to about 3000 Angstroms, and more preferably from about 1000 Angstroms to about 3000 Angstroms, and most preferably with about 2000 Angstroms of gold and about 200 Angstroms of titanium. The bump plating membrane  66  not only facilitates the silver electroplating step that forms the flip-chip bumps, but it also separates the first bump photoresist layer  64  from a second bump photoresist layer  68  that is deposited and patterned to form the profile shown in FIG. 2C. Alternative metals and/or materials for the bump plating membrane  66  include titanium, tungsten, and/or gold. The chrome film  62  has, however, exhibited excellent adhesion characteristics with the above-described evaporated gold/titanium bump plating membrane  66 .  
     [0038] Referring now to FIG. 2E, an electroplating step forms silver bumps  70  according to the profile defined by the first and second bump photoresist layers  64  and  68 . The silver bumps  70  may constitute either electrical or thermal flip-chip bumps. For example, the silver bump  70  formed over the gold bridge  60  is typically a thermal flip-chip bump, while the silver bump  70  formed over the transmission line  58  may be an electrical flip-chip bump. The silver electroplating procedure is well known to those skilled in the art and will not be further explained. Silver bumps are preferred, inasmuch as they exhibit superior thermal and electrical conductivity, but alternate metals suitable for the flip-chip bumps include gold and copper, either alone or in combination.  
     [0039] Silver migration is, in large part, inhibited by the solder used to couple the MMIC device to the assembly substrate. Migration may still occur if, for example, the solder does not entirely or adequately cover the silver bump. In that event, the titanium in the bump plating membrane  66  and the solder dam formed by the chrome film  62  further inhibits the silver from migrating to the gold metallization layer.  
     [0040] The thickness of the silver bumps  70  is preferably in a range from about 25 microns to about 105 microns, and more preferably from about 75 microns to about 105 microns, and is most preferably about  90  microns. It should be noted that the elements depicted in FIGS. 1 and 2A- 2 E are not necessarily shown to scale.  
     [0041] After formation of the silver bumps  70 , solder is applied thereto for coupling the assembly substrate to the MMIC circuit disposed on the substrate  50 . As is known to those skilled in the art, the solder is applied with a resin- or water-based flux. Both the solder and the flux may be selected from any of those typically used by those skilled in the art. In accordance with the present invention, the molten solder does not have the tendency to wet the native oxide of the exposed chrome film, thereby inhibiting its flow.  
     [0042] However, in accordance with an alternative embodiment of the present invention, a conventional dielectric passivation layer  72  may be deposited on the semiconductor structure prior to the application of solder. The passivation layer  72  may comprise silicon oxy-nitride or silicon dioxide, either alone or in combination, or any other dielectric known to those skilled in the art to be suitable for such deposition (e.g., plasma deposition techniques). The passivation layer  72  has a thickness typically in a range from about 0.5 microns to about 2.0 microns. While such dielectric materials exhibit poor bonding strengths when deposited directly on a gold surface, the chrome oxide on the surface of the rings  65  of the chrome film  62  provides an effective adhesion agent therefor. Without the adhesive characteristics of the chrome oxide, the MMIC device may become unpassivated during extreme thermal cycles and/or shocks. In this manner, the chrome film provides the flexibility to utilize both passivated and unpassivated circuit elements in a flip-chip environment.  
     [0043] It shall be noted that, as used herein, terms implying direction or orientation (e.g., “on” and “over”) should not be read to require any particular final orientation of the microwave device. For instance, if the MMIC  10  is eventually flipped for attachment to an assembly substrate, the chrome layer  26  may still be considered to be “under” the bump  22 . Likewise, the bump  22  may still be understood to be “over” the chrome layer  26 . Furthermore, the phrases “disposed over,” “deposited over,” and the like shall not be read to be limited to the context when two entities are in direct contact. For example, two layers that may be separated by a membrane or film may be referred to as “disposed over” one another.  
     [0044] Numerous other modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only. The details of the device and method may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appending claims is reserved.