Abstract:
Passivating layers methods for forming the same are provided for packaged integrated circuit devices. In particular, an integrated circuit die is mounted in a plastic leaded chip carrier, and a photosensitive material is then deposited over the surfaces to be passivated. Portions of the photosensitive material are then exposed to UV light, resulting in a crosslinked siloxane network. In this way, a low-temperature photodefinable passivation layer is provided for the package, with characteristics similar to conventional oxides. Advantageously, the photosensitive material can be patterned during the UV exposure, and unexposed portions selectively removed to leave the passivation layer only over desired portions of the package.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application is a divisional of U.S. Application Ser. No. 09/145,106, filed Sep. 2, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to passivation layers for packaged dies and methods for forming the same. 
     2. Description of the Related Art 
     Die carriers are used in the construction of integrated circuit chips or dies to connect such dies to larger circuits, such as the motherboard of personal computers. In the past, die carriers, such as lead frames, have been most commonly metal. After mounting the die of the die carrier and connecting the leads, passivation layers are generally formed over the mounted die and carrier to protect the chips from moisture, chemicals, and other environmental factors. 
     Passivation layers can be formed on lead frames using chemical-vapor deposition and thermal processes at relatively high temperatures, due to the high melting point of the metal. Recently, however, plastic has become more widely used in die carriers. Due to the low melting point of plastics, such die carriers are easily damaged by exposure to high temperature steps. 
     Consequently, there is a need for a process for forming a protective layer on integrated circuit devices and other surface mounted structures at lower temperatures. Desirably, the passivation layers formed by the process should be moisture resistant and protect the underlying die. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a process for forming a passivated integrated circuit device is provided. The process includes mounting a die onto a surface of a die carrier. An organosilane precursor is reacted to form a layer of photosensitive material on surfaces of the die and the die carrier. 
     In accordance with another aspect of the invention, a method is disclosed for packaging an integrated circuit die to form a surface mount device. The method involves mounting the die onto a surface of a plastic die carrier. A photodefinable layer is deposited on the die and the die carrier after mounting the die. This photodefinable is then converted to a passivation layer. 
     In accordance with another aspect of the invention, a method is provided for packaging an integrated circuit die for surface mounting. The die is mounted on a die carrier. A layer of plasma polymerized methylsilane (PPMS) is deposited onto a first side of the die and die carrier after mounting the die. At least a portion of the PPMS is then exposed to ultraviolet (UV) light in the presence of oxygen. 
     In accordance with still another aspect of the invention, a packaged integrated circuit device is provides. The device includes a die carrier, with an integrated circuit die mounted onto a surface of the die carrier. A passivation layer, including plasma polymerized methylsiloxane (PPMSO), extends over portions of the die and the die carrier. 
     In an illustrative embodiment of the invention, reacting methylsilane in a plasma enhanced CVD reactor leaves a layer of PPMS. Advantageously, this layer is formed at low temperatures, and then converted by exposure to UV light to the oxide-like passivation layer of PPMSO. The low temperatures of the process enables passivation of die carriers having plastic substrates, such as a plastic ball grid array. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These aspects and others will be apparent from the following description of preferred embodiments and the accompanying drawings, which are meant to illustrate and not to limit the invention, wherein: 
     FIG. 1A is a cross-sectional view showing a plastic die carrier according to an illustrative embodiment; 
     FIG. 1B is a plan view of a taped front side of die carrier of FIG. 1; 
     FIG. 2 illustrates a die mounted on a back side of the die carrier of FIG. 1; 
     FIG. 3 shows the die carrier of FIG. 1 after leads have been attached to the die and the die carrier on a front side; 
     FIG. 4 shows the die carrier after epoxy is deposited on the front side to envelope the leads; 
     FIG. 5 shows the die carrier after depositing a photosensitive material over the back side of the die and die carrier; 
     FIG. 6A shows the die carrier of FIG. 5 after exposure of the photosensitive material to developing light; 
     FIG. 6B is a plan view of the back or die side of the die carrier of FIG. 6A; 
     FIG. 7 is a cross-sectional view showing the passivated die carrier mounted on the surface of a printed circuit board; 
     FIG. 8 is a perspective view of stacked die carriers in accordance with a second embodiment; 
     FIG. 9A shows one of the die carriers of FIG. 8, with a patterned passivation layer constructed in accordance with the second embodiment; 
     FIG. 9B is a plan view of the front or lead side of the die carrier of FIG. 9A; and 
     FIG. 9C is a plan view of the back or die side of the die carrier of FIG.  9 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A process for forming a passivation layer on a packaged substrate according to an illustrative embodiment of the present invention will be described while referring to the accompanying drawings. Like parts and components are designated by the same reference numerals for the reader&#39;s ease. The drawings are schematic and not to scale. 
     FIGS. 1A and 1B show a die carrier  10  according to a first preferred embodiment. The die carrier preferably comprises a plastic substrate having wire traces formed therein. Use of plastic substrates, such as FR-4 commonly used for printed circuit boards, enables scaling of the chip or die package, due to the ability to print fine wire traces between plastic layers. As a result, plastic carriers allows integration of the chip to a printed circuit board while adding no more than about 20% to the size of the chip itself, such that the carrier and die together are said to present a chip scale package. 
     The illustrated die carrier  10  comprises a plastic ball grid array (“PBGA”), preferably comprising a BT resin plastic substrate  12  with surface solder balls  14  electrically connected to conductive traces within the substrate. While not shown, it will be understood that the traces can be formed within or at the surface of the substrate  12 . The illustrated carrier  10  includes a window  16  formed within a die recess  18 . The skilled artisan will recognize numerous other carrier configurations suitable for mounting dies to circuit boards. 
     As shown in FIG. 2, a die  20  is mounted within the recess  18 , preferably on the surface of the die carrier opposite the side on which the solder balls  14  are formed. The side of the substrate  12  on which the die  20  is mounted is typically referred to as the “die side” or “back side” of the carrier  10 . The opposite side of the substrate  12 , on the other hand, is typically referred to as the “lead side” or “front side” of the substrate  12 , which includes the solder balls  14  in the illustrated PBGA embodiment. The die  20  can comprise an integrated circuit, such as a memory chip, a microprocessor, a photodiode or other chips designed for specific purposes. 
     As shown in FIG. 3, leads  22  are attached to each of the die  20  and the substrate  12 . In particular, the illustrated leads  22  comprise wire bonds extending from contacts on the die  20  to conductive traces of the carrier  10  within or at the surface of the substrate  12 . Wire bonding or other methods of connecting an integrated circuit to the die carrier are well known in the art, and need not be discussed in detail here. 
     As shown in FIG. 4, the leads  22  and the portion of front side of the die  20  which is exposed through the window are then covered by a protective material  24 . The protective material  24  preferably comprises an epoxy deposited onto the front side of the carrier  10  such that it envelopes the leads  22  and seals the window  16 . Such protective epoxies are often referred to in the industry as “glob tops.” A die packaged in this manner may be described as an integrated circuit device  30 , including the die  20  leaded to the die carrier  10 . The IC device  30  can be surface mounted on a larger printed circuit board, as will be apparent from the discussion below of FIG.  7 . 
     As shown in FIG. 5, after the die  20  is mounted on the die carrier  10 , a layer of photosensitive material  32  is deposited on the back or die side surface of the packaged die  30 . In the illustrated embodiment, the layer  32  is deposited by reacting an organosilane precursor, such as methylsilane (CH 3 SiH 3 ), in a plasma enhanced chemical vapor deposition (PECVD) process. Desirably, the substrate can be maintained under about 300° C. during the deposition. In contrast, conventional oxide passivation layers are formed at higher temperatures (e.g., CVD at 300° C. to 500° C., or thermal oxidation at 600° to 700° C.). The preferred organosilane can be reacted in a plasma deposition chamber at between about 75° C. and 150° C., more preferably, between about 90° C. and 140° C. 
     Other preferred parameter ranges includes chamber pressures between about 1 Torr and 2.5 Torr, with an exemplary value of about 1.72 Torr; organosilane gas flow rate between about 40 sccm and 150 sccm, with an exemplary value of about 66 sccm; electrode spacing between about 400 mils and 550 mils, with an exemplary value of about 450 mils; and power between about 50 W and 200 W, with an exemplary value of about 100 W. Preferably, a non-reactive carrier gas flow is also included in the process. In the exemplary implementation, addition of a helium gas flow of about 1,000 sccm to 3,000 sccm to the deposition process was found to increase the uniformity of deposited film. The skilled artisan will recognize that parameters can fall outside these preferred ranges under different conditions, such as different plasma chambers. 
     Thickness of the deposited layer can be varied according to the specific applications. A typical deposition rate is about 2,000 Å/min. The organosilane precursor in the illustrative embodiment undergoes fragmentation and condensation to form a thin, amorphous film of plasma polymerized methylsilane (“PPMS”)  32 . 
     FIG. 6A illustrates the device  30  of FIG. 5 after the PPMS film  32  is photo-oxidized to form a crosslinked siloxane network, or “PPMSO”  34 . Preferably, crosslinking occurs by exposure of the PPMS  32  (FIG. 5) to deep ultraviolet (UV) light in air or other oxygen environment. Sources of deep UV light are widely available in commercial steppers at wavelengths of 193 nm or 248 nm. The PPMSO layer  34  comprises an oxide-like material incorporating carbon and hydrogen, of the form SiO x :C:H, where x is less than or equal to about 2, and the ratio of carbon to silicon is less than about 1. This material etches at about the same rate as silicon dioxide, and is similarly moisture and abrasion resistant. 
     Thus, the cross-linking produced by UV exposure results in a robust, moisture-resistant protective or passivation layer  34 . The siloxane network acts as a mechanical and chemical protection layer for the packaged die to protect it from physical and environmental attack. At the same time the siloxane network electrically insulates and passivates the circuitry while providing thermal expansion and other properties which are compatible with oxide layers (e.g., passivation or interlevel dielectrics) formed within the die itself. 
     Advantageously, the PPMS film  32  (FIG. 5) can be patterned during exposure to UV light and further dry developed in a halogen-based plasma to remove unoxidized organosilane material. The PPMS film  32  is thus said to be photodefinable. As will be apparent from the discussion below of FIG. 9A, for example, it may be desirable to passivate only the die, or only undesirably exposed traces on the die carrier. The process of depositing an organosilane precursor onto a substrate through plasma polymerization and exposing the precursor to UV light to form a crosslinked siloxane network has been discussed in the context of dry photoresist masks in the fabrication of integrated circuits. O. Joubert et al.,  Plasma Polymerized All - Dry Resist Process for  0.25  μm Photolithography,  12 J. Vac. Sci. Technol. B. 3909 (1994). The disclosure of Joubert is hereby incorporated by reference. 
     FIG. 6A thus shows the completed packaged die  30 , with a passivation layer  34  covering the back side of the die  20  and the substrate  12 , and wire bonds  22  on the front side electrically connecting the die  20  into the printed circuit of the die carrier  10 . The wire bonds  22  and the front side of the die  20  (otherwise exposed to the aperture  16 ) are protected at the front side by the glob top  24 . FIG. 6B shows the back side of the passivated packaged die  30 . In this view the die  20  is visible through the crosslinked siloxane passivation layer  34 . 
     As shown in FIG. 7, the packaged die  30  is thus ready to be mounted into a larger circuit. FIG. 7 shows the packaged die or integrated circuit device  30  mounted to a printed circuit board (PCB)  40 , such as the motherboard of a personal computer. The PCB  40  comprises a plastic substrate  42  having circuit traces or bond pads  44  designed to electrically communicate with the packaged die  30 . The illustrated plastic ball grid array (PBGA) is flip-chip mounted with the front side down, the solder balls  14  aligned with the contacts  44  of the PCB  40 . Such surface mounting of integrated circuit packages is well known in the art and need not be elaborated upon here. 
     With reference now to FIG. 8, the plurality of packaged dies or integrated circuit devices  50  are illustrated stacked upon one another, in accordance with a second preferred embodiment. Such stacked packages are described, for example, in U.S. patent application Ser. No. 09/072,101, filed May 4, 1998, entitled “Stackable Ball Grid Array Package,” and assigned to the assignee of the present application. The disclosure of the &#39;101 application is hereby incorporated by reference. 
     FIG. 9A illustrates one such stackable integrated circuit device  50 . As with the previous embodiment, the packaged die  50  can include a plastic substrate  12 ′, solder balls  14 ′, aperture  16 ′, die  20 ′ mounted within a die recess  18 ′, wire bonds  22 ′ and glob top  24 ′, such that the similar reference numerals are used as in the previous embodiment, with a prime (′) designation. In addition to these elements, however, the die carrier  10 ′ of the packaged die  50  includes solder ball pads  52  on the back side of the plastic substrate  12 ′, opposite to the solder balls  14 ′. The solder ball pads  14 ′ are electrically connected through the substrate  12 ′ to the solder ball pads  52 . While that electrical connection is schematically illustrated in FIG. 9A with a contact  54  extending through the substrate  12 ′, the skilled artisan will understand that, in reality, the connection may be through a complex of wiring layers through the plastic substrate  12 ′, rather than through an aligned contact. Additionally, the electrical contacts on either side of the substrate can be configured in a number of manners other than the illustrated solder balls and pads. 
     In stacking the packaged dies  50  upon one another the solder balls  14 ′ of one die carrier  10 ′ must electrically contact the solder ball pads  52  of a second die carrier  10 ′. Accordingly, insulating passivation of the back side of the die  20 ′ should not extend over the solder ball pads  52 . Accordingly, as shown, a patterned passivation layer  56  is formed over the die  20 ′ and seals the juncture of the die  20 ′ with the plastic substrate  12 ′. The patterned passivation layer  56  does not extend over the solder ball pads  52 . 
     As described with respect to the previous environment, the patterned passivation layer  56  can be formed by depositing a layer of PPMS and exposing the PPMS to deep UV light. During the exposure step, however, the portion of the PPMS exposed to the UV light can be confined to that area in which passivation is desired, in keeping with conventional photolithographic principles. As disclosed in the Joubert reference, incorporated by reference above, the portions of the PPMS over which passivation is not desired (e.g., the solder ball pads  52 ) are not exposed to the UV light. The oxide-like PPMSO passivation layer  56  is thus formed only where desired, leaving PPMS over the solder ball pads  52 . 
     The unexposed portions of the PPMS layer can thereafter be developed by exposure to a dry halide plasma treatment. An exemplary chlorine plasma treatment to develop unexposed PPMS comprises flowing about 80 sccm of chlorine gas under a pressure of about 2 mTorr, with RF power set at about 1 kW. Exposure to this chlorine plasma removes the unexposed PPMS, while leaving the exposed PPMSO passivation layer  56  unharmed. Accordingly, the solder ball pads  52  remain exposed and available for contact with solder balls  14 ′ from an overlying packaged substrate  50 . The skilled artisan will recognize other packaging schemes in which a photodefinable passivation layer over the die will be advantageous. 
     Although, this invention has been described in terms of certain preferred embodiments and suggested possible modifications thereto, other embodiments and modifications may suggest themselves and be apparent to those of ordinary skill in the art. Such modifications are intended to also fall within the spirit and scope of the present invention, which is accordingly defined by the claims which follow.