Abstract:
A semiconductor device includes: a semiconductor substrate having a main plane; a semiconductor element provided on the main plane of the semiconductor substrate; an electrode pad provided on the main plane of the semiconductor substrate and connected to the semiconductor element; a guard ring surrounding the semiconductor element and the electrode pad, and provided on the main plane of the semiconductor substrate; and an insulating film covering all region of a semiconductor of the main plane of the semiconductor substrate exposed inside the guard ring, wherein the insulating film is made of a water impermeable material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor device provided with high humidity resistance. 
         [0003]    2. Background Art 
         [0004]    Rapid progress is being made in universal use of high-frequency semiconductor devices typified by field-effect transistor elements using compound semiconductors such as GaAs, GaN. Regarding high frequency semiconductor devices, there has been no need to consider influences of water from an external environment so far by maintaining a high level of hermeticity inside packages in which the semiconductor devices are mounted. However, in recent years, since package mounting is performed using low-priced plastic mold materials, cost reduction is increasingly being achieved. Since it is difficult to prevent permeation of water into plastic mold materials, how to secure humidity resistance of semiconductor devices is becoming an important factor in securing reliability of products. 
         [0005]    A semiconductor device is proposed in which a guard ring is constructed of a conductive semiconductor in a peripheral region of a semiconductor device and part of the guard ring is exposed from an insulating film (e.g., see (FIG. 7 and FIG. 8, p. 29 of) Japanese Patent Application Laid-Open No. 2008-227116). 
       SUMMARY OF THE INVENTION 
       [0006]    When a field-effect transistor (FET) is manufactured according to Japanese Patent Application Laid-Open No. 2008-227116, although it is assumed that it is possible to prevent degeneration of a semiconductor layer formed in a peripheral portion and prevent an impurity from intruding into the semiconductor device through a gap between the semiconductor layer and a protective film, and thereby improve humidity resistance, there are actually cases where the semiconductor of the guard ring exposed from the insulating film is corroded. For this reason, almost no superiority is found regarding the corrosion phenomenon in the semiconductor layer of the region peripheral to the chip over conventional semiconductor devices that do not adopt the configuration of Japanese Patent Application Laid-Open No. 2008-227116. Moreover, corrosion of the semiconductor of the guard ring gradually advances toward a drain electrode pad, causing even a metal part of the electrode pad to be corroded. This also secondarily causes cracking or peeling of the insulating film that protects the periphery of the pad, degrading the transistor region. This problem is conspicuous in GaN devices for which high-voltage operation is required. 
         [0007]    In view of the above-described problems, an object of the present invention is to provide a semiconductor device provided with high humidity resistance. 
         [0008]    According to the present invention, a semiconductor device includes: a semiconductor substrate having a main plane; a semiconductor element provided on the main plane of the semiconductor substrate; an electrode pad provided on the main plane of the semiconductor substrate and connected to the semiconductor element; a guard ring surrounding the semiconductor element and the electrode pad, and provided on the main plane of the semiconductor substrate; and an insulating film covering all region of a semiconductor of the main plane of the semiconductor substrate exposed inside the guard ring, wherein the insulating film is made of a water impermeable material. 
         [0009]    In the present invention, the insulating film that covers the whole region where the semiconductor of the semiconductor substrate inside the guard ring is exposed is made of a water impermeable material. Thus, it is possible to obtain a semiconductor device provided with high humidity resistance. 
         [0010]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a cross-sectional view illustrating a semiconductor device according to a first embodiment of the present invention. 
           [0012]      FIG. 2  is a plan view illustrating the semiconductor device according to the first embodiment of the present invention. 
           [0013]      FIG. 3  is a cross-sectional view illustrating the semiconductor device according to the first embodiment of the present invention mounted in a package. 
           [0014]      FIG. 4  is a cross-sectional view illustrating a semiconductor device according to a second embodiment of the present invention. 
           [0015]      FIG. 5  is a plan view illustrating the semiconductor device according to the second embodiment of the present invention. 
           [0016]      FIG. 6  is a cross-sectional view illustrating the semiconductor device according to the second embodiment of the present invention mounted in a package. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    A semiconductor device according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted. 
       First Embodiment 
       [0018]      FIG. 1  is a cross-sectional view illustrating a semiconductor device according to a first embodiment of the present invention.  FIG. 2  is a plan view illustrating the semiconductor device according to the first embodiment of the present invention. A semiconductor substrate  1  is any one of a Si substrate, GaAs substrate, InP substrate, SiC substrate and GaN substrate, and has a multilayered film structure of semiconductor for performing a predetermined transistor operation. 
         [0019]    A semiconductor element  2  is provided on a main plane of the semiconductor substrate  1 . The semiconductor element  2  here is a field-effect transistor element to be used for a high output amplifier, but it may also be a bipolar transistor element or the like. The semiconductor element  2  includes a gate electrode  2   a,  a source electrode  2   b  and a drain electrode  2   c.    
         [0020]    A gate electrode pad  3   a,  a source electrode pad  3   b  and a drain electrode pad  3   c  are provided on the main plane of the semiconductor substrate  1  and respectively connected to the gate electrode  2   a,  source electrode  2   b  and drain electrode  2   c  of the semiconductor element  2 . A back metal film  4  is provided on the back of the semiconductor substrate  1  and the back metal film  4  is connected to the source electrode pad  3   b  via a via hole  5 . 
         [0021]    A guard ring  6  that surrounds the semiconductor element  2  and the electrode pads  3   a,    3   b  and  3   c  is provided in a region peripheral to the chip on the main plane of the semiconductor substrate  1 . An insulating film  7  covers all the region of the semiconductor of the semiconductor substrate  1  exposed from the electrodes  2   a,    2   b  and  2   c,  and the electrode pads  3   a,    3   b  and  3   c  inside the guard ring  6 . Note that the region covered with anything other than the semiconductor such as the gate electrode pad  3   a,  the source electrode pad  3   b  and the drain electrode pad  3   c  need not be covered with the insulating film  7 . These regions need to be electrically connected to the outside. 
         [0022]    In the present embodiment, the guard ring  6  is made of a conductive semiconductor and the insulating film  7  also covers the whole guard ring  6 . In the region on the side of the chip outside the guard ring  6 , an inter-chip boundary region to be separated from a wafer into individual chips, and the insulating film  7  in the outermost circumferential part of the chip may be opened. The interface between the metal and the semiconductor on the side of the chip inside the guard ring  6  needs to be covered with the insulating film. 
         [0023]    Here, the insulating film  7  is made of a water impermeable material or a material at least more water impermeable than a resin film. For example, it is preferable to use a silicon nitride film or the like capable of preventing permeation or diffusion of water as the insulating film  7 . The silicon nitride film is often formed using plasma CVD and preferably formed under a condition similar to Si 3 N 4  which is a stoichiometric composition. Generally, permeation or diffusion of water of the silicon nitride film fluctuates when it is deviated from a stoichiometric composition. For this reason, it is necessary to design a film thickness of the silicon nitride film enough to prevent water permeation up to each metal material making up the main plane of the semiconductor substrate  1  and transistor for the film forming condition of the silicon nitride film. 
         [0024]      FIG. 3  is a cross-sectional view illustrating the semiconductor device according to the first embodiment of the present invention mounted in a package. The back metal film  4  of the semiconductor device is joined to a package base material  8  via a solder material  9 . Since the package base material  8  is given a GND potential (0 V) which is a reference potential, the source electrode  2   b  of the semiconductor device is given the reference potential via the via hole  5  and the source electrode pad  3   b.  The guard ring  6  is also given the reference potential. No potential difference is produced in the semiconductor layer outside the guard ring  6 , which is kept to 0 V. This prevents corrosion reaction by electrochemical reaction. 
         [0025]    Although a potential difference is produced between the drain electrode pad  3   c  which is given a positive potential and the guard ring  6  which is given the reference potential, since the surface of the semiconductor is protected by the insulating film  7  or the metal electrode pads  3   a,    3   b  and  3   c,  it is possible to eliminate influences of water. Therefore, it is also possible to prevent corrosion of the metal of the electrode pads  3   a,    3   b  and  3   c,  cracking or peeling of the insulating film  7  or progression of degradation toward the transistor region, which are incidental to corrosion reaction of the semiconductor. 
         [0026]    The insulating film  7  that covers the whole region where the semiconductor of the semiconductor substrate  1  inside the guard ring  6  is exposed is made of a water impermeable material. Thus, it is possible to obtain a semiconductor device provided with high humidity resistance. 
         [0027]    When the guard ring  6  is made of a conductive semiconductor, the semiconductor of the guard ring  6  also becomes a starting point of corrosion reaction. However, the insulating film  7  covers the whole guard ring  6 , and can thereby prevent corrosion reaction. 
         [0028]    The semiconductor element  2  may also be an integrated circuit including a passive element such as resistor, capacitor or inductor. Regarding the region in which this passive element is formed, the region where the semiconductor of the semiconductor substrate  1  is exposed is also covered with the insulating film  7 . 
       Second Embodiment 
       [0029]      FIG. 4  is a cross-sectional view illustrating a semiconductor device according to a second embodiment of the present invention.  FIG. 5  is a plan view illustrating the semiconductor device according to the second embodiment of the present invention.  FIG. 6  is a cross-sectional view illustrating the semiconductor device according to the second embodiment of the present invention mounted in a package. 
         [0030]    In the present embodiment, the guard ring  6  is metallic. The source electrode pad  3   b  also serves as part of the guard ring layer. The insulating film  7  covers the guard ring  6  and the insulating film  7  on the source electrode pad  3   b  (part of the guard ring  6 ) is opened. The rest of the configuration is similar to that of the first embodiment. 
         [0031]    When the guard ring  6  is metallic, even when part of the guard ring  6  is exposed from the insulating film  7 , the semiconductor which becomes a starting point of corrosion reaction is not exposed. Therefore, it is possible to obtain a semiconductor device with high humidity resistance as in the case of the first embodiment. Furthermore, when the guard ring  6  is metallic, even when the insulating film  7  on the side of the chip outside the guard ring  6  is opened, no potential difference is produced in the region from which the semiconductor on the side of the chip outside the guard ring  6  is exposed, which is kept to 0 V, and humidity resistance never deteriorates. 
         [0032]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0033]    The entire disclosure of Japanese Patent Application No. 2014-204754, filed on Oct. 3, 2014 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.