Abstract:
A semiconductor device comprises one or more transistors and two or more layers of dielectric material encapsulating a front side of said one or more transistors. The gate of each of said one or more transistors is located within a cavity, or air-box, in at least one of the dielectric layers, so that the gate terminal is physically separated from said dielectric material. Such an arrangement may reduce parasitic capacitance. In another arrangement, a semiconductor device comprises one or more gallium nitride high electron mobility transistors and one or more dielectric layers encapsulating a front side of said one or more transistors, wherein the gate terminal of each of said one or more transistors is located within a cavity in at least one of the one or more dielectric layers, separated from said dielectric material.

Description:
STATEMENT REGARDING FEDERAL FUNDING 
     This invention was made under U.S. Government Contract No. HR0011-09-C-0126. The U.S. Government has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     TECHNICAL FIELD 
     This disclosure relates to transistors and methods of encapsulation and manufacture thereof. 
     BACKGROUND 
     High frequency active circuits, designed to operate at a frequency above 50 GHz, may be vulnerable to damage for a number of reasons. Such circuits may include features with extremely small physical dimensions, such as a few nanometers, that may be fragile, especially in areas around transistor gates. In addition, interconnects and passive components, such as capacitors and thin-film resistors, on a front-side of a circuit may be sensitive to scratches. Therefore, such circuits are at risk of damage when handled during packaging and assembly. Also, the front-side of the circuit may be exposed to humidity and other environmental conditions that may cause performance degradation or accelerate failure of active components. 
     Encapsulation, or passivation, has been used to provide some physical protection for active areas, interconnects and passive components on the front-sides of micro-electronic chips, using a layer or coating of a material having a higher dielectric constant than air. Examples of dielectric materials used for this purpose include BCB (benzocyclobutene), spin-on glass and polyimide. The protective dielectric material can also be used to provide mechanical support for multi-layer interconnects, allowing additional flexibility in the design of the circuit. However, the presence of dielectric material around gates of components of the circuit causes parasitic capacitance to increase. Parasitic capacitance can cause a significant degradation in the performance of the circuit, particularly at high frequencies, potentially affecting one or more of gain, frequency drift, noise, output power and power-added efficiency. For example, a GaN (gallium nitride) amplifier encapsulated with BCB can show a deterioration of approximately 1.5 dB in small signal gain and a shift in operational frequency of 10-15%, when compared with an unencapsulated amplifier that is otherwise identical. Such decreased performance can occur even where a low-loss dielectric material is used for encapsulation. 
     Therefore, the protection and flexibility provided by conventional methods is offset by a decrease in the performance of the circuit. In low frequency circuits, operating below 30 GHz, the benefits may outweigh the degradation in performance. However, at higher frequencies, above 30 GHz, the reduction in gain, output power and efficiency of the circuit can be problematical. Hence, the usefulness of conventional dielectric encapsulation and passivation protection methods for high frequency circuits is limited. 
     SUMMARY 
     Embodiments of the present disclosure may include a semiconductor device including a substrate and one or more transistors mounted on said substrate, each of said one or more transistors having a respective gate terminal. Two or more layers of dielectric material encapsulate a front side of said one or more transistors, at least one of said two or more layers of dielectric material having one or more cavities. The gate terminal of each of said one or more transistors is located within one of said one or more cavities, separated from said at least one layer of dielectric material. 
     Such a semiconductor device may include two or more interconnects, where a first one of said interconnects is supported by a first one of said layers of dielectric material and a second one of said interconnects being supported by a second one of said layers of dielectric material. 
     The one or more transistors may include a gallium nitride high electron mobility transistor. 
     In some embodiments, the semiconductor device may form part of a device such as a power amplifier, low noise amplifier, mixer, switch, phase-shifter or variable attenuator. Alternatively, or additionally, the semiconductor device may form part of a submillimeter-wave circuit or mixed signal circuit. 
     In another embodiment, a semiconductor device includes a substrate and one or more gallium nitride high electron mobility transistors mounted on said substrate, each of said one or more transistors having a respective gate terminal. One or more layers of dielectric material encapsulate a front side of said one or more transistors, at least one of said one or more layers of dielectric material having one or more cavities. The gate terminal of each of said one or more transistors is located within a respective one of said one or more cavities and separated from said at least one layer of dielectric material. 
     In other embodiments, a device such as a power amplifier, low noise amplifier, mixer, switch, phase-shifter or variable attenuator or a circuit, such as a sub-millimeter wave circuit or a mixed signal circuit, includes said semiconductor device. 
     In yet another embodiment, a method of encapsulating a semiconductor device includes mounting one or more transistors on a substrate, each of said one or more transistors having a respective gate terminal, applying two or more layers of dielectric material encapsulating a front side of said one or more transistors, and forming one or more cavities in at least one of said two or more layers of dielectric material, where said one or more cavities are located around a gate terminal of said one or more transistors to separate said gate terminal from said two or more layers of dielectric material. 
     In such a method, said forming one or more cavities might include, before applying a first one of said two or more layers of dielectric material, applying a layer of sacrificial material over active areas of said one or more transistors, and after applying said two or more layers of dielectric material, etching said layers of dielectric material to provide access to said layer of sacrificial material, and removing said layer of sacrificial material. 
     In certain embodiments, the method further includes, after applying a first one of said two or more layers of dielectric material and before applying a second one of said two or more layers of dielectric material, forming one or more interconnects supported by said first layer of dielectric material and, optionally, may further include, after applying a second one of said two or more layers of dielectric material, forming one or more interconnects supported by said second layer of dielectric material. In an example where interconnects are supported by the first and second layers of dielectric material, forming said one or more interconnects supported by said first layer of dielectric material may include dry-etching said first layer of dielectric material, and forming said one or more interconnects supported by said second layer of dielectric material may include photo-etching said second layer of dielectric material. 
     In a further embodiment, a method of encapsulating a semiconductor device includes mounting one or more gallium nitride high electron mobility transistors on a substrate, each of said one or more transistors having a respective gate terminal. One or more layers of dielectric material are applied to encapsulate, or passivate, a front side of said one or more transistors. Then, one or more cavities in at least one of said one or more layers of dielectric material, where said one or more cavities are located around a gate terminal of said one or more transistors to separate said gate terminal from said one or more layers of dielectric material. 
     These and other features and advantages will become further apparent from the detailed description of example embodiments that follows and the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a device according to an embodiment; 
         FIG. 2  is a flowchart of an encapsulation method for use in manufacture of the device of  FIG. 1 ; 
         FIGS. 3 to 8  depict stages in the encapsulation of the device of  FIG. 1  using the method shown in  FIG. 2 ; 
         FIG. 9  is a graph comparing DC characteristics of a GaN HEMT that has been encapsulated according to an embodiment and an unencapsulated GaN HEMT; 
         FIG. 10  is a graph comparing RF characteristics of a GaN HEMT that has been encapsulated according to an embodiment and an unencapsulated GaN HEMT; and 
         FIG. 11  depicts an encapsulated semiconductor device according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the figures and the following description, numerals indicate various features, like numerals referring to like features throughout both the drawings and description. 
       FIG. 1  depicts a device  1 , such as an IC (integrated circuit) chip according to an embodiment. In this particular example, the device  1  includes at least two transistors  2   a ,  2   b , of which the respective gate terminals  3   a ,  3   b , source terminals  4   a ,  4   b  and drain terminals  5   a ,  5   b  are shown, mounted on a substrate  6 . For example, the first transistor  2   a  may be a D-mode (depletion mode) GaN HEMT and the second transistor  2   b  may be an E-mode (enhancement mode) GaN HEMT, while the substrate  6  is formed of SiC (silicon carbide). 
     The front-side of the transistors  2   a ,  2   b  are encapsulated with at least one layer of dielectric material. In this particular example, three dielectric layers  7 ,  8 ,  9  are present. In addition to providing physical protection for the front-sides of the transistors  2   a ,  2   b , the dielectric layers  7 ,  8 ,  9  can also be used provide mechanical support for metal interconnects  10   a ,  10   b ,  10   c ,  11   a ,  11   b ,  12  at one or more levels within the device  1 . In the example embodiment depicted in  FIG. 1 , the dielectric layers  7 ,  8 ,  9  are used to support interconnects  10   a ,  10   b ,  10   c ,  11   a ,  11   b ,  12  at three different levels in the device  1 . The provision of interconnects at multiple levels can allow scaling, so that the overall physical dimensions of the device  1  can be made relatively small. 
     The active areas of the transistors  2   a ,  2   b  are separated from the material of the dielectric layers  7 ,  8 ,  9 . In this example, a cavity  13   a ,  13   b  is provided in a first one of the dielectric layers  7  to provide such separation, by forming an “air-box” around the gate terminals  3   a ,  3   b  of the individual transistors  2   a ,  2   b . Since the gate terminals  3   a ,  3   b  are separated from the dielectric layers  7 ,  8 ,  9 , parasitic capacitance between the gate terminals  3   a ,  3   b  and the source terminals  4   a ,  4   b  and/or between the gate terminals  3   a ,  3   b  and the drain terminals  5   a ,  5   b  of the respective transistors  2   a ,  2   b  can be reduced, when compared with encapsulated devices without cavities. 
     An encapsulation method for use in manufacturing the device  1  will now be described with reference to the flowchart of  FIG. 2  and to  FIGS. 3 to 7 , beginning at step s 0 . The transistors  2   a ,  2   b  are provided on a SiC wafer  6 ′ (step s 1 ). An example of a method of fabricating GaN HEMTs is disclosed in K. Shinohara et al., “ Scaling of GaN HEMTs and Schottky Diodes for Submillimeter - Wave MMIC Applications ”, IEEE Transactions of Electron Devices, October 2013, the disclosure of which is incorporated herein by reference in its entirety. 
     The active areas of the transistors  2   a ,  2   b  are coated with a layer  14  of sacrificial material (step s 2 ), as shown in  FIG. 3 . In this example, the sacrificial layer  14  is formed of a photo-resist, such as PMGI (polymethylglutarimide), spun-coated on the substrate  6  and having a thickness of approximately 2 microns. 
     The first dielectric layer  7  is then applied over the front-side of the transistors  2   a ,  2   b , covering the sacrificial layer  14  (step s 3 ). In this particular example, the first dielectric layer  7  is formed of BCB and deposited using spin-on coating. 
     The first dielectric layer  7  is then patterned and etched (step s 4 ) to provide vias  15   a ,  15   b ,  16   a ,  16   b , shown in  FIG. 4 , for example using a RIE (reactive-ion etching) tool. A metallization process is then used to form a first set of interconnects  10   a ,  10   b ,  10   c  by partially filling those vias  15   a ,  15   b ,  16   a ,  16   b  (step s 5 ), as shown in  FIG. 5 . In this particular example, the first dielectric layer is a BCB coating of approximately 3 microns in thickness, deposited using spin-coating, and dry etched to provide the vias  15   a ,  15   b ,  16   a ,  16   b  and the interconnects  10   a ,  10   b ,  10   c  are formed by Au (gold) metallization. 
     Where the further dielectric layers are to be added to the device, for example, to provide interconnects at more levels (step s 6 ), the steps of applying and etching a dielectric layer  8 ,  9  (steps s 4  and s 5 ) and forming interconnects  11   a ,  11   b ,  12  (step s 6 ) is repeated for each further dielectric layer  8 ,  9 . In this particular example, the further dielectric layers  8 ,  9  are provided in the form of spin-on BCB coatings of approximately 3 microns in thickness. 
     Each further dielectric layers  8 ,  9  is, in turn, deposited, cured, patterned and etched, in turn, to form further vias,  17   a ,  17   b ,  18  (steps s 4  and s 5 ). Since tolerances for the positioning of the interconnects  11   a ,  11   b ,  12  in the further dielectric layers  8 ,  9  is greater than the tolerance for the position of the interconnects  10  in the first dielectric layer  7 , photo-etching may be used to define the vias  17   a ,  17   b ,  18  instead of dry-etching in step s 5 . Further interconnects  11   a ,  11   b ,  12  are then formed using a process such as sputtering and electroplating (step s 6 ).  FIG. 6  depicts the device  1  following the application of second and third dielectric layers  8 ,  9  and the formation of the further interconnects  11 ,  12  thereon. 
     After the required number of dielectric layers  7 ,  8 ,  9  and interconnects  10 ,  11 ,  12  have been formed (step s 6 ), the wafer  6 ′ may then be thinned to form the substrate  6  (step s 7 ). In this particular example, the wafer  6 ′ is thinned to a thickness of 50 microns. Also, if required, backside vias (not shown) can be formed through the wafer  6 ′ at this stage. 
     The cavities  13   a ,  13   b  are then formed by removing the layer  14  of sacrificial material covering the active areas of the transistors  2   a ,  2   b  (step s 8 ), to complete the encapsulation method (step s 9 ). In this particular example, a hard mask (not shown) is placed over the device  1  and vias  19   a ,  19   b , shown in  FIGS. 7 and 8 , are formed in the dielectric layers  7 ,  8 ,  9  using a dry-etching technique such as RIE, to provide access to the layer  14  of sacrificial material covering the active areas. The layer  14  of sacrificial material is then removed using a developer, solvent or other chemical formulation, to form the cavities  13   a ,  13   b  around the gate terminals  3   a ,  3   b , to produce the device  1  as shown in  FIGS. 1 and 8 , where  FIG. 8  shows a cross-section of the device through the vias  19   a ,  19   b.    
       FIGS. 9 and 10  show DC and RF characteristics respectively, for a GaN HEMT with multi-level interconnects such as the transistor  2   a  in  FIG. 1 .  FIG. 9  is a graph showing variation of the transductance g m  of the transistor  2   a  and the current I ds  between the drain terminal  5   a  and the source terminal  4   a  against the voltage V gs  between the gate terminal  3   a  and the source terminal  4   a  for an example where the gate length L gd  is 20 nm, the gate-to-source overlap L gs  is 30 nm, the gate-to-drain overlap L gd  is 90 nm and the voltage V ds  between the drain terminal  5   a  and the source terminal  4   a  is 4.0 V.  FIG. 10  is a graph showing the maximum stable gain (MSG) and unilateral gain (U g ) of the same GaN HEMT structure at radio frequencies, where the voltage V ds  between the drain terminal  5   a  and the source terminal  4   a  is 5.0 V and the voltage V gs  between the gate terminal  3   a  and the source terminal  4   a  is −0.75 V. 
     In both  FIGS. 9 and 10 , the values for the GaN HEMT, before encapsulation, are indicated by a solid line, while the values for the GaN HEMT after encapsulation with a cavity  13   a  are indicated by open circles. As shown by  FIGS. 9 and 10 , the changes in the DC and RF characteristics of the GaN HEMT caused by encapsulation may be insignificant. In particular, in the example shown in  FIG. 9 , the differences in cut-off frequency f T  and maximum oscillation frequency f max  between the encapsulated GaN HEMT and the unencapsulated GaN HEMT appear to be negligible. Hence, in this example, the performance of the GaN HEMT is largely unaffected by multi-layer encapsulation by the method of  FIG. 2 , where the presence of the cavity  13   a  prevents a significant increase in the parasitic capacitances of the gate terminal  3   a.    
       FIG. 11  depicts a semiconductor device  20  according to another embodiment in which two GaN HEMTs  2   a ,  2   b  are encapsulated by a single dielectric layer  7  and interconnects  10   a ,  10   b ,  10   c  are supported by the single dielectric layer  7  at one level. Such a semiconductor device  20  may be provided by a method as shown in  FIG. 2 , without repetition of steps s 4 , s 5  and s 6  discussed hereinabove. 
     Embodiments of the present disclosure may provide methods for providing at least some of the advantages of dielectric encapsulation while reducing, or even avoiding, the performance degradation associated with conventional encapsulation techniques. Where multi-layer encapsulation is used, the dielectric layers  7 ,  8 ,  9  can be used to support interconnects  10   a ,  10   b ,  10   c ,  11   a ,  11   b ,  12  at multiple levels, facilitating scaling of devices  1  and increasing multi-functionality and improving flexibility when interconnecting between multiple devices  1 . Where the overall size of the device  1  is reduced, the cost of the device may also decrease. 
     Embodiments of the present disclosure may be particularly beneficial in active devices operating at high speed or at high frequencies, above 50 GHz, such as G-band frequencies (110-300 GHz), where performance degradation due to parasitic capacitances may be particularly marked. For example, the use of highly-scaled GaN HEMTs in submillimeter-wave and mixed signal circuits, power amplifiers, low noise amplifiers, mixers, switches, phase shifters, variable attenuators and so on, may be facilitated by using embodiments of the encapsulation method. 
     Encapsulation methods according to particular embodiments may be compatible with GaN T2, T3 and T4 processes. 
     In the embodiments described above, the sensitive active areas of the transistors  2   a ,  2   b  and at least some of the interconnects  10   a ,  10   b ,  10   c ,  11   a ,  11   b ,  12  are protected by one or more dielectric layers  7 ,  8 ,  9 . Protected chips are easier to handle during mounting and assembly of electronic devices, for example, where tweezers of vacuum wands are used to hold and manipulate the chips. 
     Also, depending on the details and type of the dielectric encapsulation, the embodiments can provide a hermetic or near-hermetic environment for the active areas of the transistors  2   a ,  2   b , protecting the transistors  2   a ,  2   b  from some adverse environmental conditions. This can improve the long-term reliability of the device  1  and, also, allows greater freedom in a next level of packaging of the device  1 . 
     The foregoing description of embodiments is presented for the purposes of illustration only. It is not intended to be exhaustive or to limit the disclosure to the precise form of the examples disclosed. For example, while the example embodiments disclosed with reference to  FIGS. 1 and 3  to  8  and to  FIG. 11  have two GaN HEMT transistors  2   a ,  2   b , embodiments may have other numbers, or types, of active devices. 
     While  FIGS. 1 and 11  depict embodiments in which transistors  2   a ,  2   b  are located within respective cavities  13   a ,  13   b  in a first dielectric layer  7 , other embodiments may be envisaged in which more than one transistor is located within one cavity and/or where a cavity extends into more than one dielectric layer  7 ,  8 ,  9 . 
       FIGS. 1 and 11  depict embodiments in which three dielectric layers  7 ,  8 ,  9  and one dielectric layer  7  are provided respectively, with interconnects  10   a ,  10   b ,  10   c ,  11   a ,  11   b ,  12  provided on a corresponding number of levels. Other embodiments may have different numbers of dielectric layers and/or levels of interconnects. 
     The example embodiments shown in  FIGS. 1 and 11  have a SiC substrate, gold interconnects and one or more dielectric layers  7 ,  8 ,  9  formed of BCB. However, in other embodiments, alternative materials may be used for some or all of these components. Examples of other suitable materials for the one or more dielectric layers  7 ,  8 ,  9  include spin-on glass, silicon nitride (SiN), polyimide, and so on, while suitable materials for forming some or all of the interconnects include copper. Further, other techniques may be used to deposit and/or define the layer  14  of sacrificial material and the one or more dielectric layers  7 ,  8 ,  9  than those discussed above in regard to  FIG. 2 , such as plasma-enhanced chemical vapor deposition (PECVD). 
     Other modifications and variations to the above embodiments will be apparent to persons skilled in the art, and the method steps described above might be interchangeable with other steps to achieve the same result. It is intended that the scope of the disclosure be interpreted with reference to the claims appended hereto and their equivalents. 
     Reference to an element in the singular hereinabove is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more”. Moreover, no element, component or method step in the present disclosure is intended to be dedicated to the public, regardless of whether the element, component or method step is explicitly recited in the accompanying claims. No claim element herein is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ”.