Patent Application
20100052154
1. Field of the Invention
The present invention relates generally to a method for smoothening surfaces of non-metallic and semi-metallic lids disposed over semiconductor dies for use in processor assemblies. More specifically, embodiments of the present invention relate to a processor assembly, where the provision of said surface smoothened lids allows for improved marking permanency and thermal properties of a packaged device including the surface smoothened lids.
2. Background Art
Advanced microprocessors having a relatively large die area exceeding 100 mm2 require heat dissipation often in excess of 350 watts. Such high power densities and heat dissipation requirements necessitate the use of a high thermal conductivity lid to contact the microprocessor during heat generation. Additional mechanical requirements such as coplanarity of the lid and thickness of material used to adhere the lid to the semiconductor die are factors to be considered while choosing suitable materials for a lid. With the increasing rate of semiconductor die manufacturing, a need for efficient lid marking also dictates choice of material.
Traditionally, lids made of copper have been employed for smaller semiconductor dies due to low cost and high thermal conductivity qualities of copper. However, for large semiconductor dies, the mismatch in the coefficient of thermal expansion (CTE) between that of copper and the semiconductor may result in high stress at the mechanical interface between the lid and the semiconductor. To mitigate the effect of stress, grease is added as an additional thermal interface from the lid material to the device. To avoid the complexity of attachment and the deficiency of mismatched CTE, alloys consisting of materials such as tungsten, copper, or aluminum are employed in microprocessors or power insulated-gate bipolar transistor (IGBT) devices. The alloy materials possess the advantages of economy of processing and attachment to semiconductor devices, and adequacy of mechanical protection of the semiconductor, thereby allowing for efficient conduction of heat from the semiconductor to a secondary heat rejection device such as a heat sink or a cold plate.
However, due to the inherent compromise of thermal conductivity caused by the employment of alloys, utility of such alloys in high heat dissipating semiconductor devices is limited by thermal performance. Search for a semiconductor-matched CTE in materials has recently led to the development of alternative composite materials such as non-metallic or semi-metallic lids that also possess high thermal conductivity. Typical materials consist of carbon-copper composites, Aluminum Silicon Carbide (AlSiC) composites, Silicon Carbon Diamond (ScD) composites, diamond, and tungsten carbide diamond composites. While these composite materials are more suitable than metallic lids, the composite materials pose challenges in processing and handling due to typically high hardness, difficulty in soldering or brazing, difficulty in accepting permanent marking, and in many cases difficulty in obtaining a flat surface, which is desirable for a good thermal interface.
In general, in one aspect, the invention relates to a semiconductor packaged device comprising: a semiconductor die; an ultrahigh thermal conductivity lid disposed on the semiconductor die, the ultrahigh thermal conductivity lid comprising: a coupon having at least one uneven surface; a first layer on said at least one uneven surface formed from a process comprising one of sputter coating a highly adhesive metal over said uneven surface and sputtering a metallic seed layer; and a second layer on said first layer formed from a process comprising one of sputtering a metallic diffusion barrier layer over said first layer and electroplating the metallic seed layer with a highly conductive metal; and a thermal interface layer between said semiconductor die and said ultrahigh thermal conductivity lid, wherein the ultrahigh thermal conductivity lid has a smooth outer surface formed by at least one of chemical and mechanical processing of the at least one ultrahigh thermal conductivity lid after formation of the second layer.
In general, in one aspect, the invention relates to a method for smoothening surfaces of non-metallic and semi-metallic lids disposed over semiconductor dies for use in processor assemblies, the method comprising: providing at least one lid having at least one uneven surface; forming a first layer on said at least one uneven surface; forming a second layer on said first layer, and at least one of chemical and mechanical processing the at least one lid to provide for a smooth surface finish.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
Specific embodiments of the invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency.
In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.
In general, embodiments of the present invention describe a specific method for coating non-metallic or semi-metallic lids with a metallic material or multiple metallic materials for providing a flat surface during solder attachment to a semiconductor die. In one or more embodiments, such a flat surface may allow for easy marking and may provide an ideal surface for solder attachment to a semiconductor.
In one or more embodiments, the coating process may be performed before attachment of the lid to the processor such that the coupon is processed in environments ideal for preparation of the coupon for coating, but otherwise harmful to semiconductor devices.
In one or more embodiments, Step 405 may include sputtering to deposit a seed layer of metal such as Cu to provide good adhesion. In one or more embodiments, formation of the seed layer may include formation of a sputtered barrier layer of adhesive metals such as Ti or Cr as described above, followed by a sputtered adhesion layer of metals such as Cu or Ni, and finally the seed layer of Cu.
In Step 410, a diffusion barrier layer of metals with appropriate material properties such as Ni may be sputtered over the Ti or Cr layer to minimize thermal barriers and mismatch effects. In one or more embodiments in which a seed layer is deposited in step 405, step 410 may include electroplating the seed player with highly conductive metals such as Cu.
Step 415 involves chemical and mechanical post-processing such as lapping and milling for flattening and shaping the outer surface to a desired thickness. In one or more embodiments in which electroplating is used, post-plating processes such as polishing may be performed to attain desired surface finish and properties.
In one or more embodiments, Step 415 may render the surface of the coating suitable for permanent marking via traditional etched laser marking methods or similar technologies by providing for appropriate flatness and surface finish. Step 415 may also render the composite lid suitable for efficient employment in processors by providing for appropriate surface corrosive resistance, minimal thermal conductivity and diffusivity barrier of the coating, and minimal thermal mismatch between coated composite lid, the thermal interface material, semiconductor, and the rest of the electronics package.
It will be obvious to one of ordinary skill in the art that metallic coating is not limited to vapor deposition, sputtering, and/or electroplating, and that a combination of these methods, such as the abovementioned sputtering of a seed layer followed by electroplating, is within the scope of the invention. In one or more embodiments, a goal is to coat non-metallic or semi-metallic lids with metals to reduce thermal expansion and thermal stresses between layers of a packaged server processor unit or central processing unit (CPU) comprising such lids, while maintaining high thermal diffusivity between the layers, and also allowing for improved marking permanency of the ultrahigh thermal conductivity lids.
Additional advantages of embodiments of the invention may include one or more of the following. Dimensional physical tolerance characteristics of the coated ultrahigh thermal conductivity lids may be rendered comparable or better than traditional metallic lids, while thermal conductivity of a packaged processor comprising said lids is improved such that the processor may be able to release heat to the ambience using traditional air cooling techniques, beyond the limits of traditional metal lid processor packages. In one or more embodiments, the lids are rendered lighter than traditional lids, and also possess higher stiffness. In one or more embodiments, the thermal properties may be improved by at least four times in comparison with traditional lids, with tailorable CTE, electrical resistivity, higher stiffness, and strength.
While the invention has been described with respect to an exemplary embodiment of a method for forming a semiconductor-lid structure, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
