BACKSIDE METALLIZATION FOR SEMICONDUCTOR ASSEMBLY

Backside metallization techniques for a semiconductor assembly are disclosed. In one aspect, a die, such as a radio frequency (RF) die, within a semiconductor package may include backside metallization for RF performance reasons. The metallization is generally planar and covers a surface of the RF die. Exemplary aspects of the present disclosure cause the metallization to include trenches or grooves to allow for better expansion and contraction during thermal cycling of the RF die. In particular, the trenches decrease a modulus of the metallization layer and act as a shock absorber and allow for compression and expansion of the metallization to match the compression and expansion of the non-metal substrate of the RF die. By allowing for better matching of the compression and expansion of the two heterogeneous materials, delamination may be delayed or averted.

BACKGROUND

I. Field of the Disclosure

The technology of the disclosure relates generally to semiconductor packages that include backside metallization of a semiconductor assembly for performance reasons and, particularly, for a die with backside metallization that can handle multiple thermal cycles without delamination.

Computing devices abound in modern society, and more particularly, mobile communication devices have become increasingly common. The prevalence of these mobile communication devices is driven in part by the many functions that are now enabled on such devices. Increased processing capabilities in such devices mean that mobile communication devices have evolved from pure communication tools into sophisticated mobile entertainment centers, thus enabling enhanced user experiences. The increased functionality has caused evolutions within wireless standards that support the increased flow of data to the mobile communication devices. The newer wireless standards in turn have caused changes in power amplifiers associated with transmission chains that comply with the newer wireless standards. In many instances, the power amplifiers are becoming larger in physical size which leads to various mechanical challenges. These mechanical challenges in turn provide room for innovation.

SUMMARY

Aspects disclosed in the detailed description include backside metallization techniques for a semiconductor assembly. A die, such as a radio frequency (RF) die, within a semiconductor assembly may include a backside metallization layer for RF performance reasons. The metallization layer is generally planar and covers a surface of the RF die. Exemplary aspects of the present disclosure cause the metallization layer to include trenches or grooves to allow for better expansion and contraction during thermal cycling of the RF die. In particular, the trenches decrease a modulus of the metallization layer and act as a shock absorber and allow for compression and expansion of the metallization layer to match the compression and expansion of the non-metal substrate of the RF die. By allowing for better matching of the compression and expansion of the two heterogeneous materials, die delamination may be delayed or averted.

In this regard in one aspect, a semiconductor assembly is disclosed. The semiconductor assembly comprises a die comprising a backside. The semiconductor assembly also comprises a metallization layer patterned on the backside. The metallization layer comprises at least one trench within a boundary of the metallization layer.

DETAILED DESCRIPTION

Aspects disclosed in the detailed description include backside metallization techniques for a semiconductor assembly. A die, such as a radio frequency (RF) die, within a semiconductor assembly may include backside metallization layer for RF performance reasons. The metallization layer is generally planar and covers a surface of the RF die. Exemplary aspects of the present disclosure cause the metallization layer to include trenches or grooves to allow for better expansion and contraction during thermal cycling of the RF die. In particular, the trenches decrease a modulus of the metallization layer and act as a shock absorber and allow for compression and expansion of the metallization layer to match the compression and expansion of the non-metal substrate of the RF die. By allowing for better matching of the compression and expansion of the two heterogeneous materials, delamination may be delayed or averted.

Before addressing exemplary aspects of the present disclosure, it should be appreciated that an RF die may include a backside metallization layer for a variety of reasons including providing a good ground plane for electrical elements within the die and providing a suitable surface for die-attach. Further, the metallization layer may provide hot spot mitigation or other thermal management. This metallization layer may be approximately four micrometers (4 μm), although other dimensions (e.g., as thin as 0.1 ƒm) may also be appropriate depending on use. The metallization layer may, in some cases, be used on a relatively large die (e.g., greater than two millimeters by two millimeters (2 mm×2 mm)). Such a die may be a gallium nitride (GaN) or gallium arsenide (GaAs) die having a silicon carbide (SiC), aluminum nitride (AlN), silicon (Si) or diamond substrate on which the metallization layer is formed or patterned. The metallization layer may include materials such as gold (Au), copper (Cu), silver (Ag), nickel (Ni), platinum (Pt), titanium (Ti), chromium (Cr), Tungsten (W), or a combination of these different materials and may be attached to a lead frame, chip carrier, laminate, or other suitable substrate to form a semiconductor assembly. The attachment to the lead frame may be performed through a metallic die-attach material such as a gold-tin or other suitable material. The heterogeneous collection of materials in the semiconductor packaging process may result in a high mismatch between respective coefficients of thermal expansion (CTE). Such CTE mismatch means that during thermal cycling the materials may expand and contract at different rates and/or by different amounts. Collectively, this high degree of CTE mismatch may result in delamination between the die and the die attach material or between the die attach and the lead frame. Such delamination may result in reduced performance, reduced lifetime, and/or device failure.

Exemplary aspects of the present disclosure introduce a mechanical mechanism that decreases the effective modulus of the metallization layer such that expansion and contraction forces are mitigated, lowering or eliminating the risk of delamination across many thermal cycles. In a particular aspect, this mechanical mechanism is one or more trenches formed within the metallization layer.

In this regard,FIGS.1A and1Bshow top and side views of a die100having a metallization layer102on an “underside” or “backside”104of the die100. The backside104is a planar surface of a substrate106. As used herein, backside is meant to be the side of the die100opposite a side that has active elements such as transistors and the like. Trenches108are provided within the boundary110of the metallization layer102. In the die100, the trenches108are uniformly spaced across the x-y axes, forming uniformly-sized portions of the metallization layer102. Likewise, the trenches108extend through the entire z-axis dimension of the metallization layer102exposing the substrate106.

In contrast,FIGS.2A and2Bshow top and side views of a die200having a metallization layer202on an “underside” or “backside”204of the die200. The backside204is a planar surface of a substrate206. Trenches208are provided within the boundary210of the metallization layer202. In die200, the trenches208are uniformly spaced across the x-y axes, forming uniformly-sized portions of the metallization layer202. However, unlike the trenches108, the trenches208extend only through a portion of the metallization layer202along the z-axis, leaving a fill212.

While uniformly-spaced trenches108,208are possible, such trenches are not required. In contrast,FIGS.3A and3Bshow top and side views of a die300having a metallization layer302on an “underside” or “backside”304of the die300. The backside304is a planar surface of a substrate306. Trenches308are provided within the boundary310of the metallization layer302. In the die300, the trenches308are non-uniformly spaced across the x-y axes, forming non-uniformly-sized portions of the metallization layer302. However, the trenches308extend through the entire z-axis of the metallization layer302along the z-axis, exposing the substrate306.

FIGS.4A and4Bshow top and side views of a die400having a metallization layer402on an “underside” or “backside”404of the die400. The backside404is a planar surface of a substrate406. Trenches408are provided within the boundary410of the metallization layer402. In the die400, the trenches408are non-uniformly spaced across the x-y axes, forming non-uniformly-sized portions of the metallization layer402. However, unlike the trenches308, the trenches408extend only through a portion of the metallization layer402along the z-axis, leaving a fill412.

While the trenches108,208,308, and408are rectilinear, the present disclosure is not so limited. In this regard,FIG.5shows a top view of a die500having a metallization layer502on a backside of the die500. Trenches508are provided within the boundary510of the metallization layer502. In the die500, the trenches508may have rounded corners514. The radius of curvature of the rounded corners514may be uniform or varied as needed or desired. The trenches508may delimit a large central area516, which is generally rectilinear (albeit with the rounded corners). Rounded corners may also be used with any of the previous aspects. The trenches508may extend only through a portion of the metallization layer502along the z-axis, leaving a fill, or the trenches508may extend completely through the metallization layer502in the z-axis.

FIG.6shows a top view of a die600having a metallization layer602on a backside of the die600. Trenches608are provided within the boundary610of the metallization layer602. The trenches608may delimit a large central area616, which is generally oval although trenches formed from other forms of arcuate segments may be used. Note also, that the trenches608may include both arcuate segments and linear segments. The trenches608may have uniform width or have varied width. The trenches608may extend only through a portion of the metallization layer602along the z-axis, leaving a fill, or the trenches608may extend completely through the metallization layer602in the z-axis.

By providing trenches in whatever configuration within the backside metallization layer, the effective modulus of the backside metallization layer is decreased. That is, the air gaps within the trenches give room for the backside metallization layer to flex and bend as thermal cycling causes the material of the layer to expand and contract. This air gap serves as an effective shock absorber when the metallization layer expands and contracts at different rates than other materials in the semiconductor assembly so that less strain is put on the bond, resulting in less likelihood of delamination.

FIGS.7-11illustrate various configurations of trenches followed by a graph1200inFIG.12showing comparative results about the efficaciousness of the configurations in reducing strain. In this regard,FIG.7shows a top view of a die700having a metallization layer702on a backside of the die700. Trenches708are provided within the boundary710of the metallization layer702. The die700is substantially similar to the die100ofFIGS.1A and1Bwith rectilinear trenches708and uniformly-spaced trenches708. That is, a given portion718may be square, having sides a=sides b. For the purposes of the graph1200inFIG.12, the trenches708extend all the way through the metallization layer702and a=b=50 μm.

FIG.8shows a top view of a die800having a metallization layer802on a backside of the die800. Trenches808are provided within the boundary810of the metallization layer802. The die800has rectilinear trenches808and uniformly-spaced trenches808, but with rounded corners814. A given portion818may be square, having sides a=sides b. For the purposes of the graph1200inFIG.12, the trenches808extend all the way through the metallization layer802, a=b=50 μm, and a radius of curvature of the rounded corners814is 20 μm.

FIG.9shows a top view of a die900having a metallization layer902on a backside of the die900. Trenches908are provided within the boundary910of the metallization layer902. The trenches908are not rectilinear in nature and form an x or cross (or both) creating triangular portions918. For the purposes of the graph1200inFIG.12, the trenches908extend all the way through the metallization layer902.

FIG.10shows a top view of a die1000having a metallization layer1002on a backside of the die1000. Trenches1008are provided within the boundary1010of the metallization layer1002. The die1000has circular trenches1008centered on a common center in a central portion of the metallization layer1002. For the purposes of the graph1200inFIG.12, the trenches1008extend all the way through the metallization layer1002and the pitch of the trenches1008is uniform.

FIG.12illustrates the graph1200having normalized strain illustrated for two thicknesses of metallization layers corresponding to the dies700,800,900,1000,1100. Specifically thicknesses of 2.5 μm and 3.5 μm were tested. As can be seen, the dies700,800have an almost twenty percent reduction in strain. Such reduction should help reduce or eliminate the delamination observed in conventional dies.

The metallization layers disclosed herein such as the metallization layers102,202,302,402,502,602,702,802,902, and1002may be gold (Au) or copper (Cu) or other metal as needed or desired. The metallization layers may be formed by electron beam evaporation, electroplating, chemical vapor deposition (CVD), sputtering, physical vapor deposition (PVD), or the like.

The trenches108,208,308,408,508,608,708,808,908, and1008may be formed by mechanically scoring the metallization layer or wet or dry etching of the metallization layer after patterning the die or wafer backside using a suitable mask, such that when the mask is removed, the trenches remain or other technique as needed or desired. Still other techniques may be used without departing from the present disclosure. While it is contemplated that the trenches within a die may have uniform width and uniform depth, such is not required. Thus, some trenches could be partial, leaving a fill and other trenches within a single die may be complete, exposing the substrate.

While some specific dimensions for the pitch, width, and depth of the trenches is provided, it should be appreciated that other dimensions may also be used without departing from the present disclosure, and the dimensions provided herein are for the purpose of example only.

The abundance of possible variations may be modeled for a given die design and an optimal variation selected for thermal conductivity, preservation of a ground plane, electromagnetic compatibility (EMC), electromagnetic interference (EMI) or the like. While any modeling program may be used, ANSYS is well suited for this task.