Casting mold for producing an optical semiconductor module

A method and a casting mold for producing an optical semiconductor module is provided, wherein a semiconductor body having at least one optically active element on its top is introduced into a leadframe. Then conductive connections are established between the semiconductor body and the leadframe, and then the leadframe and semiconductor body are encapsulated in a casting mold. Wherein provided in the part of the casting mold that faces the top of the semiconductor body are masking bodies, which extend from the top inner wall of the casting mold towards the optically active elements and cover the elements with their respective end face in a way that seals out casting material.

This nonprovisional application claims priority under 35 U.S.C. §119(a) on German Patent Application No. DE 102005010311, which was filed in Germany on Mar. 3, 2005, and which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a casting mold for producing an optical semiconductor module, wherein a semiconductor body having at least one optically active element on its surface is introduced into a leadframe, then conductive connections are established between the semiconductor body and the leadframe, and then the leadframe and semiconductor body are encapsulated in a casting mold.

2. Description of the Background Art

Manufacturing methods are known, in which an optical IC (PDIC) is encapsulated in a transparent casting material or is placed on an organic substrate and then covered with a casting material. The casting material must be transparent in order to permit radiation in the optical range to pass to or from the optically active element. In this connection, the casting material must be matched to the wavelength range in which the optical element transmits or receives. In addition, the casting material must have temperature stability, long-term stability, and be moisture-resistant.

Such casting materials are relatively expensive, and problems regularly occur with the optical transparency.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method for producing an optical semiconductor module in which an economical casting material can be used and the desirable optical characteristics of the semiconductor module are achieved.

The inventive method makes it possible to produce, in an especially simple and economical way, a semiconductor that on the one hand is encapsulated, and thus protected from environmental influences such as vibrations and moisture, but on the other hand in which the corresponding casting material is excluded in the area where light is to be received or transmitted.

By this method, a more economical casting material, for example an epoxy resin that is not optically transparent, can be used, resulting in great cost savings. The casting materials used can thus also be selected according to criteria other than optical transmission, thereby additionally permitting the module to be optimized for other requirements as well.

Preferably the masking bodies must be chosen of such a size that they leave open adequate areas of the semiconductor body. In this context, each optically active element on the semiconductor body can have its own associated masking body, or several of the optically active elements can be covered jointly by one masking body.

After the casting material has been cast and largely solidified, each mold can be opened and the masking bodies can be removed together with the part of the mold facing the top of the semiconductor body. During the casting process, the end faces of the masking elements are pressed against the semiconductor bodies in such a manner that the thin-bodied casting material cannot penetrate between the end face of the masking body and the corresponding region of the semiconductor body.

The semiconductor body can be elastically held in a leadframe, so that when the masking bodies have appropriate lengths they can push the semiconductor body back a distance against the elastic force, thus producing the appropriate contact force.

In an embodiment, if the top of the semiconductor body as a whole is covered with a polyimide layer prior to encapsulation, with the regions of the optically active elements left open. The polyimide layer helps to reduce the thermal stress on the semiconductor body in the emerging cast body, and makes the semiconductor module more thermally stable, and thus usable at higher temperatures. This is especially important when the semiconductor module is to be further processed or connected to other circuit units using the lead-free soldering method, since the solders used in lead-free soldering require a soldering temperature that is increased by approximately 20° C. under otherwise equivalent joining conditions.

With regard to its external structure, the casting mold corresponds largely to a casting mold that is for elastic leadframes of the QFN type (quad flat non-leaded package), wherein the masking bodies are additionally arranged on and attached to an interior wall. Such casting molds are typically made of steel, and the masking bodies may be connected to the mold as a single piece.

Typically the masking bodies may be designed as cylindrical rods, but they may also have a slight conical taper or a cross-section that is reduced in a stepwise, conical, or convex or concave manner toward the semiconductor body; this facilitates final forming after the casting process. In any case, they have an end face that can be pressed against the semiconductor body. In cross-section, the masking bodies may be round or polygonal, preferably rectangular, in design. Even in rectangular form, the masking bodies can taper conically toward the semiconductor body in order to facilitate demolding.

In order to facilitate sealing of the optically active areas of the semiconductor body requiring protection by the masking body end faces, provision can be made that said bodies have on their end face a raised edge that is flat around its circumference and forms a sealing edge with respect to the semiconductor body. Recessing the actual end face behind this edge also effectively prevents the casting material from being drawn into the intermediate space between the semiconductor body and the masking body by capillary action.

The resulting semiconductor module is characterized by its simple design and low price, a result of the simple encapsulation while leaving the optically active areas open. The corresponding recesses in the casting material make possible the emission and reception of optical signals. As a result of a polyimide coating on the semiconductor body, the inventive semiconductor module is particularly temperature-stable.

DETAILED DESCRIPTION

In the drawings, like or functionally like elements and signals are identified with the same reference labels, unless otherwise specified.

FIG. 1shows a cross-section of an encapsulated semiconductor module in which a semiconductor body1is attached to a substrate2and this unit is elastically mounted on a leadframe whose parts are labeled3,4. The casting material, which typically is an epoxy resin, is labeled5. The parts3,4of the leadframe each have separate, individual contacts, which are connected by bond wires6,7to respective bond areas8,9of the semiconductor body. After encapsulation and curing of the semiconductor module, the individual contacts3,4are produced by sawing from a contiguous metal body. Prior to the manufacture of the semiconductor module, the metal body has a contiguous frame of uniform material, for example copper, in which areas are formed by etching or pre-punching where the individual contacts can be separated simply later. In this way, this frame is simple to process but can nevertheless be divided after casting into numerous individual parts that are electrically insulated from one another.

FIG. 1shows that the semiconductor body1is only partially covered by the casting material5. An open space10remains, in which optically active elements can be arranged, which can then emit or receive radiation without being hindered by the casting material5.

Not shown in the figure is a thin polyimide layer coating the semiconductor body1, with the exception of the region10and the regions8,9, where bonding will later take place.

The substrate2can be composed of, for example, a ceramic, an organic material, or even a metal. Conductivity is of no importance here, since the electrically active zone of the semiconductor body is located solely on its top side.

FIG. 2shows the semiconductor module fromFIG. 1in a view from below, where the separated contacts3,4are exposed and visible.

FIG. 3shows an outside view of the semiconductor module from the front.

Shown inFIG. 4is a three-dimensional view of the semiconductor module, in which the casting material is shown transparent such that the bond wires6,7and the parts of the leadframe are visible. The bond wires lead to the semiconductor body1, where they end at bond areas8,9. In the casting material is shown a recess10, which leaves the optically active elements of the semiconductor body1free in order to emit and receive optical radiation.

A casting mold11,12for the semiconductor module is shown schematically inFIG. 6. Typically, a large number of such casting molds are combined in order to produce as many semiconductor modules as possible in a single casting step, although the figure shows only one individual mold, which includes a bottom part11and a top part12. On its inner side13, the top part12bears two masking bodies14,15, which are designed as cylindrical rods with end faces16,17.

After placement on the substrate of the leadframe and semiconductor body, which have already been bonded together, the casting mold is first closed. As this is done, the end faces16,17of the masking bodies14,15strike the semiconductor body and cover the optically active areas on it. After the casting mold has been closed, the epoxy resin, which has a low viscosity at high temperatures, is then injected through an inlet opening as the casting material until the interior volume of the casting mold11,12is filled. After solidification of the casting resin, the top of the casting mold12is removed and the masking bodies14,15are withdrawn from the hardened epoxy resin. There remain openings in the casting resin that extend to the semiconductor body, where they expose the optically active areas.

FIG. 7shows a longitudinal section of a cylindrical masking body18, which has an end face19with a basin-like indentation whose edge20is flat around its circumference and forms a sharp edge that constitutes a cover that seals casting material away from the semiconductor body during casting.

FIG. 8shows another masking body21, which is likewise cylindrical and which has on its end face22a circumferential ridge23, which forms a sealing surface with respect to the semiconductor body.

FIG. 9shows a conically tapering truncated cone24, which has a flat end face25for placement on the semiconductor body, whileFIG. 10shows a truncated pyramid26that is rectangular in cross-section with a flat end face27.