Method for producing a lens mold

Abstract

A method for producing a lens mold suitable for manufacturing a field of micro-lenses is disclosed. The method includes the step of molding the lens mold from a sheaf of closely-packed balls held by a hexagonal mounting.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 10/343,819, filed on Jul. 15, 2003, which is a national stage of International Application No. PCT/DE01/02874, filed on Jul. 30, 2001. Priority is claimed from German Patent Application No. 100 38 213, filed on Aug. 4, 2000. The entire content of application Ser. No. 10/343,819 is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for producing a lens mold suitable for manufacturing a field of micro-lenses.

2. Description of the Related Art

Radiation sources, such as light-emitting diodes, generally have a semiconductor chip cast integrally with a transparent lens body. It is also known for the radiation output of such radiation sources to be increased by the provision of many semiconductor chips. Such radiation sources typically have a condenser optical element, which comprises a lens. However, the radiation density of such radiation sources is often unsatisfactory if it is critically important to create a highly luminous radiation source with little three-dimensional spread.

SUMMARY OF THE INVENTION

Disclosed below is a field of lenses, forming a hexagonal lattice, which is disposed upstream, in the projection direction, of the semiconductor chips. Because of the hexagonal arrangement of the lenses, a high density of the associated semiconductor chips per unit of surface area can be attained. Since the lenses are typically formed by sphere segments, it is possible to use sphere segments with large radii for the lenses. As a result, the radiation-emitting active layer of the semiconductor chips can to a great extent be located inside the Weierstrass sphere (see Sze, Physics & Semiconductor Devices, 2nd ed., Page 694 which is incorporated herein by reference) associated with the respective sphere. This leads to a high radiation yield for the individual semiconductor chips.

It is an object of the invention to create a rational method for producing a lens mold suitable for manufacturing a field of lenses.

This object is attained according to one aspect of the invention in that the lens mold is molded from a sheaf of closely-packed balls held by a hexagonal mounting.

Because of the hexagonal mounting, the sheaf of balls in a sense puts itself into a hexagonal lattice structure, when the balls rest tightly against one another. It therefore suffices to assure that the mounting is completely filled with the balls to be molded.

Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, a plan view on a printed circuit board, equipped with semiconductor chips and already bonded, for the radiation source of the invention;

FIG. 2, an enlarged cross-sectional view of the printed circuit board taken along line I-I of FIG. 1 above which micro-lenses have been placed;

FIG. 3, a plan view on a field of lenses;

FIG. 4, a cross section through an apparatus that can be used to produce a casting mold for fabricating a field of micro-lenses;

FIG. 5, a plan view of the apparatus of FIG. 4;

FIG. 6, a cross section through a further apparatus that can be used to produce the casting mold;

FIG. 7, a cross section through the casting apparatus used to produce the field of micro-lenses; and

FIG. 8, a graph that shows the radiation output as a function of the spacing between the upper edge of the semiconductor chip and the associated hemispherical micro-lens.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a plan view on a printed circuit board 1, which is made from Al₂O₃ or Si. Embodied on the printed circuit board 1 are terminal contacts 2, from which conductor tracks 3 lead to contact points 4, to which bond wires 5 are attached that lead to, among other things, chip contact faces 6. Semiconductor chips 7 are mounted on the chip contact faces 6 and are bonded in series line by line.

FIG. 2 shows an enlarged detail of a cross section through the printed circuit board 1 provided with micro-lenses 8. It can be seen that the semiconductor chips 7 are each mounted with an underside 9 on the chip contact faces 6. The bond wires 5 are each mounted on a top side 10 of the semiconductor chips 7 and lead to an adjacent chip contact face or to one of the contact points 4.

The micro-lenses 8 are half spheres with a radius R. The geometric center point of the micro-lenses 8 is located at a spacing Δx from the top side of the semiconductor chips 7. The spacing Δx is selected such that at least half of the radiation-emitting active layer of each of the semiconductor chips 7 is located inside the Weierstrass sphere of radius R/n, where n is the index of refraction of the material used for the micro-lens 8. The centers of the Weierstrass spheres coincide with the centers of the micro-lenses 8. Radiation generated inside the Weierstrass sphere can exit the micro-lens 8. It is therefore advantageous if as large as possible a portion of the active layers of the semiconductor chips 7 come to be located inside the Weierstrass sphere. It is therefore important to select as large as possible a radius of the micro-lenses 8. Conversely, in that case the spacing between semiconductor chips 7 must also be selected as correspondingly great. However, a great spacing between the semiconductor chips 7 means a low radiation density. The attempt is therefore made to keep the spacing between the micro-lenses 8 as slight as possible. The arrangement of micro-lenses 8 shown in FIG. 3 in a hexagonal lattice structure is the tightest possible arrangement of micro-lenses 8 and makes it possible to attain a high radiation output with at the same time a high radiation density.

The micro-lenses 8 are expediently cast from synthetic resin. The production method is performed as follows:

First, a first mold plate 11 is produced, which as shown in FIG. 4 has a central arbor 12 with a hexagonal cross section that can be seen in FIG. 5. The arbor 12 is disposed on a base 13. Alignment pins 14 are located in the vicinity of the base 13. A holder frame 15 is also mounted on the first mold plate 11 and has indentations 16 on its inside. The interior defined by the holder frame 15 is filled with silicone. A silicone frame 17 is thus formed, which in its center has an opening of hexagonal cross section. The silicone frame 17 engages the indentations 16 and can therefore be mounted easily, together with the holder frame 15, on a second mold plate 19, shown in FIG. 6. The alignment pins 14, also present here, serve to align the holder frame 15 and the silicone frame 17 on the second mold plate 19. As a result, the silicone frame 17 comes to rest on the second mold plate 19 in such a way that the opening 18 in the silicone frame 17 is aligned with a mounting 20 in the second mold plate 19. The mounting 20, with its sidebars 21, occupies the space in the silicone frame 17 that has been created by the base 13 of the first mold plate 11. Furthermore, it likewise has a hexagonal cross section. Tiny balls 22 are placed close together in the mounting 20. The tiny balls 22 have a radius that is essentially equivalent to the radius of the micro-lenses 8 to be produced. Since the mounting 20 has a hexagonal cross section, and since the tiny balls 22 are located close together, these balls 22 are arranged in accordance with a hexagonal lattice structure.

Next, the opening 18 is filled with silicone. The result is the micro-lens mold 24, shown in FIG. 7 in a casting apparatus 23. The casting apparatus 23 has a suction stub 25, on which a base plate 26 that holds the printed circuit board 1 is mounted. For that purpose, a central suction opening 27 is provided that leads to the printed circuit board 1. The holder frame 15 with the micro-lens mold 24 is located above the base plate 26. The two are partly covered by a pressing plate 28, which is connected to the base plate 26 via a screw connection, not shown, and assures the secure seat of the micro-lens mold 24 on the base plate 26.

The alignment pins 14 have left leadthroughs 29 behind in the micro-lens mold 24, and these leadthroughs serve to introduce the synthetic resin into the space in the micro-lens mold 24 above the printed circuit board 1.

It should be noted that the printed circuit board 1 under the micro-lens mold 24 is understood to have already been provided with the semiconductor chips 7 and been bonded in final form.

Finally, casting resin is introduced through the leadthroughs. This fills the space between the micro-lens mold 24 and the circuit board 1, thus molding the micro-lenses 8.

In FIG. 8, finally, a graph is shown in which the radiation output φ is shown in a space angle, with a one-half opening angle of 60°, that is, an opening angle of 120°, as a function of the spacing Δx.

FIG. 8 shows the results of calculations. The calculations were performed with a semiconductor chip 7 with an outline of 200 μm×200 μm and a height of 250 μm. It was assumed that the semiconductor chip emitted 70% of its radiation output from the top side 10. The further 30% was assumed to emerge from the side faces of the semiconductor chip 7. As the spectrum, the spectrum of a black body at 2000 K was assumed. The calculations were performed for two types of casting resin in which the semiconductor chip 7 is embedded. These were first a casting resin with an index of refraction of n=1.55 and second a casting resin with an index of refraction of n=1.87. The calculated curves 30, 31 and 32 each represent the results from micro-lenses 8 with the radii of 250 μm, 300 μm, and 350 μm, for an index of refraction of n=1.55. The curves 33, 34 and 35 each represent the results from micro-lenses 8 with the radii of 250 μm, 300 μm, and 350 μm, for an index of refraction of n=1.78 of the casting resin. A line 36, finally, shows the results to be expected without a micro-lens 8.

The diameters of the micro-lenses 8 were 500 μm, 600 μm, and 700 μm. FIG. 8 clearly shows that the radiation output in the space angle enclosed assumes its greatest values at a spacing Δx of 0.1 mm. The radiation output is then approximately twice as high as without micro-lenses 8. At this spacing, a large part of the active layer of the semiconductor chip 7 is also located inside the Weierstrass sphere of the micro-lenses 8.

The advantages of the hexagonal arrangement of micro-lenses 8 will become apparent from Table 1 that follows:

	TABLE 1
	Arrangement
	Square	Hexagonal	Hexagonal
Lens diameter	600	μm	600	μm	700	μm
Area per unit	0.36	mm2	0.312	mm2	0.42	mm2
cell
Total radiation	0.4777	W	0.45919	W	0.48668	W
output at 70°
half angle
Radiation	1.3269	W/mm2	1.4718	W/mm2	1.1588	W/mm2
conduction per
unit of surface
area

From Table 1 it becomes clear that increasing the radius of the micro-lenses 8 does not necessarily lead to an increase in the radiation output per unit of surface area. This is because, although with the greater radius of the micro-lenses 8 a larger portion of the active layer of the semiconductor chips 7 comes to be located inside the Weierstrass sphere, in return the spacing of the semiconductor chips 7 increases, so that the luminance decreases.

For practical reasons, it may nevertheless be advantageous if a diameter of 700 μm is selected for the micro-lenses 8, because otherwise problems can occur in bonding the semiconductor chips 7 to the chip contact faces 6 and in bonding the bond wires 5. Moreover, conventional casting resins shrink upon curing, which is why the cured micro-lenses are smaller by about 6% anyway than the corresponding molds of the micro-lens mold 24.

Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

1. A method for producing a lens mold suitable for manufacturing a field of micro-lenses, comprising the step of molding the lens mold from a sheaf of closely-packed balls held by a hexagonal mounting.

2. The method of claim 1, wherein the lens mold is cast from silicone.

3. The method of claim 1, wherein the mounting is surrounded by a silicone frame comprising an opening, said opening being aligned with the mounting.

4. The method of claim 3, wherein the silicone frame is produced by filling silicone into the interior defined by a holder frame mounted on a first mold plate and surrounding an arbor of the first mold plate.

5. The method of claim 1, wherein the balls have a radius that is essentially equivalent to the radius of the micro-lenses to be produced.