IES20010597A2 - Laser machining - Google Patents

Abstract

A UV laser beam is used for machining. It has an average power greater than 4W, and is used for machining both sides of the substrate. A high resolution vision system controls the beam. Ablation is assisted by a gas blowing device.

Description

According to the invention, there is provided a method of machining a 10 semiconductor comprising the steps of directing a UV laser beam at the wafer for laser ablation.

In one embodiment, the laser beam has an average power greater than 4 W.

In another embodiment, the method comprises the further steps of flipping the wafer and machining the other side.

Preferably, a vacuum is applied for flipping the wafer.

In one embodiment, the beam is controlled in response to a high resolution vision system.

In another embodiment, laser ablation is assisted by a gas blowing device.

In one embodiment, the invention comprises the steps of removing debris during machining.

In another embodiment, the invention comprises the further steps of chemical etching to machine very fine structures with a high aspect ratio.

INT CL ,¾..._____________OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 JNL Nn. OF IE010597 -2In one embodiment, the laser beam is a Q-switched laser beam.

In another embodiment, the laser beam power is adjusted to adjust depth of machining.

Detailed Description of the Invention The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which :Fig. 1 is a perspective view from above of a machining apparatus of the invention; Fig. 2 is a plan view of a wafer; and Fig. 3 is a diagram illustrating machining directions.

Fig. 1 illustrates a system for micro-machining semiconductor wafers by means of UV laser ablation. The system allows for complicated shapes, blind and cut-out, to be machined on both sides of semiconductor wafers at high speed. The method employs a UV laser source and an accurate scanning and positioning system that involves a high resolution x-y stage and a two-axis galvanometer. The X-Y stage incorporates a flipping mechanism that allows the machining of the wafer of both sides. A high-resolution two-camera imaging system is used for the accurate positioning of the wafer and the inspection of the micro-machined features. The machine also includes a sophisticated gas blowing and debris extraction system.

A Q switched UV laser source of average power larger than 4 Watts is used for the laser ablation. The beam is delivered to the wafer using dielectric mirrors designed IE010597 -3for the appropriate wavelength, laser power and angle of incidence. A beam expander can be used to increase the diameter of the beam. The beam is then directed into a two-axis galvanometer. Attached to the galvanometer is a telecentric lens that delivers uniformly a focused beam to an area of up to 50mm by 50 mm. In order to machine the whole area of the wafer, an x-y stage is used. This x-y stage incorporates a vacuum- operated flipping device, which acts also as a wafer holder. This allows the wafer to be machined on both sides. The wafer holder is designed for wafer of specific diameter. It can however be easily changed to accommodate wafers with smaller or larger diameters. Two high-resolution cameras, one at each side of the wafer are used for the alignment of the wafer and the inspection of the machined structures. A gas blowing and a debris-removing system assist the laser ablation.

An example of a semiconductor wafer is shown in Fig. 2. In this example, and without limiting the generality of the application, the wafer pattern is divided in rectangles. An example of a cutting pattern, a rectangular slot, is shown in Fig. 3. This is done again without loss of generality. It can be seen that the slot is machined using a combination of rastering and rectangle cutting. The number and the length of the rastering beams depends on the width and the length of the desired microstructure. The distance between the rastering beams depends on the dimensions of the focused laser beam. The speed at which the beam is scanned depends on the dimensions of the focused beam and the repetition rate of the laser, as it is these factors that govern the spatial overlap between each laser spatial pulse profile. The rectangles smoothen the comers and edges of the slots. The number of the rectangles and the distance between them depends on the shape and size of the via or slot. The number of repetitions of the pattern depends on the depth of the blind slot or, in the case that cut-out slot is desired, on the thickness of the wafer. The net number of repetitions of rastered and rectangular passes, and their repetition combination depends on the width and the depth of the slots.

IE010597 -4It has been found experimentally that the ablation rate reduces deeper in the wafer . This is mostly due to the deposition of some of the debris in the machined part. This is overcome by an efficient debris-removal system, etching of ’’side channels” for debris escape and by drilling the wafer on both sides.

The basic criterion on which the machining speed is improved is that by etching the slot width to allow escape and forced removal of debris, the overall “cut rate” is increased. The cut rate is defined by the scan rate divided by the number of passes required to achieve the cut.

This method is a considerable improvement over mechanical sawing and chemical etching of wafers in micromachining application and for the purpose of singulation of semiconductor dies. Compared to mechanical sawing, this method has a very high resolution - the shape and size of the micro-machined structures is only limited by the diameter of the focused beam, and that can be changed using the appropriate focusing optics.

The depth of the microstructures can be adjusted by changing the power of the laser.

The depth of focus of third and fourth harmonic YAG and YLF type lasers is large because the spatial output is Gaussian . Also, the plane of focus can be controlled. In addition, the only debris from the micro-machined wafer is in powder form, incapable of causing mechanical damage to the wafer. And finally, as laser ablation does not apply any mechanical force on the wafer, it does not cause any chipping or cracking of the semiconductor.

An advantage of laser ablation over chemical etching is speed, particularly where cut out or high aspect ratio shapes are required. When high aspect ratio shapes with very fine features are desired, laser ablation can be used in combination with chemical etching.

IE010597 -5The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims (5)

1. A method of machining a semiconductor comprising the steps of directing a UV laser beam at the wafer for laser ablation.

2. A method as claimed in claim 1, wherein the laser beam has an average power greater than 4 W, wherein the method comprises the further steps of flipping the wafer and machining the other side, and wherein a vacuum is applied for flipping the wafer.

3. A method as claimed in any preceding claim, wherein the beam is controlled in response to a high resolution vision system, wherein laser ablation is assisted by a gas blowing device, and wherein the method comprises the steps of removing debris during machining.

4. A method as claimed in any preceding claim, comprising the further steps of chemical etching to machine very fine structures with a high aspect ratio, wherein the laser beam is a Q-switched laser beam, and wherein the laser beam power is adjusted to adjust depth of machining.

5. A method substantially as described with reference to the drawings.