Tool Simplifies Mask Repair Inspection at 193 nm

Mask repair is, to put it mildly, a delicate and critical operation. But the process does not end there. The mask must be measured after it has been repaired to verify whether the repair has been properly carried out.

Typically, mask repair is done using an ion beam system or an atomic force microscope, which remove material where the defect exists. If material is removed on a binary mask, for example, the result can be zero attenuation. In other cases, material must be added to ensure that the light is blocked. The proper quantity of material must be added, in this instance, to match the surrounding area. In the case of a phase-shift mask, the material is typically around 10% transmission. This means it is necessary to add similar transmission material to preserve phase properties.

Currently, there are phase measurement tools that enable the technician to conclude that broad areas of the mask have been properly repaired. In the case of smaller (<1 µm) features, however, a tool capable of doing the actual phase-shift measurement would be ideal. In lieu of this, the next best thing is to measure the transmission that corresponds to a phase shift.

A mask has a characteristic phase shift, and if there is a defect that requires the removal of material to repair, the expected result is a clear area. However, a residual stain often is left by the repair tool and some attenuation (and associated phase shift) occurs. Adding material to repair the mask and then verifying that the repaired spot has the same transmission properties is a more complicated task, particularly because the process takes place at such minute scales.

	On the top is a MOSI phase-shift mask at 100× magnification. A 200 nm probe spot is positioned above 200 and 300 nm isolated features. The bottom image shows a binary mask at 200× magnification; a 150 nm probe spot (circled in red) is positioned on a 200 nm bridge. (Source: Actinix)

The laser's light is directed through sealed beam tubes to the back of the platform, where it is split into two components. One component is reflected through a periscope that directs it to the upper microscope area. This is the probe beam used to measure the transmission. The second beam component goes through a beam homogenizer assembly and through the back and bottom of a modified Zeiss Axiotron microscope. This beam is used to image the photomask onto the 193 nm camera.

The camera is cooled in order to further enhance sensitivity, and runs in a pulsed mode, triggered by a pulse generator that synchronizes the camera to the laser. The camera requires a computer to display the images it receives. A PCI card frame takes the images and a dedicated software package controls the camera and the image processing.

The probe beam back-illuminates a user-selectable pinhole in the upper microscope area that attenuates the laser and provides a circular spot that is imaged down to the mask plane by the tube lens and objective combination, providing 100× magnification. The beam then traverses the mask at the desired measurement site, passes through the condenser lens and is focused on the detector. The pinholes are interchangeable from 18 to 500 µm.

At the heart of the platform is its signal processing system. The signal detected by the photomultiplier under the mask on the microscope is sent to an amplifier and a sample/hold in the signal processor, providing a voltage corresponding to the light level transmitted through the photomask. A second sample/hold receives a signal from a reference detector monitoring the light source's power level. These integrated signals are digitized and sent to the computer, which normalizes the transmitted light sample, shot by shot, to the laser energy. Signal statistics are then output in real time, and the user can decide when an adequate number of samples are collected to achieve a given measurement precision, usually 1% in less than 30 sec.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt