A scanning laser microscope is a kind of photoelectric microscope which focuses a light beam emanating from a laser onto a point on an object of interest, receives the light scattered from the point by a photoelectric converter, and obtains a signal indicating the condition of the point. Some scanning laser microscopes can collect information regarding the surface of an object by varying the distance between the object and the focused laser beam. Scanning laser microscopes are expected to find wider application.
The features of a scanning laser microscope are now described briefly. The optical path in the prior art microscope is schematically shown in FIG. 1. In this microscope, the illuminating light emitted from a light source 30 such as a tungsten lamp passes through a condenser lens 32 and illuminates the whole field of view of a sample 34. An image of the illuminated sample is focused onto an image plane 38 by an objective lens 36. It is assumed that the sample does not transmit light and that the sample is provided with an infinitely small hole on the optical axis. Light going out of this hole is focused onto the image plane by the objective lens. If no diffraction occurs, an infinitely small light spot should be produced. In practice, however, the light spot has some size because the light behaves as waves. This is graphically shown in FIG. 2, which represents the distribution of intensities of Fraunhofer diffraction light due to a circular opening. As can be seen from FIG. 2, the center (x=0) of the diffraction image relative to the geometrical image is brightest. Completely no light exists at the position x=3.832 (the unit is normalized with the radius of the opening, the wavelength, or other factor). From this position on, dark portions and bright portions alternate with each other but the intensities are quite small. Therefore, these portions present almost no problem. Let us now consider a disk having a radius x=3.832. This is defined as the size of the diffracted image created by the stigmatic optical system. This is called an Airy disk and determines the limit of resolution. Light corresponding to 16.2% of the whole light intensity exists outside the Airy disk. This is called halo. Normally, the halo poses no problems. However, when a dark image located beside a bright spot is observed, the halo cannot be neglected.
A method contemplated to remove the halo is illustrated in FIG. 3, where a pinhole 40 is located on the side of the diffracted image. This method permits one to improve the resolution without suffering from a deterioration in the contrast.
However, the adoption of this method means giving up a two-dimensional image. Therefore, "scan" is needed to obtain a two-dimensional image. Hence, a scanning microscope is necessary. Three types of scanning microscopes exist, and they are termed the type 1a, the type 1b, and the type 2, respectively, after Doctor Sheppard of Oxford University who is a forerunner of this research. The optical paths of these three types are schematically shown in FIGS. 4, 5, and 6, respectively. Of these three types, the types 1a and 1b scan 44 the light source or the detector 42. These two types have a single condenser lens and so the diffraction function on the sample surface is the same as the diffraction function of FIG. 2.
On the other hand, a scanning microscope of the type 2 scans both light source and detector This type has two condenser lenses. In this microscope, light is detected, based on a function (or convolution) that is a square of the diffraction function shown in FIG. 2. The halo is removed by a pinhole. Therefore, this kind of microscope is called a confocal scanning microscope. The aforementioned convolution function takes the form indicted by the solid line in FIG. 7 whereas a normal optical microscope takes the form indicated by the dotted line. As can be seen from the graph of FIG. 7, the confocal scanning microscope has a smaller Airy disk and a higher resolution.
The most important factor of the confocal scanning microscope is the pinhole installed to remove the halo. This pinhole is helpful in improving the resolution. The pinhole plays another great role. This is now described by referring to FIG. 8. Light rays coming from surfaces located off the focus cannot pass through the pinhole. Consequently, the pinhole acts to remove unwanted scattering light, or flare, produced by the objective lens and to make the depth of focus shallower. A shallower depth of focus improves the contrast and permits observation of a three-dimensional image. For these reasons, this microscope enables one to observe an image which cannot be seen by a normal optical microscope.
In this way, the scanning laser microscope has a higher resolution, higher contrast, and shallower depth of focus than the prior art optical microscope. For example, when an object such as a semiconductor device is observed, its fine structures have low contrast. Therefore, if the output signal from the detector is processed directly as an analog signal and visualized as in the prior art techniques, then it is impossible to make full use of the high resolution. Also, if the laser beam is scanned continuously, or in an analog fashion, then it is difficult to locate the illuminated position. Another problem is that it is difficult to correlate the illuminated position with information obtained from this position.