horizontal deflection mirror H(7,8) (see Appendix B for a complete description of the lens system represented by L1). Refocusing the vertical deflection onto mirror H allows H to be small so that faster scanning rates can be used. Mirror H is located at the exit-pupil point of the microscope camera tube. The beam diverges from the horizontal mirror H to form the scanning raster. The raster is focused onto the specimen by the microscope optics. The scan raster can cover all of the field of view that can be seen with the eye when the microscope is used in its usual manner. Additional details of the mechanical construction are given in Appendix C.

The laser scanner incorporates a reflected light circuit to enable one to obtain a map of the topography of the specimen being scanned. This circuit makes use of a half-silvered mirror that is built into the microscope for vertical illumination purposes when the microscope is used in a normal manner. Some of the laser light that is reflected from the specimen as it is scanned is collected by the microscope's objective and subsequently reflected from the half-silvered mirror in the microscope. This light passes out of the microscope through the vertical illuminator. In order that the microscope could retain its vertical illumination capability, a second half-silvered mirror (not shown in figure 1) was placed in the vertical illuminator tube (see figure C 17 in Appendix C) to direct the reflected light out of the tube and through lens L2 and onto a germanium photodiode. The second half-silvered mirror makes it possible to use either the vertical illumination or the reflected light circuit without changing any parts. The lens L2 focuses the raster of the reflected light to a point on the photodiode. ing to a point is necessary so that irregularities in the photodiode do not distort the electrical signal obtained from that device when the scanning mirrors V and H are driven to fairly high excursions. A germanium photodiode was chosen over a silicon photodiode because the germanium responds to both the 1.15 um laser and the 0.633 μm laser.

Refocus

The photocell provides an electrical signal which can be used to obtain a spatial map of the reflectance of the specimen being scanned by the laser radiation. Another electrical signal can be obtained from the specimen at the same time, and this signal is extracted by simple electrical connections to the device being scanned. This is the photoresponse signal, and it can be used to display spatially the electrical response of the specimen to the laser radiation. This photoresponse signal is generally of greater importance in studying semiconductor devices than the reflected light signal, the reflected light signal being mainly used to identify areas on the specimen that give a photoresponse signal. Both the response signal and the reflected light signal are generated at the same time and are thus synchronized.

3.2 Electrical Operation

Electrical signals generated within the specimen as a result of the laser scanning the device, as well as the electrical signal developed by the reflected light photodiode, are amplified and modulate the electron

beam of a cathode ray display. The electron beam of the display is deflected horizontally and vertically at the same rates, and in synchronism to, the deflection of the scanning mirrors. A point-to-point correspondence is thus established between the position of the laser spot on the specimen and the position of the electron beam on the display. As the light spot and the electron beam move, the electrical signals from the device and/or reflected-light photodiode, in modulating the electron beam, produce a picture of the response of the specimen on the display

screen.

Figure 2 is a block diagram showing the important elements in the electrical section of the system. The mixer is the heart of the electrical system. Its signal processing capabilities include signal amplification from up to three sources, addition and subtraction of these signals (mixing), and superimposing the signals onto the vertical deflection waveform driving the display for the vertical deflection type presentation. The mixer allows signals from the specimen and the reflected light photodiode to be mixed in any desired proportions so that a display of the specimen's photoresponse, for example, can be superimposed on a topological display generated by the photodiode. Any signal, or combination of signals from the specimen can be mixed with the deflection waveform that is used for vertical deflection of the electron beam on the display screen. This is a display technique which provides a picture which appears to have a three-dimensional quality. An advantage of this presentation mode is that it allows quantification of the specimen's photoresponse, since the deflection of the spot on the cathode ray tube display, as compared to a change in spot intensity, can more easily be measured. With the mixer, either the conventional spot intensity modulation, or the vertical deflection modulation, or both, can be used. The mixer provides separate outputs for each of the three channels, and each channel has a voltage gain of up to 50. These outputs can be used for separate processing of the signals derived from the specimen and the reflected light circuits. For example, when a color display is used instead of a monochromatic display (see Appendix D for details) different signals from the device under investigation can be displayed at the same time without losing their identity of origin. Even with the monocromatic display, two of these signals can be separated by using intensity modulation for one signal, and vertical modulation for the other. The mixer has provisions for specimen bias, so that a separate power supply for the specimen is often not needed. however the pre-amplifier (shown in figure 4) has the same provision and is usually used for this purpose. If the specimen signal is very strong the pre-amplifier is not needed and the specimen can be connected directly to the mixer. The schematic diagrams for both the mixer and the preamplifier are included in Appendix E along with a more detailed description of the internal circuitry.

The pre-amplifier has two channels which allow monitoring of two different signals that might be generated by the specimen. For example, if the specimen requires two power supplies, variations in both of these supplies can be displayed at the same time. The pre-amplifier has a 0

to 20 V power supply and a 0 to -20 V supply for specimen excitation. There is also a selection of load resistors for each supply. The preamplifier has a variable-cutoff high-pass filter which can be used when the specimen is noisy or has a tendency to drift. All of the circuits in both the mixer and pre-amplifier are dc coupled so that slow scanning rates can be used when the specimen requires a long recovery time from the scanning light spot. One of the channels in the pre-amplifier is a special FET amplifier circuit with no loop feedback. This circuit features very fast recovery from transients and severe overload conditions. This feature allows an accurate observation of weak signals when strong ones are present.

The reflected light amplifier is a very simple one which amplifies the signal developed by the reflected light photodiode. The circuit for this amplifier is given in Appendix E.

Two signal generators are used for the laser scanner. One generator, designated horizontal signal generator in figure 2, provides both a square wave and a sine wave. It is used at frequencies between 10 Hz and 1.2 kHz. This generator determines the horizontal sweep rate. The other generator, designated vertical signal generator, provides both a square wave and a triangle wave, and is used at frequencies between 0.01 Hz and 100 Hz for the frame rate. The square waves of both generators provide the blanking signals, and are fed into the mixer where a composite blanking signal is developed. This composite signal is then fed to the blanking input on the display. The sinusoidal waveform from the horizontal generator is fed directly to the horizontal deflection amplifier in the display. This waveform is also fed to the horizontal scanning mirror through an attenuator which serves as a picture magnifier, and thence to the scan power amplifier which drives the mirror galvanometer assembly. Operation of the magnifier will be discussed in section 4.3. The triangular waveform from the vertical generator is fed to the vertical deflection amplifier in the display; however, before it goes to the display, it first goes through the mixer where specimen signals can be added to the vertical deflection as discussed earlier. The composite signal, consisting of the triangular waveform, and the specimen modulation signal if any, is then fed to the vertical deflection amplifier in the display. The triangular waveform from this generator is also fed to the scanning mirror through a second channel of the attenuator and its scan power amplifier.

The scanning mirrors have a resonant frequency of approximately 1 kHz and do not respond to frequencies much higher. Furthermore, there is a substantial phase shift between the electrical signal driving the mirror and the actual mirror movement at even 100 Hz. It is desirable to use the fastest scanning rates possible for most specimens, simply for viewing ease. If operation at the resonant frequency were used, then obviously some kind of phase shift network would be needed to cause the mirror to move in step with the spot on the cathode ray display. A convenient solution to this problem has been found, and it is to operate the mirror slightly above resonance so as to cause

a 180 degree phase shift between the electrical driving signal and the movement of the mirror. Maximum spatial linearity of the presentation is achieved either with an in-phase, or a completely out-of-phase relationship. The picture is reversed by operating at 180 degrees out of phase, however, the picture is linear, and the picture can be restored to its proper perspective by simply interchanging the leads to the scanning mirror. The 180 deg. point can be found by scanning anything that has regularly spaced elements or markings, and using the reflected light circuit to tune the horizontal frequency for a linear picture. The horizontal sweep frequency is about 1.2 kHz, for the scanning mirrors used, when the scanner is operated under these conditions. The galvanometer responds only to the fundamental frequency when driven at resonance, and so the horizontal generator need only furnish a simple sinusoidal signal. A 3-or 4-Hz frame rate usually yields adequate definition with the 1.2 kHz sweep rate. This combination gives a 300 to 400 line scan. A triangular waveform can be used for the vertical scan because the vertical deflection galvanometer can follow this waveform at the rather slow frame frequencies. Much slower scan rates must be used if the specimen cannot recover quickly from the photo-generated signal. For these situations both the horizontal and vertical frequencies are dropped by one or two orders of magnitude. The vernier frequency adjustment on the horizontal generator is usually not disturbed because of the rather critical adjustment needed for the 180 deg. phase shift at 1.2 kHz. Unfortunately, when the scanning mirror is switched out of its 180 deg. phase shift mode to a lower frequency operation, the display not only reverses, but the amplitude of the laser deflection changes by approximately a factor of two. Both of these effects are easily correctable. A switch was added to the display to make it possible to reverse the deflection signals fed to the CRT, thus reversing the display, and a separate attenuator in the signal path between the horizontal generator and the main attenuator shown in figure 2 allows an amplitude correction to be made.

This

The attenuator shown in figure 2 serves as a display magnifier. attenuator simply changes the amplitude of the drive signal going to the scanning mirrors, and thus causes a change in the raster size incident on the specimen without changing the raster size on the display. The display sweep signals are kept at constant amplitude. As the area on the specimen scanned by the light spot is reduced, a magnification of the area that is being scanned occurs. The attenuator controls both the vertical and the horizontal mirror excursions at the same time, and is actuated by a rotary switch arrangement with a 2-5-10 sequence.

The scan power amplifiers shown in figure 2 are commercially available units and were purchased with the scanning mirrors. These amplifiers are essentially audio amplifiers with dc coupling, and are capable of delivering 5 watts into a 5 to 10

load.

In this section the operation of the scanner will be described with particular emphasis on the operation of the electronics that were designed for the scanner. The mixer and pre-amplifier are described in detail, however, descriptions pertaining to the operation of purchased equipment are omitted, as these are assumed to be well known.

For many applications, the electronics of the scanner are capable of performing all of the desired measurements on a particular device or specimen. Power supplies, both negative and positive, are built into the scanner's electronics, as well as a choice of load resistors. When higher voltage or current is needed to energize a particular specimen, external power supplies can be easily interfaced with the scanner electronics.

4.1 Operation of the Electronics

As pointed out previously, the mixer is the heart of the electronic portion of the scanner. The mixer can be used with or without the preamplifier; it will be described as if it is to be used by itself. The front panel of the mixer is shown in figure 3. The mixer can be divided into four separate sections as shown by the dashed lines in the figure. The first section starting from the left, designated channel 1, contains the controls necessary to energize a specimen, extract the signals generated by the device when the laser spot is swept over it, and amplify these signals so that they can be displayed. This channel is for specimens that require positive voltages for excitation when the specimen is energized with the built-in supply. The supply that is built in does not have to be used and this channel can simply be used as an amplifier. The next section designated channel 2, is identical to channel 1 except that it contains a negative voltage supply for specimens that require a negative supply. Channel 3 is to the right of channel 2 and contains no power supplies. It is intended for use as just an amplifier. Signals developed by the photodiode and amplified by the reflected light amplifier are usually fed into this channel, although they could equally well go into either of the other two channels. The fourth section, which is on the right-hand side of the chassis, is the mixing section. This can be subdivided into two sections. The four controls on the top mix signals from the three channels with the vertical waveform used for deflection on the cathode ray display. The use of these controls allows the user to obtain quantitative information from the scanner, as the specimen's response can be made to appear as a vertical displacement which can be measured, whereas it is much more difficult to assign numbers to a change in spot intensity on the face of the cathode ray display. The three controls on the bottom mix the signals from the three channels so that they can be displayed in the desired proportion by intensity modulation of the beam on the cathode ray display.

It is useful to specifically identify the controls and describe their function. In figure 3, the controls are labeled for the first