The results discussed in this paper are related to the signal measured by a specially developed prototype system with 0 bias voltage applied to the sensor. That is, the sensor works in the photovoltaic mode [14
]. This mode is suitable for low light level and low frequency applications and it allows simplicity in system design and development. Response speed and linearity could improve via the application of a reverse bias, however, dark and noise currents as well as response variations due to temperature are likely to increase.
When the micro object is moving in the direction perpendicular to the sensor lines, the 1D/3D PSD detector line numbering adopted is that one depicted in , where the micro object movement started at the highest number. However, when the micro object was moving in the direction parallel to the sensor lines, movement was taking place along a few sensor lines usually in the middle of the sensor (e.g., line number 17, 18).
All three different experimental results presented in show the response of the sensor as the micro cantilever enters in parallel to its line sensor. According to the numbering scheme in , the micro cantilever enters the sensor at line detector numbers 17, 18 for the case of or at line detector number 7 for the case of . Movement detection happens as it moves in parallel to them. In , a focusing lens is used and channels 17 and 18 (C17, C18) detect, in , no lens is used and channel 7 (C7) detects and in , the micro cantilever is immersed in liquid and channels 17, 18 and 19 (C17, C18, C19) detect. The latter signal responses are a measurement of current (nA) versus position movement (μm). The instability presented on some signals is believed to be related to the flickering caused by the microscope lamp.
Figure 3. (a) Sketch of the micro cantilever entering parallel to the sensor lines. It enters approximately at the centre of the sensor at detector 17 or 18 (b) Sketch of results according to when no focusing lens is used (c) Sketch of results for when the micro (more ...)
Liquid made it difficult to maintain the focus. In , the relevant three line sensors or channels (C17, C18, C19), obtain a signal response more than double that given by the noise level. The reason why there is instability coming from the response of at least one detector is because the slightest movement of the liquid was causing at some instances the loss of image focus on the sensor.
The set of results shown in derive from the signal related to the movement detection coming from just the micro cantilever, which is being reflected on the viewing area of the microscope and do not derive from the signal linked to the movement detection coming from its holding structure. Now the micro cantilever was moved from side to side perpendicular to the detector lines. All channels except channel 7 (C7) show the response when the focusing lens is present and among these, channel 19 shows the response when the micro cantilever was moving inside a liquid. Channel 7 (C7) represents the behavior when no focusing lens is present. The viewing area of the microscope is projected around channel 7 when no focusing lens is present and around channels 17 or 18 when a focusing lens is present. An interesting observation from the results of is that all channels are detecting for approximately 30 μm and this happens to be the same as the width of the micro cantilever.
Sketch of the micro cantilever moving sideways, after having entered parallel to the sensor lines as in .
The set of results in , show the response of the sensor as the micro cantilever enters perpendicular to it and therefore perpendicular to its channels too. These results also show the difference in terms of current (nA), between the signal linked to the movement detection coming from the micro cantilever and the signal related to the movement detection coming from its holding structure. As previously stated, the micro object was moving in the direction perpendicular to the sensor lines and the line numbering adopted is that one indicated in , where movement started at the highest number. For these particular results, the movement of the micro cantilever and its holding structure were both scanned at steps of 5 μm.
Figure 5. (a) Sketch of the micro cantilever and its holding structure entering perpendicular to the sensor lines (b) Sketch of the response of each detector at the 3 nA threshold level, when just the holding structure of the cantilever is present (c) Sketch of (more ...)
In , the signal related to the movement detection coming from the micro cantilever extends from 100 μm to about 300 μm. This indicates that only the first 200 μm from the length of the micro cantilever are detected, as opposed to 400 μm, which is the real length of the micro cantilever. The remaining 200 μm of micro cantilever are believed to be included (and masked) in the signal detecting the holding structure which starts after 300 μm. It is at that point that the holding structure starts to enter the field of view and the signal response boosts rapidly stabilizing at that level. The difference between the light reflected by the micro cantilever and the holding structure is therefore clearly seen. As shown in , the holding structure has a much bigger cross sectional area than the micro cantilever and therefore reflects much more light. The instability present on some signals is believed to be related to the flickering caused by the microscope lamp.
shows the response (as a function of position/movement) obtained by all line sensors just for the signal related to the holding structure (not the signal related to the micro cantilever) at the 3 nA line, which cuts across in order to help determining linearity of the sensor in this particular situation, which is next sketched in .
presents the measured data by all sensors lines when they are detecting exactly at the 3 nA threshold. As the holding structure moves along (position in μm), the line sensors are detecting sequentially and in the right order as it should be. The linearity of the sensor is thus represented by the linear fit and the correlation coefficient is 0.99 (99%).
The set of measurements shown in derive from the signal related to the movement detection coming from just the micro cantilever, and do not derive from the signal linked to the movement detection coming from its holding structure. Once the micro cantilever is in the viewing area of the microscope, its movement is recorded as it is moved from one side to the other side of the sensor lines, in the indicated direction of movement. This time, the micro cantilever is moved along the lateral dimension of the detector lines and a focusing lens was used. The position signal response coming from several line sensors (C27, C28 and C29), clearly shows a difference between moving from left to right of the sensor. Each detector line measures 7mm and therefore the range extends from −3.5 mm to +3.5 mm. The reason why some channels perform better than others is because of the internal material characteristics of each detector resultant from the fabrication.
(a) Sketch of the micro cantilever moving sideways, after having entered perpendicular to the sensor lines as in . (b) Sketch of the best channel response from and its related calculated linearity.
shows only channel 28 (C28) from because it is the sensor line that responds best to position detection. The sketched black line represents the linearity of this particular sensor, in this particular situation, in which the linear correlation coefficient is 0.97 (97%).