Light quantity controlling apparatus

Abstract: A position detecting apparatus (Fig. 1) usable for aligning a mask (15) and a semiconductor wafer (16), wherein a laser beam (14) produced by a semiconductor laser (1) is projected through a predetermined optical system to alignment marks (15a,16a) formed on the mask (15) and the wafer (16), and the light (17) reflected by the marks is detected by an accumulation type sensor (4) to produce an electric signal, from which the relative positional relation between the mask (15) and the wafer (16) is detected on the basis of the electric signal. To obtain proper mark signals, the quantity of light incident on the accumulation sensor (4) is controlled. In this apparatus, the beam emitting strength of the semiconductor laser (1) is made constant, and the control of the amount of light incident on the accumulation sensor (4) is effected by controlling (2) the operation period of the semiconductor laser (1). In addition, the actuation timing (7) of the semiconductor laser (1) is advanced from the accumulation start of the accumulation type sensor (4) by the time (Ts) required for the semiconductor laser (1) to be thermally stabilized after its actuation. The mark detection signal produced by the accumulation sensor (4) is precise.

Description: The present invention relates to a light quantity controlling apparatus as defined in the preamble of claim 1, particularly a light quantity controlling apparatus for controlling the quantity of light incident on a photoelectric transducer element through an optical system from a light emitting element in a high precision optical measurement apparatus such as a position detector wherein the relative positional relation between a mask and a wafer is photoelectrically detected using alignment marks formed on the mask and the wafer. This invention also relates to a method of detecting radiation emitted from a semiconductor laser as defined in the preamble of claim 10. In a conventional system, the quantity or strength of the light in a high precision optical measurement apparatus has been carried out by controlling emitting output of a light emitting element such as a semiconductor laser source, more particularly by continuously changing the strength of the emitted light to control the light amount incident on the photoelectric transducer element. Referring first to Figure 9, there is shown a block diagram illustrating an example of a conventional optical apparatus. The apparatus in this Figure comprises a semiconductor laser 101 functioning as the light emitting element, an LD driver 102 for driving the semiconductor laser 101, a light output controller 103 for controlling the emitted output of the semiconductor laser 101, an optical system 104 for precise optical measurement, an accumulation type line sensor 105 such as CCD element for receiving the light having passed through the optical system 104, and a signal processor for receiving the electric signal to which the line sensor 105 converts the information of light received thereby. A reference character S designates a strength distribution of the light rays incident on the line sensor 105. The semiconductor laser 101 is driven by the LD driver 102 through a light output controller 103 so as to provide a predetermined output. The beam emitted from the semiconductor laser 101 carries light information (position information) by passing through the optical system 104. The information is photoelectrically converted by the line sensor 105. The output of the line sensor 105 is processed by the signal processor 106, by which various electric information (position information) is provided for example. In such an optical measurement apparatus, when the quantity of light incident on the line sensor 105 is small, the level of the electric signal provided by the line sensor 105 is low, so that the so-called S/N ratio is low, with the result that the accuracy of the result of the signal processing by the signal processor 106 is degraded. On the other hand, when the quantity of light incident on the line sensor 105 is large, the electric signal provided by the line sensor 105 is saturated, with the same result, that is, the accuracy of the result of the processing is degraded. In the conventional measurement system, in order to compensate for changes in the quantity of light which are due to change or changes in transmissivity, reflection index, refractive index, diffraction index or the like, the light emitting output of the light emitting element (light source), that is, the strength of the produced light, is continuously adjusted, so as to provide a proper quantity of light incident on the photoelectric transducer element. Since in the conventional system, the current through the semiconductor laser 101 is changed to continuously change the light emitting output, it has been found that there is a problem that the oscillation mode, the oscillation wavelength, the coherence length, the position of the light emitting point and the shape of the emitted light are changed due to heat produced. This results in changes in transmissivity, reflection index, refractive index and diffraction index, and is a significant problem in a high precision optical measurement apparatus. The present invention is intended as a solution to the problem just mentioned. In accordance with the present invention there is provided a light quantity controlling apparatus as defined in the preamble of claim 1 which is characterised in that the control means for controlling the semiconductor laser is effective to control the operation period of the semiconductor laser as also the timing of activation of the semiconductor laser so that it is activated in advance of the start of photoelectric conversion by the photoelectric transducer element. In use the on timing of the semiconductor laser is advanced from time of start of detection by the photoelectric transducer element by that time which is required for thermal stabilisation of the semiconductor laser from actuation thereof. Accordingly, the quantity of light can be controlled without change in the oscillation mode, the oscillation wavelength, the coherence length, the position of emitting point or the shape of emitted light of the semiconductor laser (light source). Therefore, the influence to the transmissivity, the reflection index, the refractive index or the diffraction index of the optical system is eliminated to accomplish high precision optical measurement. The light quantity controlling apparatus just mentioned can be applied to the detection of the positional relationship between two objects, for example a mask and a wafer. In particular a plurality of such apparatus may be combined and the respective lasers may then be activated at different times so as to avoid mutual interference. This is particularly the case where the lasers are arranged opposite one another. According to another aspect of the present invention there is provided a method of detecting radiation omitted from a semiconductor laser. This method is defined in claim 10. Other features and advantages of the present invention will become apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. In the drawings: Figure 1 is a perspective view of a position detecting apparatus embodying the present invention; Figure 2 is a front view showing the general arrangement of the position detecting apparatus of Figure 1; Figure 3 is a sectional view of the arrangement shown in Figure 2; Figure 4 is a perspective view, partly broken away, of a semiconductor laser; Figure 5 is a graph indicating temperature change of a semiconductor laser chip upon supply of power thereto; Figure 6 is a time chart illustrating operational timing of the apparatus of this embodiment; Figure 7 is a time chart illustrating the operational timing in an apparatus which is another embodiment of the present invention; Figure 8 is a time chart illustrating the operational timing of an apparatus which is a further embodiment of the present invention; and Figure 9 is a schematic diagram illustrating a conventional light quantity controlling apparatus. The following description is given by way of example only. Referring to Figure 1, there is shown a block diagram of a position detecting apparatus using a light quantity controlling apparatus embodying the present invention. In this example, the position detecting apparatus is used for detecting misalignment between the mask 15 and the semiconductor wafer 16. In manufacturing a semiconductor device, an alignment error between the alignment mark 15a of the mask and the alignment mark 16a of the wafer is photoelectrically detected, and the error is corrected, and thereafter, the wafer 16 is exposed through the mask 15 to a predetermined exposure energy such as ultraviolet rays, excimer laser beam or synchrotron orbit radiation, by which the semiconductor element pattern formed in the mask 15 is transferred onto the wafer 16. In this Figure, the arrangement comprises a semiconductor laser 1 which is a light emitting element, an LD driver 2 for driving the semiconductor laser 1, a pick-up optical system 3 for photoelectrically detecting the alignment marks 15a and 16a. The optical system 3 is constituted by a semiconductor laser 1, a collimator lens 12 for collimating the laser beam emitted from the semiconductor laser 1, a half mirror 13 for reflecting the beam from the collimator lens 12 toward the alignment marks 15a and 16a to illuminate them (illuminating beam is designated by a reference 14) and for transmitting a mark position information carrying beam 17 which has been diffracted by the alignment marks 15a and 16a, and an accumulation type line sensor 4 such as CCD for photoelectrically converting the information beam 17 having passed through the half mirror 13 into an electric signal. The alignment marks 15a and 16a each have a Fresnel zone plate pattern which is effective to change the position at which the information beam 17 is incident on the light receiving surface of the line sensor 4 in accordance with the relative positional relationship therebetween in an X-Y plane. An example of the pick-up optical system is disclosed in U.S. Serial No. 264,084 corresponding to a European Patent Application No. 89301471.2. A line sensor driver 5 serves to produce accumulating time or the like for the line sensor 4, and to drive the line sensor 4 in accordance therewith. A signal processor 6 processes an output of the line sensor 4 (picture element signals from the respective receiving elements constituting the line sensor), and determines position of gravity of the light strength distribution on the light receiving surface of the line sensor 4 by the information beam 17, for example. A timing controller 7 is effective to synchronize the LD driver 2 with the line sensor driver 5 and to control the energization and deenergization of the semiconductor laser 1. A timing selector 8 functions to set operation control timing of the timing controller 7. The signal processor 6 determines the position of the centre of gravity in the light strength distribution of the information beam incident on the light receiving surface of the line sensor 4 which is constituted by a number of photoreceiving elements arranged in the direction Y, and thereafter, it determines the relative deviation between the mask 15 and the wafer 16 in the plane X-Y on the basis of the gravity center data. By the alignment marks 15a and 16a, the positional deviation in the direction Y between the mask 15 and the wafer 16 can be obtained. The detailed procedure is given in the above mentioned prior applications. In Figure 1, the reference character S designates an example of the strength distribution of the information light 17 on the light receiving surface of the line sensor 4. Referring to Figure 2, the position detecting apparatus includes four pick-up optical systems 3 (3U, 3D, 3L and 3R) at regular intervals around the mask 15 the pick-up optical systems 3L and 3R are arranged along the X axis, and the pick-up optical systems 3U and 3D are arranged along Y-axis. Referring to Figure 3, each of the pick-up optical systems photoelectrically detect the alignment marks 15a and 16a which correspond to the pick-up optical systems. On the basis of the photoelectric detection from the alignment marks 15a and 16a, the pick-up optical system 3L and 3R determines the positional deviation of the mask 15 and the wafer 16 in the direction Y, whereas the pick-up optical systems 3U and 3D determine the relative positional deviation between the mask 15 and the wafer 16 in the direction X by the photoelectric detection thereby of the alignment marks 15a and 16a. The four alignment marks 15a of the mask 15 are disposed at regular intervals along the circumference of the semiconductor element manufacturing pattern 15b formed in the mask 15. More particularly, they are respectively disposed outside the square pattern region 15b along the respective sides thereof. The pick-up optical system 3L, 3R, 3U and 3D are independently movable along X-Y plane. To accomplish this, they are respectively supported on X-Y stages 21L, 21R, 21U and 21D. The systems are the same as those described in conjunction with Figure 1 in the other respects, and therefore, the detailed description thereof is omitted. Figure 6 shows operational timing of the apparatus. The operation of the pick-up optical systems 3L, 3R, 3U and 3D are timed by the timing controller 7. More particularly, the time T l of actuation of the semiconductor laser 1, the time Td of deactuation of the semiconductor laser 1, the accumulation time Tc of the line sensor 4 and the transfer time Tt, are common to the pick-up optical systems 3L, 3R, 3U and 3D, that is, they are simultaneously operated. The timing controller 7 controls the timing so that Tc + Tt = T l + Td is satisfied in each of the pick-up optical systems 3L, 3R, 3U and 3D. The accumulation-period Tc and the transfer period Tt are controlled to be constant, respectively. The light emitting strength of the semiconductor laser 1 is controlled by the LD driver 2 so as to be constant during the effective emitting duration Te. The positional error between the mask 15 and the wafer 16 is determined on the basis of the mark detection signal of each of the line sensors 4 in which the information is accumulated by one accumulation period Tc. As will be understood from Figure 6, the on-duration T l partly overlaps with the accumulation period Tc and the transfer period Tt of the line sensor. In the on-duration T l , the portion overlapping with the transfer period Tt is a pre-emitting period Tp, and the portion overlapping the accumulation period Tc is an effective emitting period Te. The pre-emitting period Tp is determined on the basis of the structure of the semiconductor laser 1. Figure 4 shows the structure of the semiconductor laser 1. In this semiconductor laser 1, the temperature change of the laser chip 31 is dependent on the thermal resistance, the thermal conductivity and heat capacity of a system 32 comprising a laser chip 31 and a heat sink (not shown) thermally coupled thereto. The pre-exposure period Tp is so determined that it is longer than the time period Ts (Figure 5) required for the temperature of the laser chip 31 to be stabilized from the start of energization thereof. Figure 5 is a graph showing the temperature change of the laser chip 31 after actuation of the semiconductor laser 1. The temperature of the laser chip 31 changes in the manner indicated by (b) depending on the thermal resistance, the thermal conductivity and the sheet capacity. More particularly, when the semiconductor laser is turned on, the temperature of the laser chip 31 increases from the ambient temperature ta and is finally saturated to a temperature ta′ due to the heat transfer to the system 32. This change is dependent on the system 32 of the semiconductor laser 1 and the material and structure of the cap 34. In the structure of the currently available semiconductor laser, the time Ts required for the temperature to saturate to within the range of + and -10 % of the temperature ta′ is 0.1 - 5 msec. After this time, the temperature of the laser chip 31 is stable. In view of this, the minimum of the pre-emitting period Tp is determined in consideration of the structure of the semiconductor laser 1, so as to satisfy: Ts < Tp. The effective emitting period Te is variable in accordance with the users desire provided that 0 < Te < Tc is satisfied. Therefore, the practical quantity of light can be provided depending on the individual line sensors 4. In this embodiment, the effective emitting period Te is controlled by the timing controller 7, by which the effective quantity of light incident on the line sensor 4 is properly controlled. For example, the accumulation time Tc is divided into equal 100 segments and the timing controller 7 controls the effective emitting period Te, the effective quantity of light of the rays incident on the line sensor 4 is controlled with 1/100 step, by which the effective quantity of light can be properly controlled. Referring to Figure 7, a further embodiment will be described. This embodiment is similar to the foregoing embodiment, but the operational timing is different. As shown in Figure 7, the effective emitting period Te and the accumulation period Tc begin at the same timing and end at the same timing. That is, Tc + Tt = T l + Td, Te = Tc. The quantity is controlled with the above conditions. The fundamental aspect of the light quantity control is similar to that of the foregoing embodiment, even after the semiconductor laser 1 stops its emitting operation, the line sensor 4 receives the incident stray light and accumulates the photo-electric converted signals during the accumulation period Tc. However, in the present embodiment, this is avoided. Accordingly, the advantage of better S/N ratio signal obtainable from the line sensor 4, is provided, in addition to the advantage of the foregoing embodiment that the effective light quantity incident on the line sensor 4 can be properly controlled. Referring to Figure 8, a third embodiment of the present invention will be described. The structure of the apparatus is similar to those described in the foregoing. However, the operational timing is different as shown in Figure 8. In the foregoing embodiments, the semiconductor lasers 1 for the pick-up systems 3L, 3R, 3U and 3D are simultaneously turned on and turned off. In doing so, a problem can arise, as shown in Figure 3. For example, the illumination beams 14 from the pick-up systems 3L and 3R are parallel with the X-Z plane and are inclined relative to the Z-axis as shown in Figure 3, and are projected onto the alignment marks. When the illumination beam 14 from the pick-up system 3L is diffracted by the marks 15a and 16a to become information beam 17, a part of the beam can be so scattered and diffracted as to be directed to the pick-up system 3R (light designated by reference 25). If this occurs, the line sensor 4 of the pick-up system 3R receives in addition to the information beam 17 the scattered light 25, with the result of the change of the distribution of the light intensity on the light receiving surface of the line sensor 4. This embodiment pays attention to this problem, and the semiconductor laser 1 of the pick-up systems 3L, 3R, 3U and 3D are actuated in a time-shared manner. In this case, each of the pick-up systems 3L, 3R, 3U and 3D may be operated at different times. Alternatively, the semiconductor lasers 1 of the pick-up systems 3L and 3U may be operated at the same timing, and the semiconductor lasers 1 of the pick-up systems 3R and 3D are operated at the same timing as each other, but at the different timing from the pick-up systems 3L and 3U. Figure 8 illustrates the latter case. In this Figure, reference (a) designates the accumulation and transfer timing of the line sensor 4 of the pick-up systems 3L, 3R, 3U and 3D; (b) designates the turn-on and turn-off timing of the semiconductor lasers 1 for the pick-up systems 3L and 3U; and (c) designates turn-on and turn-off timing of the semiconductor lasers 1 of the pick-up systems 3R and 3D. In this embodiment, the signal processor 6 for the pick-up systems 3L and 3U determines the deviation between the mask 15 and the wafer 16 on the basis of the mark detection signal accumulated in the line sensor 4 during the period A of the accumulation period Tc, and signal processor 6 for the pick-up systems 3R and 3D determines the deviation between the mask 15 and the wafer 16 on the basis of the mark detection signal accumulated in the line sensor 4 during the period B. This embodiment is the same as that described with Figure 6 in the other respects. In the foregoing embodiments, the photo-electric transducer element is a CCD line sensor, but another type line sensor such as CCD area sensor or the like is usable. As described in the foregoing, according to the present invention, the quantity of the light received by the photoelectric transducer element is not continuously adjusted (analog), but the light emitting period of the emitting element having a constant output is controlled to be optimum. Therefore, the quantity of light incident on the photoelectric transducer element can be controlled without variation of the oscillation mode, oscillation wavelength, coherency length, light emitting position, configuration of light emitting spot or the like. During the transient condition in which the temperature of the light emitting element is changing after it is actuated, the control is effected such that the sensor does not accumulate the photo-electric converted signal, and therefore, the optical measurement such as the position measurement can be performed without influence of the variation of the oscillation mode, the oscillation wavelength, coherency length, light emitting point, configuration of the light emitting point. Accordingly, the present invention makes it possible to control the quantity of light without being influenced by variation in the oscillation mode, the oscillation wavelength, coherency length, the light emitting position, the configuration of the light emitting spot or the like of the light source. Therefore, the transmissivity, the reflection index, the refractive index, the diffraction index of the optical system do not change from the designed values, and therefore, the optical measurement can be performed with high precision.

Claims: A light quantity controlling apparatus, comprising: an optical system (13, 15a, 16a); a semiconductor laser (1); a photoelectric transducer element (4) for receiving light from said semiconductor laser through said optical system to produce an electrical signal; and first control means (2, 7, 8) for controlling said semiconductor laser; and second control means (5) for controlling said photoelectric transducer element; characterized in that: said first control means controls an operation period of said semiconductor laser so as to control a quantity of light which is incident on said photoelectric transducer element, and also controls the timing of activation of the semiconductor laser so that it is activated in advance of the start of photoelectric conversion by said photoelectric transducer element. An apparatus for detecting a positional relationship between two objects (15, 16), comprising: plural detecting means (3U, 3D, 3R, 3L) each including the apparatus as defined in claim 1; and means (7, 8) for controlling said plural detecting means to actuate said semiconductor lasers so that at least one of said semiconductor lasers is activated during a different time period from the other semiconductor lasers. Apparatus according to claim 2, wherein the means for controlling the detecting means causes the lasers of detecting means which are positioned around said two objects and are opposite to each other to be activated at different time periods from one another. Apparatus according to claim 2 or 3, wherein the detecting means are such that they detect misalignment between alignment marks (15a, 16a) associated with respective areas of said two objects. An apparatus according to claim 1-4, wherein said photoelectric transducer element is of an accumulation type. An apparatus according to claim 1-5, wherein said second control means controls said photoelectric transducer element so that it is only responsive to said semiconductor laser when the output of the laser is substantially constant. An apparatus according to claim 1-5, wherein the actuation of said semiconductor laser is in advance of the actuation of said photoelectric transducer element by at least the time required for thermal stabilization of said semiconductor laser. An apparatus according to claim 4, wherein said alignment marks comprise grating marks. An apparatus according to any preceding claim, wherein the first control means controls the deactuation of the semiconductor laser to be in advance of or concurrent with the finishing of photoelectric conversion by said photoelectric transducer element. A method of detecting radiation emitted from a semiconductor laser, the method comprising the steps of: providing an optical system (13, 15a, 16a); providing a photoelectric transducer element (4) for receiving light from said semiconductor laser through said optical system to produce an electrical signal; characterised in that: said semiconductor laser and said photoelectric transducer element are periodically activated and deactivated in such a way that concurrent operation of both the semiconductor laser and the photoelectric transducer element only occurs when the output of the semiconductor laser is stable.

Inventors: ABE NAOTO, NOSE NORIYUKI, SHIMODA ISAMU, UDA KOJI, UZAWA SHUNICHI, ABE, NAOTO, NOSE, NORIYUKI, SHIMODA, ISAMU, UDA, KOJI, UZAWA, SHUNICHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: H01S3
IPC codes: H01S5, G01D5, H01L31, H01L33, G01B11, H01L21, G03F9