WO2016121081A1 - Semiconductor inspection device - Google Patents

Abstract

In power control using an electro-optic element such as a Pockels cell, it has been possible for a sample to be damaged when there is a failure. This point has not been taken into consideration in the prior art. In the present invention, a waveplate is added between an electro-optic element (Pockels cell) and polarizing beam splitter, and the disposition of the same is switched between 0° and 45° according to the voltage applied to the electro-optic element (Pockels cell). As a result, even if there is a failure and the voltage applied to the electro-optic element (Pockels cell) becomes zero, it is possible to attenuate the inspection power beyond the maximum intensity in a state where voltage is being applied to the electro-optic element (Pockels cell).

Description

Semiconductor inspection equipment

The present invention relates to a semiconductor inspection apparatus, for example, an optical inspection apparatus that inspects a semiconductor wafer having a fine pattern, a wafer before pattern formation, a photomask (exposure mask), a liquid crystal substrate, and the like.

The semiconductor wafer foreign matter inspection device detects minute defects existing on the wafer surface and outputs the number, coordinates, and size. With the miniaturization of the semiconductor process, the foreign substance inspection apparatus is required to improve detection sensitivity. As one means for improving the detection sensitivity, there has been a method of increasing the intensity of illumination light. However, large objects exceeding several hundred nm are destroyed when irradiated with high illumination light intensity. In this specification, this phenomenon is called explosion. The debris generated by the explosion diffuses to the sample surface and enlarges the defective area of the sample. Therefore, the inspection power (the intensity of illumination light used for the inspection) must be limited.

Patent Document 1 discloses a technique for dynamically controlling inspection power during inspection using a Pockels cell. In Patent Document 1, normally, inspection is performed with high sensitivity by high power irradiation, and when a large foreign object exists, the inspection power is reduced only to the foreign object and its periphery to avoid explosion.

U.S. Pat. No. 7,787,114

When inspecting with high sensitivity by high-power laser irradiation as in Patent Document 1, if the Pockels cell itself or the Pockels cell control unit fails and abnormally stops, the sample is irradiated with excessive power laser. There is a possibility. The prior art did not consider this point.

Furthermore, since the control voltage changes depending on the temperature state of the Pockels cell itself, there is a possibility that the laser power cannot be controlled even if a predetermined voltage is applied. That is, there is a possibility that an excessively high power laser is unintentionally irradiated onto the sample due to a change in voltage characteristics of the Pockels cell itself.

An object of the present invention is to enable safe control of laser power in an apparatus that controls laser power using an electro-optical element such as a Pockels cell.

In order to solve the above-described problems, the present invention provides a semiconductor inspection apparatus that inspects a sample with laser light. A laser light source that emits laser light and laser light from the laser light source are incident, and the phase of the laser light is adjusted. An illumination optical system having an electro-optical element that changes into at least two states, and a wave plate that rotates the phase of the laser light;
The wavelength plate is configured such that the intensity of the laser light irradiated to the sample is attenuated more than the maximum intensity in a state where a voltage is applied to the electro-optic element without applying a voltage to the electro-optic element. Provided is a semiconductor inspection apparatus that generates a large phase difference.

Here, the electro-optical element may be a Pockels cell.

Further, a half-wave plate may be used as the wave plate.

Further, the illumination optical system includes a storage unit that changes a voltage applied to the Pockels cell, acquires and stores the intensity modulation characteristic of the illumination optical system, and based on the stored intensity modulation characteristic, the Pockels The phase fluctuation of the cell itself is corrected.

Here, the timing for storing in the storage unit and the timing for setting the laser light intensity to be attenuated in a state where no voltage is applied to the Pockels cell using the phase difference due to the wave plate are set before the inspection of the sample. It becomes.

Furthermore, a static attenuator may be added to the illumination optical system.

According to the present invention, it is possible to minimize the risk of damaging the sample even when the power control system fails.

Issues, configurations, and effects other than those described above will be clarified by the following description of the embodiments.

The figure explaining the whole device configuration of an optical inspection device The figure explaining the illumination optical system which is the 1st Embodiment of this invention The figure explaining another structural example of the power monitor system of the illumination optical system which is the 2nd Embodiment of this invention. The figure explaining the relationship (intensity modulation characteristic) between the Pockels cell applied voltage and the inspection power The figure explaining the applied voltage of the Pockels cell Diagram explaining the effect of temperature on the voltage characteristics (intensity modulation characteristics) of Pockels cells Optimization flow diagram of Pockels cell voltage characteristics (intensity modulation characteristics) and half-wave plate placement angle before inspection The figure explaining the effect of a half-wave plate

Examples will be described below with reference to the drawings.

Hereinafter, an example of a semiconductor inspection apparatus using an optical inspection apparatus will be described. However, this is merely an example of the present invention, and the present invention is not limited to the embodiments described below. In the present invention, the optical inspection apparatus widely includes apparatuses using a laser. In the following, the optical inspection apparatus includes a system in which the optical inspection apparatus is connected by a network and a composite apparatus of the charged particle beam apparatus, which are collectively referred to as an optical inspection system. is there.

In this specification, “sample” widely includes a semiconductor wafer having a fine pattern, a wafer before pattern formation, a photomask (exposure mask), a liquid crystal substrate, and the like.

FIG. 1 is a diagram showing a schematic configuration of an optical inspection apparatus 1 which is an example of a semiconductor inspection apparatus to which the present invention is applied. The optical inspection apparatus 1 includes a transport system, a stage system, an optical system, and a data processing system, which are controlled by the overall control unit 100.

Specimen 2 has a robot arm inside and is transported to the stage in the examination room by a transfer device 101 adjacent to the examination room, and is fixed to the stage by vacuum suction or edge clip.

The sample stage includes a rotation stage 102, a vertical axis stage 103, and a horizontal axis stage 104, which are controlled by a stage control unit 105. The vertical axis stage 103 uses a distance sensor (not shown) fixed to the reference plane of the illumination optical system 106 to make the focus surface of the illumination optical system 106 and the detection unit 107 coincide with the sample surface. The entire surface of the sample can be inspected by moving the stage in the radial direction while rotating the stage.

The optical system includes an illumination optical system 106 and a detection unit 107. The illumination optical system 106 includes a laser light source, a lens, and a diaphragm, and details will be described later with reference to FIGS. The illumination optical system 106 modulates the illumination beam to an appropriate power, shapes the illumination beam to an appropriate spot size, and illuminates the sample 2. The detection unit 107 includes a detection optical system including a plurality of lenses and a detector, and the scattered light from the sample 2 is collected on the detector, converted into an electric signal, and sent to the signal processing unit 109. The illumination optical system 106 and the detection unit 107 are controlled by the optical system control unit 108, and adjust the arrangement of the optical elements and the gain of the detector according to the inspection conditions.

The signal processing unit 109 appropriately processes the signal input from the detection unit 107 and determines a defect. At this time, the size of the defect is determined based on the intensity of the signal. Further, the coordinates of the defect are determined using encoder signals input from the rotary stage 102 and the horizontal axis stage 104.

The data processed by the signal processing system unit 109 is transmitted to the overall control unit 100 and displayed on the display 110 or stored as a data file in the storage unit 111.

The system configuration is not limited to this, and some or all of the devices constituting the system may be a common device.

The overall control unit 100, the stage control unit 105, the optical system control unit 108, and the signal processing unit 109 can be realized by either hardware or software. When configured by hardware, it can be realized by integrating a plurality of arithmetic units for executing processing on a wiring board or in a semiconductor chip or package. When configured by software, a program for executing desired arithmetic processing by a central processing unit (CPU) mounted on a device constituting the system or a general-purpose CPU mounted on a general-purpose computer connected to the system. It can be realized by executing.

FIG. 2 shows an example of the configuration of the illumination optical system 106. The illumination optical system includes a light source 200, a power control unit 201, and a beam shaping unit 202.

As the light source 200, for example, a laser light source is used. In order to detect minute defects in the vicinity of the sample surface, a high-power light source that oscillates a short-wavelength ultraviolet or vacuum ultraviolet laser beam and has an output of 1 W or more is used as a wavelength that does not easily penetrate into the sample.

The beam shaping unit 202 is an optical unit that forms a predetermined illumination shape, and includes, for example, a beam expander.

The power control unit 201 is mainly configured by a Pockels cell 204, a half-wave plate 205, a polarizing beam splitter 206, and an attenuator (static attenuator) 207. Hereinafter, an example in which a Pockels cell is used as an example of an electro-optical element will be described, but any element that can electrically switch the polarization direction of light may be used. The laser light incident on the Pockels cell 204 is phase-modulated according to the voltage applied from the Pockels cell control unit 208. Further, a certain amount of phase is modulated by the half-wave plate 205 arranged at the rear stage of the Pockels cell, and is incident on the polarization beam splitter 206. Here, it is important that the half-wave plate 205 is in the front stage of the polarization beam splitter. The half-wave plate 205 is fixed to the rotary stage 218 and can be arranged at an arbitrary angle. The arrangement angle is controlled by the half-wave plate control unit 209. There are two types of arrangement angles used in this example, 0 ° and 45 °. In order to obtain the phase modulation effect of these two types of arrangement angles, a half-wave plate arranged at 45 ° is used as an optical path with a linear motion stage or the like. It may be configured to be taken in and out.

The light phase-modulated by the Pockels cell 204 and the half-wave plate 205 is branched into two optical paths by the polarization beam splitter 206. The light that passes through the polarizing beam splitter 206 is referred to as inspection light, and the reflected light is referred to as non-inspection light. The ratio of inspection light to non-inspection light can be adjusted by the Pockels cell applied voltage and the arrangement angle of the half-wave plate.

Non-inspection light passes through the beam sampler 210 and enters the diffuser 211. Here, as the beam sampler 210, an element in which reflected light is larger than transmitted light is used. Part of the light reflected by the beam sampler 210 is incident on the power monitor 212 to constantly monitor its power level.

The power monitor signal is input to the Pockels cell control unit 208, and is used to optimize the Pockels cell applied voltage and to detect Pockels cell abnormality.

The Pockels cell control unit 208 includes an interlock circuit that compares the control signal with the power monitor signal and closes the optical path shutter 213 when power that does not match the control signal is detected.

It is possible to confirm whether the optical path shutter 213 is correctly controlled before illuminating the sample 2 by disposing the optical path shutter 213 downstream of the polarization beam splitter 206 that forms a branch to the power monitor 212.

In addition, the maximum value of the irradiation power is limited by the attenuator 207 at the end of the power control unit, and even if intensity modulation by the Pockels cell becomes uncontrollable, the irradiation power to the sample can be limited to realize fail safe. ing.

Since the Pockels cell 204 changes its characteristics depending on the temperature, the power control unit 201 is provided with a thermometer 215 and a heater 216, and is managed at a constant temperature by the temperature control unit 217. Instead of the heater 216, a cooler may be used, or a function of performing both heating and cooling may be provided.

As described above, it is possible to handle a constant temperature control for a relatively long-term temperature fluctuation, which is effective in improving the reliability of the inspection apparatus performance.

FIG. 3 shows another example of the configuration of the illumination optical system 106. The difference from FIG. 2 is a configuration in which a beam sampler 310 is disposed on the optical path of the inspection light and a part of the inspection light is monitored by the power monitor 212. Here, as the beam sampler 310, an element in which transmitted light is larger than reflected light is used. In this case, the optical path shutter 213 is arranged at a stage subsequent to the beam sampler 310. According to the configuration of FIG. 3, unlike the configuration of FIG. 2, since the inspection light itself is branched and monitored, the state of the inspection light can be grasped more directly.

Next, power modulation using a Pockels cell will be described with reference to the drawings.

Fig. 4 shows the relationship between Pockels cell applied voltage and inspection power (intensity modulation characteristics). When a voltage is applied to the Pockels cell, the polarization axis is rotated with respect to the transmission axis of the polarization beam splitter 206, so that a voltage characteristic (intensity modulation characteristic) 400 as shown in FIG. When the voltage VLmin 401 is applied, it rotates by 90 degrees with respect to the transmission axis of the polarization beam splitter 206, and the inspection light power is minimized. When the voltage VHmax 402 is applied, the polarization axis coincides with the transmission axis of the polarization beam splitter 206, and the inspection light power becomes maximum. The voltage at which the polarization axis rotates 180 ° is called a half-wave voltage VHW403, which is an indicator of the voltage characteristics of the Pockels cell.

FIG. 5 (a) shows an example of the Pockels cell applied voltage. The Pockels cell applied voltage has a reference first level VL501 and a second level VH502. This level can be arbitrarily specified by the Pockels cell control unit. For example, if the first level is set to the voltage VLmin that minimizes the inspection light power and the second level is set to the voltage VHmax that maximizes the inspection light power, the applied voltage is switched between VLmin and VHmax. The power can be switched. The ratio between the minimum power and the maximum power is determined by the extinction ratio of the Pockels cell, and is generally 1:50 to 1: 1000. For example, when a Pockels cell with an extinction ratio of 1:50 is used, the maximum power can be set to 100% and the power can be switched to 2%.

Also, if multiple levels are set between VLmin and VHmax as shown in Fig. 5 (b), any power between 2% and 100% can be extracted.

For example, if a large foreign object is on the sample surface and its position is known in advance by any method, switching the control voltage in the vicinity of the large foreign object to lower the inspection power, while suppressing the explosion of the large foreign object, The area can be inspected with high sensitivity by increasing the inspection power. As a method of detecting a large foreign material in advance, a method of performing a preliminary inspection before the main inspection, a method of detecting immediately before using an end portion of the inspection light beam, or the like can be considered.

The phase modulation effect of the Pockels cell changes not only with the applied voltage but also with temperature. FIG. 6 shows how the voltage characteristics of the Pockels cell change with temperature. The voltage characteristic 600 at a certain reference temperature (T = T0) changes to 601 due to a temperature drop (T = T1), and changes to 602 due to a temperature rise (T = T2). That is, the voltage that maximizes the power of the inspection light varies with temperature. Therefore, it is necessary to control the temperature around the Pockels cell at a constant level. However, since the temperature control is low in accuracy and the reaction speed is slow, it is preferable to correct the influence of a relatively short-term temperature fluctuation by the applied voltage.

FIG. 7 is a diagram illustrating a voltage correction flow. Before starting the inspection, first, the optical path shutter 213 is closed to prevent erroneous irradiation (step 701). In order to obtain the voltage characteristics as shown in FIG. 4, the non-inspection optical power data is obtained by changing the voltage applied to the Pockels cell in a certain range and pitch, and the intensity modulation characteristic storage unit shown in FIGS. (Step 702, intensity modulation characteristic storing step) VLmin is obtained from the acquired voltage characteristic data (step 703). It is confirmed whether the obtained VLmin value satisfies the conditional expression 704. If not, the arrangement angle of the half-wave plate is changed (step 705, wave plate angle adjustment step), and optimization is performed again. Here, the conditional expression 704 is a condition for suppressing the inspection light power to 50% or less of the maximum power in a state where a voltage is applied in a state where the applied voltage is 0 (the reason why it can be suppressed to 50% or less is shown in FIG. 8). When VLmin satisfying conditional expression 704 is determined, VHmax is calculated from VLmin and registered as a constant representing the voltage characteristic of the Pockels cell (step 706). The calculation formula here is based on the relationship shown in FIG. From these VLmin and VHmax, VL and VH actually used as control voltages are obtained and registered (step 707). Here, the fixed values 1 and 2 when calculating VL and VH are constants determined by how much the minimum and maximum power are to be set, respectively.

Finally, VL is set as the voltage value at the start of inspection (step 708), and after a safe state is prepared, the shutter is opened (step 709) and inspection is started.

That is, by always performing the voltage correction flow shown in FIG. 7, in particular, the intensity modulation characteristic storing step (step 702) before the inspection, the influence of the above-mentioned relatively short-term temperature fluctuation is expressed as the fluctuation of the voltage characteristic of the Pockels cell. Therefore, the influence of the temperature variation can be corrected by applying a voltage to the Pockels cell at the time of inspection based on the stored intensity modulation characteristic.

Here, the time of inspection means that the sample 2 is illuminated with the laser light of the illumination optical system 106, the scattered light from the sample 2 is received by the detection unit 107, and then the defect is determined by the signal processing unit 109. Indicates the middle of running on sample 2. The term “before inspection” includes not only before the inspection is performed, that is, immediately after the sample 2 is transported into the inspection chamber, but also before the next predetermined inspection after the predetermined inspection is completed.

FIG. 8 illustrates that the inspection power when the applied voltage becomes 0 can be suppressed to 50% or less of the maximum power when the voltage is always applied by the rotary wave plate (wave plate angle adjustment process). belongs to. The horizontal axis is a value obtained by normalizing the applied voltage with a half-wave voltage, and the vertical axis is the inspection light power. State 1 and state 2 indicate different temperature states.

In state 1, half-wave plate arrangement angle 0 ° (800), the transmission power is less than 50% even if voltage cannot be applied during inspection (ie V = 0), but state 2, half-wave plate arrangement When the angle is 0 °, 801, the transmission power exceeds 50% at the moment when no voltage is applied. Therefore, optimization is performed before the inspection, and in the case of the state 801, since the conditional expression 704 in FIG. 7 is not satisfied, the half-wave plate is rotated. Then, since the state of 801 is switched to the state of 803, the inspection power falls below 50% in a state where voltage control is impossible. Similarly, since the state of 802 does not satisfy the conditional expression 704, the half-wave plate is rotated and switched to the state of 800. In other words, when the temperature is in state 1, the half-wave plate is 0 ° so that the characteristic is 800, and when the temperature is state 2, the half-wave plate is 45 ° so that the characteristic is 803. . In other words, an offset may be added to the phase with a wave plate so that the inspection light power is minimized (preferably 0) when the voltage applied to the Pockels cell is zero. According to the present embodiment, even if some abnormality occurs in the Pockels cell itself or the Pockels cell control unit, and the voltage is no longer applied, the transmitted power is reduced to 50% or less of the maximum power with the voltage applied. Can be limited. This minimizes the risk of damaging Sample 2.

In addition to that, the voltage characteristic data acquisition / storage 702 (intensity modulation characteristic storage process) before inspection shown in FIG. 7 or the registration 706 of VLmin and VHmax associated therewith can be performed in a relatively short period of time that changes for each inspection. The effect of temperature fluctuations can be corrected, thereby not only suppressing explosions of large foreign objects with high accuracy but also maximizing the inspection light power during normal inspection with high accuracy and improving inspection sensitivity. .

In addition, this invention is not limited to the above-mentioned Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described.

Also, the control lines and information lines indicate what is considered necessary for the explanation, and not all the control lines and information lines on the product are necessarily shown. Actually, it may be considered that almost all the components are connected to each other.

DESCRIPTION OF SYMBOLS 1 ... Semiconductor inspection apparatus (optical inspection apparatus), 2 ... Sample, 106 ... Illumination optical system, 200 ... Light source, 204 ... Pockels cell, 205 ... Half wave plate (wave plate), 206 ... Polarizing beam splitter, 207 ... Static Attenuator, 208 ... Pockels cell control unit, 209 ... half wave plate control unit, 212 ... power monitor, 214 ... intensity modulation characteristic storage unit, 400 ... intensity modulation characteristic, 702 ... intensity modulation characteristic storage process, 705 ... wave plate angle Adjustment process

Claims (6)

In semiconductor inspection equipment that inspects samples with laser light,
A laser light source for emitting laser light;
An electro-optic element that receives laser light from the laser light source and changes the phase of the laser light into at least two states;
A wavelength plate that rotates the phase of the laser light, and an illumination optical system having
The wavelength plate is configured such that the intensity of the laser light irradiated to the sample is attenuated more than the maximum intensity in a state where a voltage is applied to the electro-optic element without applying a voltage to the electro-optic element. Semiconductor inspection device that generates a large phase difference. The semiconductor inspection apparatus according to claim 1,
A semiconductor inspection apparatus, wherein the electro-optic element is a Pockels cell. The semiconductor inspection apparatus according to claim 1,
The wave plate is a half wave plate;
Due to the phase difference generated by the half-wave plate, the intensity of the laser light applied to the sample without applying a voltage to the Pockels cell is 50% of the maximum intensity when a voltage is applied to the Pockels cell. Semiconductor inspection equipment that attenuates to less than%. The semiconductor inspection apparatus according to claim 1,
The illumination optical system includes an intensity modulation characteristic storage unit that changes a voltage applied to the Pockels cell, acquires and stores an intensity modulation characteristic of the illumination optical system,
A semiconductor inspection apparatus that corrects a phase variation of the Pockels cell itself based on the intensity modulation characteristic stored in the intensity modulation characteristic storage unit. The semiconductor inspection apparatus according to claim 4,
The timing for storing in the intensity modulation characteristic storage unit and the timing for setting the intensity of the laser beam to be attenuated in a state where no voltage is applied to the Pockels cell using the phase difference due to the wavelength plate are both the sample. Semiconductor inspection equipment before inspection. The semiconductor inspection apparatus according to claim 1,
The illumination optical system is a semiconductor inspection apparatus including a static attenuator.

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