JP4091698B2 - Recombination lifetime measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a recombination lifetime measuring apparatus.
[0002]
[Prior art]
In general, as a method for measuring the amount of heavy metal contamination that adversely affects the semiconductor manufacturing process, a method for measuring the excess carrier recombination lifetime using the fact that the amount of excess carrier attenuation is proportional to the amount of heavy metal contamination has been proposed. ing.
[0003]
For example, as shown in FIG. 4, light having a frequency larger than the forbidden bandwidth, that is, a wavelength of 500 nm or more and 1000 nm or less is applied to a wafer 30 placed on the wafer stage 10 by a photodiode constituting the light source device 12. Irradiate pulsed light of 100 Hz to 1000 Hz.
[0004]
By irradiation with this pulsed light, electrons absorb energy in the wafer 30 and are excited from the valence band to the conduction band. As a result, the same number of electrons and holes (hereinafter referred to as “electron hole pairs”) are generated, and excess carriers having a density exceeding the density of the thermal equilibrium state of 1.0 ^{12 to} 1.0 ¹⁸ / cm ³ are generated.
[0005]
When the irradiation of light is stopped, excess conduction electrons return to the valence band, combine with excess holes and disappear, and the carrier concentration returns to the original thermal equilibrium state (recombination). When contaminated, some electrons (or holes) cannot be recombined but are trapped by heavy metals and attenuated.
[0006]
That is, the amount of excess carrier attenuation is proportional to the heavy metal content (in other words, the amount of heavy metal contamination), so after the light irradiation by the light source device 12 is stopped, the microwave is simultaneously transmitted to the wafer surface by the microwave transmitter 14. The amount of attenuation of excess carriers is measured by irradiating and reflecting the microwave reflected by the microwave receiver 16, and the recombination lifetime of excess carriers is calculated from the obtained result.
[0007]
In this case, the start and stop of the light irradiation by the light source device 12, the microwave irradiation by the microwave transmitter 14, the measurement of the microwave by the microwave receiver 16, and the data based on the obtained measurement result The calculation is controlled by the control unit 18.
[0008]
[Problems to be solved by the invention]
However, in the apparatus having the configuration shown in FIG. 4, the electrons 32 and the holes 34 generated in the bulk by the light irradiation from the light source device 12 are recombined through impurity levels formed by heavy metals or the like in the bulk. Instead, as shown in the electron band diagram of FIG. 5, the wafer surface diffuses and recombines on the wafer surface via surface levels formed by crystal discontinuities on the wafer surface (surface recombination). ). Although FIG. 5 describes the n-type silicon wafer, the same applies to the p-type silicon wafer.
[0009]
Due to this phenomenon, there is a problem that it is difficult to accurately measure the recombination lifetime in the silicon bulk.
[0010]
Conventionally, the wafer surface is thermally oxidized, the wafer surface is chemically oxidized with a chemical such as iodine, or HF (hydrofluoric acid) is applied to the wafer surface to remove the discontinuous ends of the silicon crystal on the wafer surface with H ( A method has been proposed in which a treatment such as hydrogen) is performed on the wafer surface in advance to prevent recombination of electrons and holes through the surface level.
[0011]
However, when the wafer surface is thermally oxidized, there is a problem that the surface state changes each time depending on the atmosphere at the time of thermal oxidation and the cooling method at the time of drawing out the wafer after the thermal oxidation. In addition, when the wafer surface is chemically oxidized with a chemical such as iodine, there is a drawback that the wafer cannot be reused.
[0012]
Furthermore, when HF (hydrofluoric acid) is applied to the wafer surface and the discontinuous ends of the silicon crystal on the wafer surface are replaced with H (hydrogen), the change over time of the H-substituted wafer surface is large. Even if the recombination lifetime is measured, there is a difficulty that the evaluation cannot be performed stably and with good reproducibility.
[0014]
From the above, the present invention aims at providing a recombination lifetime measuring apparatus which can be significantly suppressed from the electron and hole to surface recombination via surface states of the wafer.
[0015]
[Means for Solving the Problems]
Recombination lifetime measuring apparatus of the invention of claim 1 in order to achieve the above object, from a wafer is charged to a conductivity type opposite to the charge of the wafer, generating the excess carriers by irradiating light to the wafer surface The lifetime of the Si bulk is measured by measuring the amount of attenuation of the excess carriers.
[0016]
That is, according to the first aspect of the present invention, the surface potential of the wafer is changed by charging the wafer to a charge opposite to the conductivity type of the wafer, whereby both electrons and holes are diffused on the wafer surface. It is preventing.
[0017]
That is, when an n-type conductive silicon wafer is positively charged, as shown in FIG. 2A, electrons 32 in the wafer are attracted to the surface, and the holes 34 repel. Therefore, only the electrons 32 exist on the wafer surface, and the electrons 32 can be prevented from recombining with the holes 34 via the surface levels on the wafer surface. Similarly, when a p-type conductive silicon wafer is negatively charged, as shown in FIG. 2B, the holes 34 in the wafer are attracted to the surface, and the electrons 32 repel. Therefore, only the holes 34 exist on the wafer surface, and the holes 34 can be prevented from recombining with the electrons 32 via the surface levels on the wafer surface.
[0018]
In this way, the surface potential of the wafer is changed as described above in accordance with the conductivity type of the wafer to prevent recombination through the surface level of the electron-hole pair, and thereafter, excess carriers are generated in the wafer. When the amount of attenuation is measured, the amount of excess carrier attenuation is based on the amount of heavy metal contamination in the wafer. Therefore, the recombination lifetime obtained by measurement is based on the heavy metal contamination amount, and the heavy metal contamination amount can be detected with high accuracy.
[0019]
In order to achieve the above object, the recombination lifetime measuring apparatus according to claim 1 includes a wafer holding means including a conductive stage having an insulating film on a surface thereof, and an n-type conductive wafer. A negative voltage is applied to the conductive stage when placed on the insulating film, and a positive voltage is applied to the conductive stage when a p-type conductive wafer is placed on the insulating film of the stage. Voltage applying means for applying a voltage, carrier generating means for excessively generating carriers on the surface of the wafer charged by applying a negative voltage or a positive voltage to the stage by the voltage applying means , and the carrier generating means And an attenuation amount measuring means for measuring the attenuation amount of the carrier excessively generated on the wafer surface.
[0020]
Voltage applying means in the recombination lifetime measuring apparatus of the invention of claim 1, Der those for charging the wafer is opposite to the conductivity type of the charge of the wafer is, on the surface of the conductive stage having an insulating film formed on the surface when placed on the wafer in the conductivity type of n-type by applying a negative voltage to the conductive stage, when a p-type conductivity is a positive voltage is applied to the conductive stage.
[0021]
Since the voltage application means changes the surface potential of the wafer according to the conductivity type of the wafer, the carrier generation means generates excess carriers for recombination lifetime measurement. Can be remarkably suppressed from surface recombination via the surface level of the wafer.
[0022]
Therefore, the attenuation amount of excess carriers measured by the attenuation amount measuring means is based on the heavy metal contamination amount in the wafer, and the recombination lifetime can be measured with high accuracy.
[0023]
As an attenuation measurement means for measuring the attenuation amount of excess carriers, for example, a microwave irradiation means for irradiating the wafer surface with microwaves absorbed in a region where crystal defects are caused by impurity implantation, and not absorbed by the wafer. A microwave receiving unit that measures the amount of microwaves, and a microwave attenuation measuring device that detects the amount of heavy metal contamination by detecting the amount of microwaves absorbed by the wafer.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 3.
(First embodiment)
As shown in FIG. 1, the recombination lifetime measuring apparatus according to the first embodiment includes a wafer stage 10 on which a wafer 30 is placed and a ground 22 is connected, and a wafer placed on the wafer stage 10. A light source device 12 configured by a photodiode that irradiates light to 30, a microwave transmitter 14 that irradiates microwaves to the wafer 30 placed on the wafer stage 10, and a microwave that is not absorbed by the wafer 30. And an ionizer (ion irradiation means) for irradiating the surface of the wafer 30 placed on the wafer stage 10 with ions generated by energization of several tens of μA to several mA at a voltage of 100 20).
[0025]
The light source device 12, the microwave transmitter 14, the microwave receiver 16, the ionizer 20, and the earth 22 are respectively connected to the control unit 18, and the control unit 18 controls each operation.
[0026]
Hereinafter, a method for measuring the recombination lifetime using the recombination lifetime measurement apparatus according to the first embodiment will be described.
[0027]
First, the ionizer 20 irradiates the surface of the wafer 30 with ions in accordance with information on the conductivity type of the wafer 30 input to the control unit 18 to charge the surface of the wafer 30. That is, when the conductivity type of the wafer 30 placed on the wafer stage 10 is n-type, positive ions are irradiated, and when the conductivity type is p-type, negative ions are irradiated.
[0028]
The ion dose at this time is adjusted so that the surface potential of the wafer falls within a range of about 1V to 100V.
[0029]
When the surface of the wafer 30 is charged, the ionization by the ionizer 20 is stopped, and the surface of the wafer 30 whose surface is charged by the photodiode constituting the light source device 12 has a wavelength of 500 nm to 1000 nm and an amplitude of 100 Hz to 1000 Hz. Irradiate the pulsed light. As a result, the electrons absorb energy and are excited from the valence band to the conduction band, so an electron hole pair is created, and the density exceeds the density of the thermal equilibrium state of 1.0 ¹² / cm ³ or more and 1.0 ¹⁸ /. Excess carriers of about cm ³ or less are generated.
[0030]
When the conductivity type of the wafer 30 is n-type, the surface of the wafer 30 is positively charged by irradiation of positive ions from the ionizer 20, so that the energy band diagram is as shown in FIG. That is, in the electron hole pair, the electrons 32 are attracted to the surface of the wafer 30, and the holes 34 repel from the wafer surface.
[0031]
When the conductivity type of the wafer 30 is p-type, the surface of the wafer 30 is negatively charged by irradiation of negative ions from the ionizer 20, so that the energy band diagram is as shown in FIG. That is, in the electron hole pair, the hole 34 is attracted to the surface of the wafer 30 and the electron 32 repels from the wafer surface.
[0032]
From the above, it can be seen that the electrons 32 and the holes 34 are not easily recombined via the surface level of the wafer 30. Therefore, excess carriers are attenuated almost via heavy metal levels.
[0033]
After generating excess carriers on the wafer 30, the control unit 18 stops the light irradiation by the light source device 12, and the microwave transmitter 14 irradiates the surface of the wafer 30 with the microwave, and the microwave receiver 16. Is used to measure the amount of microwaves not absorbed by the wafer 30.
[0034]
The control unit 18 calculates the recombination lifetime based on the obtained result, and when the microwave measurement is completed, the ground 22 is connected to the wafer stage 10 to remove the charge on the surface of the wafer 30.
[0035]
Since the recombination lifetime calculated based on the measurement result is not affected by the surface level of the wafer, the amount of excess carrier attenuation according to the heavy metal contamination amount can be measured with high accuracy.
[0036]
In the first embodiment, the control unit controls individual devices (light source device 12, microwave transmitter 14, microwave receiver 16, ionizer 20, and earth 22). The operation of each of the individual devices 12, 14, 16, 20, and 22 may be manually controlled without providing 18.
[0037]
(Second Embodiment)
As shown in FIG. 3, the recombination lifetime measuring apparatus according to the second embodiment includes a conductive wafer stage 11 in which an insulating film 13 is formed on a wafer 30 mounting surface, and the wafer stage 11. Light is applied to a power supply device 24 having a power supply unit 23 (voltage applying means) for applying a positive voltage or a negative voltage of several volts to several hundred volts and a ground 22, and the wafer 30 placed on the wafer stage 11. A light source device 12 including a photodiode, a microwave transmitter 14 for irradiating the wafer 30 placed on the wafer stage 11 with microwaves, and a microwave receiver for measuring microwaves not absorbed by the wafer 30 Instrument 16.
[0038]
The power supply device 24, the light source device 12, the microwave transmitter 14, and the microwave receiver 16 are respectively connected to the control unit 18, and the control unit 18 controls each operation.
[0039]
Hereinafter, a method for measuring the recombination lifetime using the recombination lifetime measurement apparatus according to the second embodiment will be described.
[0040]
First, the power supply device 24 applies a positive voltage or a negative voltage to the wafer stage 11 in accordance with the conductivity type information of the wafer 30 input to the control unit 18 to charge the surface of the wafer 30. That is, when the conductivity type of the wafer 30 placed on the wafer stage 10 is an n-type applies a negative voltage, if a p-type a positive voltage is applied.
[0041]
The voltage at this time is adjusted so that the surface potential of the wafer falls within the range of about 1V to 100V.
[0042]
When the surface of the wafer 30 is charged, the application of voltage by the power supply device 24 is stopped, and the surface of the wafer 30 whose surface is charged by the photodiode constituting the light source device 12 has a wavelength of 500 nm to 1000 nm and an amplitude of 100 Hz to 1000 Hz. The following pulsed light is irradiated. As a result, the electrons absorb energy and are excited from the valence band to the conduction band, so an electron hole pair is created, and the density exceeds the density of the thermal equilibrium state of 1.0 ¹² / cm ³ or more and 1.0 ¹⁸ /. Excess carriers of about cm ³ or less are generated.
[0043]
When the conductivity type of the wafer 30 is n-type, the energy band diagram is as shown in FIG. 2A because the surface of the wafer 30 is positively charged by applying a negative voltage by the power supply device 24. That is, in the electron hole pair, the electrons 32 are attracted to the surface of the wafer 30, and the holes 34 repel from the wafer surface.
[0044]
When the conductivity type of the wafer 30 is p-type, the surface of the wafer 30 is negatively charged by the application of a positive voltage by the power supply device 24, so the energy band diagram is as shown in FIG. That is, in the electron hole pair, the hole 34 is attracted to the surface of the wafer 30 and the electron 32 repels from the wafer surface.
[0045]
From the above, it can be seen that the electrons 32 and the holes 34 are not easily recombined via the surface level of the wafer 30. Therefore, excess carriers are attenuated almost via heavy metal levels.
[0046]
After generating excess carriers on the wafer 30, the control unit 18 stops the light irradiation by the light source device 12, and the microwave transmitter 14 irradiates the surface of the wafer 30 with the microwave, and the microwave receiver 16. Is used to measure the amount of microwaves not absorbed by the wafer 30.
[0047]
The control unit 18 calculates the recombination lifetime based on the obtained result. When the microwave measurement is completed, the control unit 18 switches the power supply 24 to the ground 22 and connects the ground 22 to the wafer stage 11 to connect the wafer 30. Remove surface charge.
[0048]
Since the recombination lifetime calculated based on the measurement result is not affected by the surface level of the wafer as in the first embodiment, excess carriers in accordance with the heavy metal contamination amount are used. Therefore, it is possible to accurately measure the attenuation amount.
[0049]
In the second embodiment, the control unit controls the individual devices (the power supply device 24, the light source device 12, the microwave transmitter 14, and the microwave receiver 16). The operation of the individual devices 24, 12, 14, and 16 may be manually controlled without providing them.
[0050]
【The invention's effect】
As described above, according to the first aspect of the present invention, there is an effect that it is possible to greatly suppress the surface recombination of electrons and holes through the surface level of the wafer .
[Brief description of the drawings]
FIG. 1 is a schematic explanatory diagram of a recombination lifetime measuring apparatus according to a first embodiment of the present invention.
FIG. 2A is an energy band diagram in the wafer when the surface of the wafer 30 is positively charged, and FIG. 2B is an energy in the wafer when the surface of the wafer 30 is negatively charged. It is a band diagram.
FIG. 3 is a schematic explanatory diagram of a recombination lifetime measuring apparatus according to a second embodiment of the present invention.
FIG. 4 is a schematic explanatory diagram of a conventional recombination lifetime measuring apparatus.
FIG. 5 is an energy band diagram in the wafer when the surface of the wafer is not charged.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Wafer stage 12 Light source device 14 Microwave transmitter 16 Microwave receiver 18 Control part 20 Ionizer 22 Ground 30 Wafer

Claims (1)

Wafer holding means having a conductive stage having an insulating film on the surface;
When an n-type conductive wafer is placed on the insulating film of the stage, a negative voltage is applied to the conductive stage, and a p-type conductive wafer is placed on the insulating film of the stage. A voltage applying means for applying a positive voltage to the conductive stage,
Carrier generating means for generating excessive carriers on the surface of the wafer charged by applying a negative voltage or a positive voltage to the stage by the voltage applying means ;
Attenuation measuring means for measuring the attenuation of carriers excessively generated on the wafer surface by the carrier generating means;
A recombination lifetime measuring device comprising:

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