JPH07176289A - Method and device for implanting ion - Google Patents

Abstract

PURPOSE:To provide a method and a device for implanting an ion wherein an amount of impurities injected to a semiconductor wafer can be high accurately measured an a throughput can be improved by eliminating suspending a process caused by generating gas. CONSTITUTION:In an ion implantation method for implanting an ion by irradiating a wafer W, arranged in a pressure reducing chamber 30, with an ion beam, control is performed, so that a beam current measured by an ammeter 35 obtains the first beam current value L1, for a prescribed time after starting ion implantation, and so that the beam current obtains the second beam current value higher than the first beam current value L1, after the time T. Thus, a pressure in the chamber 30, increased due to generating gas from a resist film of the semiconductor wafer W within the prescribed time T just after starting ion implantation, is generated equal to or less than a pressure in the chamber 30 after the time T. As a result, the pressure in the chamber 30 becomes a pressure less influencing the measuring accuracy of the beam current in the ammeter 35.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ion implantation method and an apparatus therefor.

[0002]

In the ion implantation technique, impurity ions generated in an ion source are accelerated by a high electric field, and the kinetic energy of the ion beam is used to introduce impurities into a semiconductor wafer. It is a technology. This ion implantation technique is now the mainstream of the impurity introduction technique, replacing the thermal diffusion technique. The most important reason is that the total amount of impurities introduced into the wafer can be measured purely physically as the amount of electric charge, and the situation can be measured. That is, the ion beam charge amount can be monitored as a current amount based on the number of ions that have jumped into the farader cup arranged on the upstream side of the semiconductor wafer, and the dose amount can be measured by a dose counter connected to this ammeter. Can be monitored.

However, studies conducted by the present inventors have revealed that the number of impurities implanted into the wafer cannot be accurately measured, especially immediately after the start of ion implantation. As a result of diligent research by the present inventors regarding this cause, for example, when performing ion implantation into a semiconductor wafer on which a resist film has been formed by coating, the resist film is vaporized by the heat at the time of ion collision, and the decompression chamber in which the semiconductor wafer is arranged It turned out that it was due to the rise of the pressure inside.

FIG. 9 (A) shows general ion implantation conditions of a conventional ion implantation apparatus. The beam current measured by an ammeter is almost constant from immediately after the ion implantation is started to when the ion implantation is completed. The characteristic diagram is as follows. FIG. 9 (B)
FIG. 4B is a characteristic diagram in which the pressure in the decompression chamber in which the semiconductor wafer is placed is monitored under the condition of FIG. As is clear from FIG. 9B, the chamber pressure rises immediately after the start of ion implantation, which is due to the gas pressure generated by the vaporization of the resist film as described above.

Thus, if the pressure in the decompression chamber rises immediately after the start of ion implantation, the probability that the gas and the ions collide with each other before reaching the semiconductor wafer increases, and some of the ions are atoms or molecules. Then, it is driven into a semiconductor wafer. The impurities implanted in the semiconductor wafer in the state of molecules or atoms increase the impurity concentration of the semiconductor wafer like the impurities implanted as ions, but they are measured by an ammeter connected to the Faraday cup. It is never done. For example, although 90 ions and 10 molecules are implanted into a semiconductor wafer and a total of 100 impurities are implanted, in a measurement that depends only on the number of ions, 90 impurities are implanted. It will be erroneously measured.

The following reason is considered to cause an error in the measurement of the amount of implanted impurities due to an increase in the pressure in the decompression chamber immediately after the start of ion implantation. In general, to prevent secondary electrons from jumping out of the Faraday cup,
For example, a suppress electrode for applying a voltage of about -1000V is provided. However, when the pressure in the decompression chamber in which the suppress electrode is arranged rises, discharge from the suppress electrode is likely to occur. When this discharge occurs, secondary electrons in the Faraday cup are emitted to the outside. When the secondary electron discharge destination is a potential for counting the ion beam charge amount, this secondary electron amount is counted as the ion beam charge amount, and erroneous measurement occurs.

When the above-mentioned gas is generated, the measured beam current value is lowered. Some ion implanters interrupt the ion implantation when the monitored beam current value falls below a certain value, and this interruption increases the time required for the ion implantation and lowers the throughput. The generation of this kind of gas becomes more remarkable in the ion implanter in which the beam current value is increased to shorten the processing time, but if the above-mentioned processing is interrupted frequently, it is meaningless to increase the beam current. Will end up.

Therefore, it is an object of the present invention that the amount of impurities injected into the object to be processed can be measured with high precision, and the interruption of processing due to gas generation can be eliminated to improve the throughput. An object is to provide an ion implantation method and an apparatus therefor.

[0009]

The method of the present invention is an ion implantation method for implanting ions by irradiating an object to be processed arranged in a decompression chamber with an ion beam to reach the object to be processed per unit time. The amount of ions is made different between a predetermined time after the start of ion implantation and other times, and the chamber internal pressure that rises due to the gas generated from the object to be processed within the predetermined time is set to the predetermined time. It is characterized in that the pressure in the chamber is equal to or lower than the latter pressure.

The apparatus of the present invention irradiates an ion beam extracted from an ion source to an object to be processed placed in a decompression chamber to inject ions, and measures a beam current based on the amount of ions reaching the ion beam. In an ion implantation apparatus for controlling the implantation amount, a arriving ion amount control means for increasing the arriving ion amount per unit time to the object to be processed stepwise or continuously is provided for a predetermined time after starting the ion implantation, and the predetermined time It is characterized in that the pressure in the chamber, which rises in the chamber due to the gas generated from the object to be processed, is set to be equal to or lower than a pressure value that has little influence on the measurement accuracy of the beam current.

In the method and apparatus of the present invention, the amount of ions reaching the object to be processed per unit time can be monitored by any one of the ion beam charge amount, the beam current, the dose counter amount and the like.

In the apparatus of the present invention, the reaching ion amount control means may include means for changing and controlling a parameter for controlling the amount of ions generated in the ion source. Alternatively, the reaching ion amount control means may include means for varying the passing amount of the beam on the way of the ion beam path from the ion source to the object to be processed. At this time, the reaching ion amount control means sets the parameter or the beam passage so that the beam current value measured within the predetermined time is lower than the beam current value measured during the other time. You can set the amount.
Alternatively, a detection means for detecting the pressure in the chamber and a means for comparing the pressure value from this detection means with a preset pressure value are provided, and the arrival ion amount control means is provided with a comparison result from the comparison means. The parameter or the beam passing amount can be changed based on

[0013]

According to the method and apparatus of the present invention, the amount of ions reaching the object to be processed per unit time is made different between the predetermined time after the start of ion implantation and the other time, or after the ion implantation is started. By increasing the amount of ions reaching the object to be processed per unit time stepwise or continuously over a predetermined time, the chamber internal pressure that rises due to the gas generated from the object within the predetermined time can be controlled. It can be set to be equal to or lower than the pressure in the chamber after a predetermined time has elapsed.
In other words, it is possible to maintain the chamber internal pressure, which rises due to the gas generated from the object to be processed within a predetermined time, below a pressure value that has little influence on the measurement accuracy of the beam current. Therefore, even immediately after the start of ion implantation, the frequency of collision between the gas generated from the object to be processed and the ion beam is reduced, and the amount of impurities implanted in the object to be processed in the state of molecules or atoms is reduced. It is possible to measure the amount of impurities implanted into the wafer with high accuracy.

In order to control the amount of arriving ions, conditions are set so that the beam current value measured within a predetermined time immediately after the start of ion implantation becomes lower than the beam current values measured during other times. Alternatively, the pressure in the chamber can be constantly monitored, and the reaching ion amount can be variably controlled based on the result of comparison between the detected pressure value and the preset pressure value.

[0015]

Embodiments of the ion implantation method and apparatus to which the present invention is applied will be described below in detail with reference to the drawings. As for the ion implantation apparatus to which the present embodiment is applied, the higher the gas generation speed that adversely affects the measurement accuracy, the greater the effect. From this point, rather than the low current type with a beam current of less than 100 μA, 1st Example which can be suitably implemented with a thing with a comparatively large beam current value, such as a medium current type whose electric current is 100 μA to 2 mA, and a large electric current type whose beam current is 200 μA to 30 mA. FIG. 1 shows a first embodiment of the present invention. It is a schematic diagram showing the whole ion implantation device concerning an example. In the figure, the ion source 10 is
For example, the gas sublimated from the solid raw material in the paperizer 12 is turned into plasma. As the ion source 10, either a hot cathode discharge type ion source (Freeman type) using a hot filament or a microwave ion source using microwave plasma utilizing a magnetron may be adopted. In this embodiment, the amount of ions extracted from the ion source 10 is made variable by changing the plasma density, for example. The parameters for changing the plasma density include an arc current, an arc voltage, a current of the source magnet (magnetic flux density), and the like. By changing one or more of these parameters, the amount of extracted ions can be changed. is there.

Ions in the plasma of the ion source 10 are extracted as an ion beam to the outside by the extraction voltage applied between the extraction electrode 14 and the main body of the ion source 10. A mass spectrometer 20 is arranged on the downstream side of the extraction electrode 14 via a slit 16, and only desired ions are extracted here. As shown in FIG. 1, the variable slit 18 may be arranged on the ion beam path from the ion source 10 to the semiconductor wafer W, for example, on the upstream side of the mass spectrometer 20. By changing the slit width of the variable slit, the amount of ions reaching the semiconductor wafer W can be changed. The ion beam that has passed through the mass spectrometer 20 then enters the accelerator 24 through the slit 22 and is accelerated by an acceleration voltage given in advance.

A decompression chamber 30 is arranged downstream of the accelerator 24. In the decompression chamber 30, a rotary disk 34 that is rotationally driven by a spin motor 32 is provided, and a plurality of semiconductor wafers W are mounted and held on the rotary disk 34. A resist film is applied and formed on the semiconductor wafer W. A Faraday cup 36 is provided on the upstream side of the semiconductor wafer W mounted and held on the rotating disk 34, and is connected to an ammeter 35. Further, a dose counter 37 for counting the ion irradiation amount to the semiconductor wafer W is arranged between the ammeter 35 and the earth. The Faraday cup 36 confines secondary electrons generated at the time of ion implantation so as not to flow outside, and accurately measures the beam current measured by the ammeter 35 and the dose amount measured by the dose counter 37. To do. A suppress electrode 38 to which a voltage of, for example, −1000 V is applied is provided on the upstream side of the Faraday cup 36 so that secondary electrons do not fly out of the Faraday cup 36.

Further, in the first embodiment, the CPU
40, an operation panel 42, and a memory 44 are provided. On the operation panel 42, in addition to general ion implantation conditions such as implantation energy and dose, it is possible to input different ion implantation conditions unique to the first embodiment within a predetermined time immediately after the start of ion implantation and after that. is there. CPU 40
Based on the information on the ion implantation conditions peculiar to the embodiment input through the operation panel 42 and stored in the memory 44,
Parameters for changing the amount of ions of the ion beam extracted from the ion source 10 are set, or the slit width of the variable slit 18 is changed.

Next, the operation of the first embodiment apparatus will be described with reference to FIGS. 2 (A), 2 (B) and 3. FIG. 2A shows the conditions (L1, L2, T) set by the CPU 40 based on the condition information peculiar to the first embodiment input from the operation panel 42 in the relationship between the time axis and the beam current. It represents. As shown in FIG. 2 (A),
The beam current detected by the ammeter 35 is set to be the first beam current value L1 (for example, 5 mA) within a predetermined time T after the start of the ion implantation. 2 beam current value L2 (eg 15m
A) is set. Therefore, CPU4
0 indicates the first beam irradiation condition as shown in step 1 of the flowchart shown in FIG. 3, that is, various parameters of the ion source 10 or variable so as to obtain the first beam current value L1 in FIG. 2 (A). The slit width of the slit 18 is set and controlled. After that, the CPU 40 determines whether or not the elapsed time immediately after the start of the ion implantation exceeds the set time T (step 2), and moves to step 3 only when this determination is YES. In step 3, the CPU 40 sets the second beam irradiation condition, that is, as shown in FIG. 2 (A), the ion beam of the ion source 10 is adjusted so that the second beam current L2 is higher than the first beam current value L1. The parameter or the slit width of the variable slit 18 is set and controlled. Then, in step 4, the dose counter 37
It is determined whether or not the amount of ions implanted into the semiconductor wafer W detected in 1 has reached a set value, and when the set amount has been reached, the ion implantation operation ends.

The pressure state in the decompression chamber 30 during the ion implantation operation performed under the above-mentioned conditions is shown in FIG.
As in (B). As shown in the figure, it can be seen that the pressure in the chamber 30 within the predetermined time T after the start of the ion implantation is almost equal to the pressure in the chamber 30 after the time T has elapsed. This is because by controlling so that a relatively low first beam current value L1 is obtained within the time T, gas is gradually generated from the resist film of the semiconductor wafer W, and due to the balance with the exhaust of the chamber 30, This is because the pressure in the chamber 30 can be made lower than in the conventional case. Further, after the lapse of time T, the amount of gas generated from the resist film becomes smaller than that immediately after the start of ion implantation, so control is performed so that a second beam current value higher than the first beam current value L1 is obtained. However, the pressure in the chamber 30 is not increased so much.

Therefore, the first beam current value L1 is set to the chamber 30 within the time T after the start of ion implantation.
The internal pressure is relatively low such that the internal pressure becomes equal to or lower than the chamber internal pressure after the lapse of time T even if the resist film formed on the semiconductor wafer W is pressurized by a gas generated by vaporization by heat of ions. The current value is selected. In other words, by setting the ion implantation conditions so that the first beam current value L1 is measured, the chamber 3 within the time T
The pressure within 0 is set to a pressure at which the frequency of collision with the ion beam is low, such as a vacuum higher than that of the 10 ⁻⁵ Torr level, and is set to a pressure that does not affect the measurement accuracy of the ammeter 35 or the dose counter 37. It Thereby, the amount of impurities implanted into the semiconductor wafer W can be controlled with high accuracy. This control is ensured by making the pressure in the chamber 30 within the time T lower than that in the conventional case and by making the measurement accuracy of the beam current measured by the ammeter 35 high. Also, time T
Since a relatively bulky vacuum can be maintained inside, the suppress electrode 3
No discharge from 8 also occurs.

As the time T during which the first beam current value L1 is maintained, a time is selected by which the beam current value is lowered and the adverse effect of the gas generated from the resist film is gradually eliminated. For example, if the total time required for ion implantation is 10 minutes, this time T may be about 2 to 3 minutes.

Although the beam current value is controlled in two steps in the above embodiment, it may be set in a plurality of steps within the time T, or the rising speed of the measured beam current value may be slower than in the prior art. Alternatively, the beam current value may be controlled to continuously increase.

Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIGS. 4 and 5. This second embodiment apparatus also includes a CPU 50, an operation panel 55, and a memory 54 as the reaching ion amount control means. The operation panel 52 is similar to the first embodiment in that it inputs the first and second beam current values L1 and L2 and the time T shown in FIG.
The PU 50 stores this information in the memory 54. This second
The CPU of the embodiment is different from that of the first embodiment.
0 is that the beam current from the ammeter 35 is monitored and feedback-controlled. Therefore, the CPU 50
Is a comparator 56 that compares the beam current value from the ammeter 35 with the first and second beam current values L1 and L2 input through the operation panel 52 and stored in the memory 54, and the comparison result. And a controller 58 for changing the parameters of the ion source 10 or the slit width of the variable slit 18 based on the above. This controller 5
8 can also control the end of the ion implantation operation based on the information from the dose counter 37. Further, a timer 59 is connected to the controller 58, and it is determined whether or not a time T has elapsed after the start of ion implantation.

The operation of this second embodiment device is shown in the flow chart of FIG. In step 1 of FIG.
First, the CPU 50 sets the first beam irradiation condition, that is, the condition that the preset first beam current value L1 is obtained, in the ion source 10 or the variable slit 18. The ion implantation operation is started under this condition, the beam current is measured by the ammeter 35 based on the ion beam passing through the Faraday cup 36, and the measured beam current value is sequentially input to the CPU 50. CPU50
In step 2, the measured beam current value is compared with the beam current value L1 as the first beam irradiation condition. Then, when the measured beam current value deviates from the first beam current value L1, the CPU 50 changes the parameter of the ion source 10 in step 3 or changes the slit width of the variable slit 18.

When the operation of step 3 is completed or when the determination of step 2 is NO, the process proceeds to step 4, and the CPU 50, based on the information from the timer 59, elapses after the start of the ion implantation operation. It is determined whether or not the time exceeds the preset time T input in advance via the operation panel 52. If the determination in step 4 is NO, the process returns to step 2, and if the determination is YES, the process proceeds to step 5. If the determination in step 4 is YES, in step 5, the CPU 50 sets the parameters of the ion source 10 or the slit width of the variable slit 18 to the second beam irradiation condition, that is, the condition that the second beam current value L2 is obtained. To set. After that, the CPU 50
It is determined whether or not the beam current value obtained from the ammeter 35 has deviated from the second beam current value L2 (step 6). If this determination is YES, in step 7 as in step 3 Change the parameter or slit width. When the determination in step 6 is NO, or after the operation in step 7 is completed, the CPU 50 determines in step 8 whether or not the preset impurity implantation amount has been reached based on the information from the dose counter 37. To do. If the determination in step 8 is NO, the process returns to step 6, and if the determination in step 8 is YES, all the operations are completed.

The pressure change in the decompression chamber 30 during the operation of the apparatus of the second embodiment is almost the same as that of the first embodiment as shown in FIG.
As in (B), the same effects as those of the first embodiment can be obtained. In addition to this, in the second embodiment, the parameter for controlling the reaching ion amount is changed so as to reduce the level difference between the measured value of the beam current and the set value, so that the decompression chamber 30 within the set time T is changed. It is possible to reliably suppress an increase in internal pressure. In the second embodiment, the feedback control is performed based on the beam current measured by the ammeter 35. However, the beam current value which is the basis of this control has a small measurement error even within the set time T. Therefore, reliable control can be executed.

Third Embodiment Next, a third embodiment of the present invention will be described with reference to FIGS. The third embodiment device has a CPU 60, an operation panel 62 and a memory 64 similarly to the second embodiment device, but a pressure sensor 66 arranged inside the decompression chamber 30 is connected to the CPU 60. . Unlike the second embodiment, the operation panel 62 sets the target pressure value L3 in the decompression chamber 30 shown in FIG. 7A, and the CPU 60 stores this pressure value L3 in the memory 64. The pressure value L3 is set to a pressure value that has little influence on the measurement accuracy of the beam current measured by the Faraday cup 36. The CPU 60 compares the pressure value L3 stored in the memory 64 with the measurement value monitored by the pressure sensor 66, and the ion source 10 in a direction to reduce the pressure level difference which is the comparison result. And a controller 69 for changing the parameter or the slit width of the variable slit 18.

In the third embodiment, unlike the first and second embodiments, the set time T is not set, and the decompression chamber 3 which directly affects the measurement error of the beam current is used.
The pressure inside 0 is monitored. The operation is shown in FIG.
Is shown in the flowchart. First, the CPU 60
In step 1 of FIG. 8, control is performed so that ion beam irradiation is performed under initial setting conditions. This initial setting condition can be set to the first beam current value L1 as shown by the solid line in FIG. 7B, as in the first and second embodiments. Alternatively, as shown by the broken line in FIG. 7B, it may be set to a value higher than the first beam current value L1.

Next, the CPU 60 compares the pressure value L3 stored in the memory 64 in step 2 with the measurement value measured by the pressure sensor 66. And CPU60
Changes the parameters of the ion source 10 or the slit width of the variable slit 18 only when there is a difference in this pressure level (step 3). The CPU6
For 0, the level difference may be registered in advance, and control may be performed without changing the parameters and the like if the level difference between the measured value and the set value is within the registered level difference. The operations in steps 2 and 3 are performed until the amount of impurities implanted in the semiconductor wafer in step 4 reaches the set value, and all operations are completed when the determination in step 4 is YES.

According to the third embodiment, the decompression chamber 3 is raised due to the gas generated from the resist film of the semiconductor wafer W within the time immediately after the start of the ion implantation operation.
The pressure within 0 can be set to be equal to or lower than the pressure within the chamber after a predetermined time has elapsed immediately after the start of ion implantation.
Moreover, the pressure level L3 to be monitored is the ammeter 35.
Since the pressure value is set so as to have a small influence on the measurement accuracy of the beam current in the above, it is possible to reliably prevent the measurement error of the amount of impurities implanted into the semiconductor wafer W.

The present invention is not limited to the above-described embodiment, and various objects such as ion-implanted objects to be processed are not limited to semiconductor wafers. In addition, the present invention provides a device without an accelerating tube and a Faraday cup 36.
It can also be applied to a device in which is arranged on the back side of the rotating disk. Furthermore, although each of the above-described embodiments is a batch processing type ion implantation apparatus, it can be applied to a single wafer processing apparatus that implants ions one by one into an object to be processed.

[0033]

As described above, according to the present invention,
Even immediately after the start of ion implantation, the pressure in the decompression chamber in which the object to be processed is placed can be maintained at a pressure value that has little effect on the measurement accuracy of the beam current. Even with the apparatus, it is possible to accurately measure the amount of impurities implanted into the object to be processed.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the overall configuration of an ion implantation apparatus according to a first embodiment of the present invention.

FIG. 2A is a characteristic diagram showing the reaching ion amount control condition in terms of the relationship between the time axis and the beam current value, and FIG.
FIG. 6 is a characteristic diagram showing a pressure change in the decompression chamber when the operation is executed under the condition of.

FIG. 3 is a flow chart for explaining the operation of the first embodiment device shown in FIG.

FIG. 4 is a block diagram showing a main part of an ion implantation apparatus according to a second embodiment of the present invention.

FIG. 5 is a flow chart for explaining the operation of the second embodiment device shown in FIG.

FIG. 6 is a block diagram showing a main part of an ion implantation apparatus according to a third embodiment of the present invention.

FIG. 7A is a characteristic diagram showing the reaching ion amount control conditions of the apparatus of the third embodiment in terms of the relationship between the time axis and the pressure value in the chamber, and FIG. 7B is the condition of FIG. It is a characteristic view of the beam current measured below.

FIG. 8 is a flow chart for explaining the operation of the device of the third embodiment.

FIG. 9A is a characteristic diagram showing the reaching ion amount control condition of a conventional ion implanter in terms of the relationship between the time axis and the beam current, and FIG. 9B is under the condition of FIG. It is a characteristic view of the pressure in the chamber when it is carried out.

[Explanation of symbols]

 10 ion source 14 extraction electrode 16 slit 18 variable slit 20 mass analyzer 22 slit 24 accelerator 30 decompression chamber 35 ammeter 36 Faraday cup 37 dose counter 40, 50, 60 CPU 42, 52, 62 operation panel 44, 54, 64 memory 56,68 Comparator 58,69 Controller 59 Timer 66 Pressure sensor

Claims (6)

[Claims] 1. An ion implantation method in which an object to be processed placed in a decompression chamber is irradiated with an ion beam to inject ions, wherein the amount of ions reaching the object to be processed per unit time is The chamber internal pressure that rises due to the gas generated from the object to be processed within the predetermined time is made equal to or less than the chamber internal pressure after the predetermined time elapses, which is different between the predetermined time and the other time. The ion implantation method is characterized in that 2. An object to be processed placed in a decompression chamber is irradiated with an ion beam extracted from an ion source to implant ions, and a beam current based on the reached ion amount is measured to determine the ion implantation amount. In the controlled ion implantation apparatus, a reaching ion amount control means for increasing the amount of ions reaching the object to be processed per unit time stepwise or continuously is provided for a predetermined time after the start of ion implantation, and within the predetermined time. An ion implantation apparatus, wherein the chamber internal pressure, which rises due to the gas generated from the object to be processed, is set to a pressure value or less that has little influence on the measurement accuracy of the beam current. 3. The ion implantation apparatus according to claim 2, wherein the reaching ion amount control unit includes a unit that changes and controls a parameter that controls an amount of generated ions in the ion source. 4. The arriving ion amount control means according to claim 2, characterized in that it comprises means for varying a beam passage amount in the course of an ion beam path from the ion source to the object to be processed. Ion implanter. 5. The reaching ion amount control means according to claim 3 or 4, wherein the beam current value measured within the predetermined time period is lower than the beam current value measured during other time periods. The ion implantation apparatus is characterized in that the parameter or the beam passage amount is set so that 6. The detection device according to claim 3, further comprising: a detection unit that detects the pressure in the chamber, and a comparison unit that compares a pressure value from the detection unit with a preset pressure value. The reaching ion amount control means changes the parameter or the beam passing amount based on the comparison result from the comparison means.