JPH08124886A - Method and apparatus for grinding semiconductor device - Google Patents

Abstract

PURPOSE: To remove irregularities generated in a wafer surface when forming a wiring. CONSTITUTION: First, a metal film 15 is ground using abrasive liquid for a metal, and when a torque signal is gradually increased and reaches a reference signal level a supply material of the abrasive liquid is exchanged from the abrasive liquid for a material to neutral abrasive liquid. In the neutral abrasive liquid, the metal film 15 is ground at the almost same degree of grinding speed as an insulating film 13. The grinding by the neutral abrasive liquid is continued until rotation torque becomes in a balanced state. When the torque becomes a specific value in a balanced state, the supply material of the abrasive liquid is exchanged from the neutral abrasive liquid to alkali abrasive liquid for an insulating film, and the insulating film 13 is ground for a specified period.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chemical mechanical polishing (CMP) method for flattening irregularities generated on a surface of a substrate wafer when wiring is formed in the middle of a wafer process for manufacturing a semiconductor integrated circuit device.
g) The present invention relates to a polishing method and a polishing apparatus used therefor.

[0002]

2. Description of the Related Art In a semiconductor integrated circuit device having multi-layered wiring, a lower layer wiring is formed on a substrate, an insulating film is formed on the lower layer wiring, and the lower layer wiring and the upper layer wiring are connected to the insulating film. Therefore, after forming the connection hole, a metal film is deposited on the insulating film, and the metal film is patterned by a lithography process and an etching process to form an upper wiring. At that time, the surface of the insulating film has unevenness due to the existence of the lower layer wiring, and if the upper layer wiring is formed as it is, the lithography process for forming the upper layer wiring must be performed on the uneven metal film. Therefore, not only is it difficult to form a fine pattern, but there is a possibility that a leak may occur in the insulating film and the upper wiring may be cut. for that reason,
It is preferable that the surface of the insulating film which is the base for forming the upper wiring is as flat as possible.

As one of such flattening techniques, a connection hole is formed in an insulating film, a metal film is deposited, and then the metal film is subjected to chemical mechanical polishing to form the metal film only in the connection hole. It has been proposed to leave the surface of the insulating film flat (see Japanese Patent Application Laid-Open No. 62-102543). In the method introduced in the cited example, a trench groove or a connection hole is formed in an insulating film having a flat surface, a metal film is deposited on the insulating film, and the metal film is chemically etched to a height at which the insulating film is exposed. It is a method of flattening the surface by mechanical polishing.
The present invention relates to a method and an apparatus for flattening a base for forming an upper layer wiring by embedding a metal film in a connection hole formed in an insulating film as in this reference, but in the reference, the metal film is embedded. It is premised that the insulating film is entirely composed of flat surfaces having the same height.

[0004]

Considering the wiring of the current semiconductor integrated circuit device, the surface of the insulating film forming the connection hole is not always formed by a plane having the same height. Since the patterned lower layer wiring exists in the lower layer of the insulating film, the surface of the insulating film has unevenness.
Therefore, when the method of the cited example is applied to the one in which a metal film is deposited on an insulating film having unevenness on the surface, polishing may be performed so that the metal film remains in the connection hole at the connection hole portion. Since there is also unevenness in the portion, the metal film partially remains in the concave portion, and cannot be used as a base for the upper layer wiring.

According to the present invention, a connection hole is formed in an insulating film formed on a substrate wafer, a metal film is deposited from the connection hole onto the insulating film by a technique such as blanket tungsten, and then a metal is embedded in the connection hole. It is an object of the present invention to provide a chemical mechanical polishing method for flattening the surface of an insulating film together with a metal buried in a connection hole and an apparatus therefor.

[0006]

According to the polishing method of the present invention, an insulating film covering a substrate wafer is formed, and the insulating film has a connection hole with an upper layer wiring at a predetermined position in a region to be connected to a lower layer. After forming a metal film for filling the connection hole from the connection hole to the insulating film, a load is applied between the surface of the substrate wafer and the polishing pad to bring them into contact with each other, and a polishing liquid is supplied between them. While at least one of the substrate wafer and the polishing pad is rotated at a constant speed to flatten the surface of the substrate wafer, the polishing step first uses a polishing liquid as a metal polishing liquid, and After continuing polishing until the rotation torque of the pad reaches a predetermined level,
The feature is that the polishing liquid is switched to a neutral polishing liquid containing no chemically reactive additive, and polishing is continued until the increase of the rotation torque is stopped. After polishing with a neutral polishing liquid, it is preferable to switch the polishing liquid to an alkaline polishing liquid for insulating film to carry out polishing for a certain period of time so that the metal embedded in the connection hole protrudes from the surface of the insulating film. Come to do. When switching the polishing liquid, it is preferable to wash the polishing pad before switching.

In the polishing apparatus of the present invention, the upper surface is a horizontal surface, a polishing pad is provided on the horizontal surface, the polishing surface plate that rotates in the horizontal surface and the back surface of the wafer substrate are held, and the front surface of the wafer substrate is polished. The rotation speed of the head mechanism is kept constant by inputting the head mechanism that presses against the pad and rotates in the horizontal plane under load, the rotation speed detection unit that detects the rotation speed of the head mechanism, and the detection signal of the rotation speed detection unit. The drive control unit that controls the rotation so as to maintain the output signal from the drive control unit to the head mechanism is detected, and the detected output signal is compared with a predetermined reference signal or reference condition to determine the reference signal. A comparison unit that generates an output signal when it exceeds or exceeds a reference condition, and at least one of a metal polishing liquid and a neutral polishing liquid is selected and supplied onto the polishing platen, and the output of the comparison unit In response to the And a polishing liquid supply section having means for switching the type of the supplied polishing liquid.

In a preferred mode, a constant level is set as the reference signal in the comparison section, and an output signal is generated when the detection signal of the rotation speed detection section reaches the reference signal level. The unit first supplies the polishing liquid for metal, receives the output signal from the comparison unit, and switches the supply of the polishing liquid to the neutral polishing liquid.

In a further preferred aspect, the comparison section is further set so that the signals become balanced as a reference condition, and the first output generated when the detection signal of the rotation speed detection section reaches the reference signal level. After the signal, the second output signal is generated when the detection signal of the rotation speed detection unit reaches the equilibrium, and the polishing liquid supply unit uses the polishing liquid for metal as the polishing liquid, the neutral polishing liquid, and the insulating liquid. It supplies one selected from the film-forming alkaline polishing liquids, switches from the metal polishing liquid to the neutral polishing liquid in response to the first output signal from the comparison unit, and outputs the second output from the comparison unit. Upon receiving the signal, the supply of the polishing liquid is switched to the alkaline polishing liquid for the insulating film.

In another preferred mode, the comparison section sets that the differential signal is 0 as a condition for generating the second output signal, and after the generation of the first output signal, the detection signal of the rotation speed detection section. Is differentiated and a second output signal is generated at the time when the differentiated signal changes from negative to positive.

In the present invention, at least two types of polishing liquids, that is, a metal polishing liquid and a neutral polishing liquid are used, and preferably three types of polishing liquids including an insulating film alkaline polishing liquid are used. The metal-polishing liquid is a polishing liquid prepared so that the polishing rate for the metal film is higher than that for the insulating film. As such a metal-polishing liquid, an acid is used to p.
It is preferably adjusted to pH 3.0 or less so that H becomes acidic, and about 5 wt% of alumina fine particles having an average diameter of about 80 nm are suspended as a polishing agent, or a metal polishing accelerator according to a metal to be polished. There is an abrasive in which silica fine particles having an average diameter of about 30 nm are suspended in an aqueous solution added with.

The alkaline polishing liquid for an insulating film is a polishing liquid having a higher polishing rate for the insulating film than that for the metal film, and the pH is adjusted to 7 by adding potassium hydroxide (KOH) or ammonia (NH ₃ ). It is prepared to have a larger size, preferably pH 10 to 11, and SiO ₂ fine particles having an average diameter of about 30 nm are suspended therein. The metal polishing liquid or the insulating film alkaline polishing liquid chemically alters the surface of the object to be polished, and the altered layer is mechanically removed by the fine particles of the abrasive,
The polishing rate of an object to be polished which is affected by the pH of the polishing liquid and the metal polishing accelerator is increased. When chemically modified, the affinity of the suspended abrasive with the fine particles is increased and the polishing rate is increased.

On the other hand, the neutral polishing liquid does not have such a chemical action, and only mechanical polishing by the suspended polishing material works, and the polishing rate difference between the metal film and the insulating film is small. And both are polished at the same speed.
The metal polishing liquid or the insulating film alkaline polishing liquid can obtain a good polishing rate even if the average particle diameter of the abrasive particles is 100 nm or less, but the neutral polishing liquid has a chemical action. The polishing rate is slower because it is not present. for that reason,
In order to increase the polishing rate of the neutral polishing liquid, the abrasive of the neutral polishing liquid has a larger particle size than that of the metal polishing liquid or the insulating film alkaline polishing liquid, and the average particle size is 10
Alumina particles having an average particle size of 0 nm to 1 μm, for example, about 250 nm. The connection hole in which the metal is embedded in the present invention includes not only the via hole but also the contact hole. Therefore, the insulating film to be planarized includes not only the interlayer insulating film between the metal wirings but also the insulating film in which the contact hole is formed.

FIG. 1 shows the polishing apparatus of the present invention. Reference numeral 1 is a polishing surface plate, the upper surface of which is a horizontal surface, and is rotated in the horizontal surface by the rotating shaft 1a. The polishing pad 1b is attached to the upper surface of the horizontal surface. The back surface side of the semiconductor wafer 2 to be polished is adsorbed and held by the wafer chucking head 3. A load is applied to the wafer chucking head 3 so as to press the surface of the wafer 2 against the polishing pad 1b, and the head 3 is rotated by a motor 4 to polish the surface of the wafer 2. A rotation monitor 5 is provided on the rotation shaft of the head 3 as a rotation speed detection unit that detects the rotation speed of the head 3 by an encoder or the like. 6 is a motor 4 so that the rotation speed of the motor 4 is constant.
Is a drive control unit for controlling the rotation speed of the head 3 by inputting a detection signal of the rotation monitor 5 and performing negative feedback control of the current flowing to the motor 4. 6a is a power supply for supplying a drive current to the motor 4 via the drive control unit 6.

Reference numeral 8 denotes a polishing liquid supply unit, which can supply at least the metal polishing liquid and the neutral polishing liquid by switching. In this example, the insulating film alkaline polishing liquid is also switched to supply the nozzle 9 with the polishing liquid. Can be supplied onto the polishing pad 1a. The polishing liquid supply unit 8 is the comparison unit 7.
Is provided with a solenoid valve for switching and supplying the polishing liquid according to the output signal. The drive control unit 6 of the comparison unit 7 is the head 3
The signal 10 corresponding to the current flowing to the motor 4 is maintained as a voltage value, for example, in order to keep the rotation speed of the motor constant. The signal 10 is a torque signal for rotating the head 3 at a constant speed. A reference signal or a reference condition is set in advance in the comparison unit 7, and the comparison unit 7 uses the drive control unit 6
Signal 10 is compared with a reference signal or a reference condition, and an output signal is generated in the polishing liquid supply unit 8 to switch the type of polishing liquid supplied by the polishing liquid supply unit 8.

In order to explain the operation of the present invention, a process of flattening an interlayer insulating film for a first-layer metal wiring to a second-layer metal wiring of a semiconductor device having multi-layer wiring will be described with reference to FIGS. 2 and 3. Explain. FIG. 2A shows a semiconductor substrate (wafer) 1 on which semiconductor devices such as MOS transistors are built.
1, a base metal wiring 12 is formed, an interlayer insulating film 13 covering the base metal wiring 12 is formed, and a connection hole 14 with the upper metal wiring is formed in the interlayer insulating film 13 at a predetermined position on the lower metal wiring. The metal film 15 for filling the connection hole is formed from the connection hole 14 to the interlayer insulating film 13. As an example of the process up to this state, a metal film such as an aluminum alloy is formed on the underlying substrate 11 by a sputtering method or the like, and patterned by lithography and etching to form the metal wiring 12. It is covered with SiO ₂ or PS by the CVD method or the like.
An interlayer insulating film 13 such as a G film is deposited, and a connection hole 14 is formed in the interlayer insulating film 13 by lithography and etching. A blanket metal film 1 formed by a CVD method of tungsten or the like so as to cover the interlayer insulating film 13 from the connection hole 14.
5 is formed.

First, the blanket metal film 15 is polished using a metal polishing liquid. When polishing is performed while keeping the number of revolutions of the head 3 in FIG. 1 constant, the torque signal 1
0 gradually increases as shown in FIG. FIG.
The period up to a in (A) is a period in which the rotational torque increases as the convex portion of the metal film 15 is polished and the contact area with the polishing pad increases, and eventually the convex portion of the metal film disappears. When it becomes flat, the torque becomes constant (sections a and b). After that, the convex portions of the underlying interlayer insulating film 13 are eventually exposed. At this time, the rotational torque applied to the head 3 changes as the material to be polished changes. The torque change varies depending on the material of the object to be polished, the type of pad, or the type and concentration of the polishing liquid.When a polishing liquid for metals is used, the polishing speed for metal films is high, but the polishing speed for insulating films is low When the state of FIG. 2 (B) is reached, the rotational torque of the head begins to rise as in the section after b in FIG. The L1 level is set as the reference signal level for generating the first output signal in the comparison unit 7, and when the rotational torque of the head fetched as the output signal 10 of the drive control unit 6 reaches the L1 level. The comparison unit 7 outputs a first output signal to the polishing liquid supply unit 8, and the polishing liquid supply unit 8 receives the first output signal and switches the supply of the polishing liquid from the metal polishing liquid to the neutral polishing liquid. .

In the neutral polishing liquid, the metal film 15 and the interlayer insulating film 1
3 is polished at the same polishing rate, and the state shown in FIGS. 2B to 2C is obtained. Until this state (C) is reached, the rotational torque of the head continuously changes according to the abundance ratio of the metal film and the insulating film. 2B to 2C, the ratio of the metal film decreases and the polishing ratio of the insulating film increases. Since the insulating film has a slightly lower polishing rate, the rotational torque is as shown in FIG. It increases like the increase curve ("proportional signal") of B).

When the stage shown in FIG. 2C is reached, the polishing rate of the metal film and the insulating film becomes constant, so that the rotational torque of the head becomes a constant value and becomes a constant value. The fact that the rotational torque has reached a constant value in the equilibrium state is detected by comparing it with the condition set in the comparison unit 7, and a second output signal is output to the polishing liquid supply unit 8 so that the polishing liquid supply unit 8 outputs the second output signal. The supply of the polishing liquid is switched from the neutral polishing liquid to the alkaline polishing liquid for the insulating film. Since the polishing rate of the interlayer insulating film 13 is higher than the polishing rate of the metal film 15 in the alkaline polishing liquid for insulating film, if polishing with this alkaline polishing liquid is continued for a certain period of time, as shown in FIG. 2 (D). The metal embedded in the contact hole is projected from the surface of the interlayer insulating film 13, the surface roughness of the interlayer insulating film 13 is reduced, and the metal film on the interlayer insulating film 13 is completely removed.

The polishing after the polishing liquid is switched to the insulating film alkaline polishing liquid is controlled by the time, and as shown in FIG. 3C, the polishing is set to be performed for a certain time t. In the state of FIG. 2D, the metal film embedded in the connection hole is effective in increasing the contact area between the upper layer metal and the metal in the connection hole, and good electrical contact can be obtained. .

In the polishing with the neutral polishing liquid shown in FIGS. 2B to 2C, the polishing area of the metal film 15 gradually decreases and the polishing area of the insulating film 13 gradually increases. The rotational torque of the head begins to rise as shown by the curve ("proportional signal") in FIG. 3 (B) and becomes constant when the metal film 15 is in the state shown in FIG. 2 (C) only in the connection hole. Becomes The lower curve in FIG. 3 (B) represents the differential waveform of the upper torque signal. The differential waveform changes from positive to negative when the rising gradient of the torque signal decreases, and then the differential signal rises from negative to zero again as the torque signal reaches a balanced state. When the differential signal rises from negative to zero and passes through 0, it is detected by the condition set in the comparison unit 7, and the second output signal is supplied to the polishing liquid supply unit 8 to supply the polishing liquid to the alkaline polishing for insulating film. You may make it switch to a liquid. This makes it possible to more reliably output the second output signal for switching the polishing liquid to the alkaline polishing liquid for insulating film.

[0022]

The present invention will be described as a more specific example.
The polishing sample shown in FIG. 2A is as follows. Approximately 300 SiO ₂ film is deposited on a silicon wafer by the CVD method.
After depositing to a thickness of nm, an aluminum alloy film (Al-Si-Cu film) containing a small amount of Si and Cu is deposited to a thickness of about 500 nm by a sputtering method in order to form the lower metal wiring 12. , And patterned into a line and space of 1.0 μm by photolithography and etching. After that, CVD is performed as the interlayer insulating film 13.
A SiO ₂ film was deposited to a thickness of about 1.2 μm by the method, and a square connection hole 14 having a side of 0.6 μm was formed on the lower metal wiring 12 by photolithography and etching.
Then, a blanket metal film 15 of tungsten was deposited on the interlayer insulating film 13 by the CVD method to a thickness of about 700 nm to obtain a polishing sample shown in FIG.

As the metal polishing liquid for polishing the metal film 15, a polishing liquid prepared to have a pH of 3.0 and having 5 wt% of alumina fine particles having an average particle diameter of 80 nm suspended therein was used. 2B is detected from the rotational torque of the head 3, the comparison unit 7 obtains a first output signal, and the polishing liquid supply unit 8 switches the polishing liquid to the neutral polishing liquid. As the neutral polishing liquid, pure water in which alumina fine particles having an average particle diameter of about 250 nm were suspended was used.

After that, the state shown in FIG. 2C is reached, and the polishing liquid supply unit 8 switches the supply of the polishing liquid to the alkaline polishing liquid for the insulating film by the second output signal from the comparison unit 7. As the alkaline polishing liquid for use, one prepared by suspending SiO ₂ fine particles having a pH of 10.3 and an average particle diameter of 30 nm was used. At the stage of switching the polishing liquid from the neutral polishing liquid to the alkaline polishing liquid for insulating film, the switching may be performed as it is, but since the particle size of the fine particles in the polishing liquid changes from large to small, the polishing liquid is insulated. When switching to the film-forming alkaline polishing liquid, it is preferable to stop the supply of the polishing liquid once before the switching and clean the polishing pad 1b while flowing pure water to the polishing pad 1b. As a result, the finishing accuracy of the surface polishing of the interlayer insulating film 13 is improved.

The final polishing performed by supplying the alkaline polishing liquid for the insulating film was set for a fixed time. In this final polishing, there is almost no change in the rotational torque of the head, so control is performed only by time.
Polishing of the interlayer insulating film 13 of the iO ₂ film can perform more stable polishing than polishing both the metal film 15 and the interlayer insulating film 13 with a neutral polishing solution at the same time. It will be of high quality. When switching the polishing liquid, the polishing pad does not have to be washed, but not only when switching from the neutral polishing liquid to the alkaline polishing liquid for the insulating film as described above, but also when switching from the metal polishing liquid to the neutral polishing liquid. The polishing may be performed at any time, and a better polishing result can be obtained.

[0026]

According to the present invention, a contact hole is formed in an insulating film having irregularities, and a metal film for filling the contact hole is formed on the insulating film. Initially, metal polishing is mainly performed to polish the metal film. The metal film and the insulating film are both polished by polishing with a liquid and switching to a neutral polishing liquid when the insulating film is exposed, so when the connection holes used for the multilayer wiring of the semiconductor device are filled with metal, It is possible to remove the metal on the insulating film in the part other than the connection hole by a series of processes, which leads to simplification of this kind of process,
Furthermore, since the insulating film can be flattened, the reliability of the upper layer wiring formed thereon is also improved.

When the embedded metal film and the insulating film are simultaneously polished to flatten the surface, and then the polishing liquid is changed to an alkaline polishing liquid for an insulating film to polish the insulating film mainly, the embedded metal is projected from the surface of the insulating film. When the upper layer wiring is formed, the area of the contact portion between the embedded metal and the upper layer wiring metal can be increased, and the resistance value at this portion can be reduced to improve the reliability. If the polishing pad is washed when the polishing liquid is switched, the polishing material in the previous polishing step can be removed and polishing can be performed with the desired polishing liquid, and the polishing quality can be improved. In particular, when the final insulating film is polished with the insulating film alkaline polishing liquid, the roughness of the insulating film surface can be improved by cleaning the polishing pad when switching from the neutral polishing liquid to the insulating film alkaline polishing liquid. Can be reduced, and the finishing accuracy at the process stage can be improved.
The polishing apparatus of the present invention has a function for carrying out the polishing method of the present invention, and can achieve the above-described effects.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an apparatus of the present invention.

FIG. 2 is a sectional view of a semiconductor integrated circuit device showing a polishing step in the present invention.

FIG. 3 is a diagram showing a rotation torque in the present invention.

[Explanation of symbols]

 1 polishing surface plate 1b polishing pad 2 substrate wafer 3 wafer chucking head 4 motor 5 rotation monitor 6 drive control unit 7 comparison unit 8 polishing liquid supply unit 10 rotation torque signal 11 substrate 12 lower layer wiring 13 insulating film 14 connection hole 15 connection hole Metal film for embedding

Claims (7)

[Claims] 1. An insulating film covering a substrate wafer is formed, and a connecting hole for connecting an upper layer wiring is formed at a predetermined position in a region where a lower layer is connected to the insulating film. After forming the metal film for burying the connection holes over the surface of the substrate wafer and the polishing pad, a load is applied to bring them into contact with each other, and at least one of the substrate wafer and the polishing pad is supplied while supplying a polishing liquid between them. In the polishing method of rotating the substrate wafer at a constant speed to flatten the surface of the substrate wafer, the polishing liquid is first used as a metal polishing liquid, and after polishing is continued until the rotation torque of the substrate wafer or the polishing pad reaches a predetermined level, A polishing method characterized in that the polishing liquid is switched to a neutral polishing liquid containing no chemically reactive additive, and polishing is continued until the increase of the rotation torque stops. 2. The polishing method according to claim 1, wherein after polishing with a neutral polishing solution, the polishing solution is further switched to an alkaline polishing solution for an insulating film and polishing is performed for a certain period of time. 3. The polishing method according to claim 1, wherein when the polishing liquid is switched, the polishing pad is washed before the switching. 4. An upper surface is a horizontal surface, a polishing pad is provided on the horizontal surface, a polishing surface plate that rotates in the horizontal surface, and a back surface of a wafer substrate are held, and the front surface of the wafer substrate is pressed against the polishing pad. A head mechanism that applies a load to rotate in the horizontal plane, a rotation speed detection unit that detects the rotation speed of the head mechanism, and a detection signal from the rotation speed detection unit are input, and rotation is performed to keep the rotation speed of the head mechanism constant. The drive control unit that controls the output signal from the drive control unit to the head mechanism is detected, and the detected output signal is compared with a predetermined reference signal or reference condition, and the reference signal is exceeded or the reference condition is exceeded. A comparison unit that generates an output signal when it is satisfied, and at least one of a metal polishing liquid and a neutral polishing liquid is selected and supplied onto the polishing surface plate, and is also supplied according to the output of the comparison unit. Polishing liquid Polishing apparatus characterized by comprising a, a polishing liquid supply section having means for switching the type. 5. The comparison unit sets a constant level as a reference signal and generates an output signal when the detection signal of the rotation speed detection unit reaches the reference signal level, and the polishing liquid supply unit The polishing apparatus according to claim 4, wherein the polishing liquid for metal is first supplied, and the supply of the polishing liquid is switched to the neutral polishing liquid in response to the output signal from the comparison unit. 6. The comparison unit is further set as a reference condition that the signals are balanced, and after the first output signal generated when the detection signal of the rotation speed detection unit reaches the reference signal level. The second output signal is generated when the detection signal of the rotation speed detection section reaches the equilibrium. The polishing solution supply section uses the polishing solution for metal as the polishing solution, the neutral polishing solution, and the alkaline solution for the insulating film. It supplies a selected one of the polishing liquids, switches from the metal polishing liquid to the neutral polishing liquid in response to the first output signal from the comparison unit, and receives the second output signal from the comparison unit. The polishing apparatus according to claim 5, wherein the supply of the polishing solution is switched to the alkaline polishing solution for an insulating film. 7. The comparison unit sets, as the condition for generating the second output signal, that the differential signal becomes 0 instead of the equilibrium condition, and after the generation of the first output signal, the rotation speed detection unit The polishing apparatus according to claim 6, wherein the detection signal is differentiated, and the second output signal is generated when the differential signal changes from negative to positive.