JP2953395B2 - Sputtering equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sputtering apparatus for forming a thin film of a metal or an insulator on a semiconductor substrate, and more particularly to a sputtering apparatus having a substrate temperature control mechanism for controlling the temperature of the semiconductor substrate during sputtering. Things.

[0002]

2. Description of the Related Art Along with the high integration of semiconductor integrated circuits, in a sputtering process in a semiconductor manufacturing process, it is required to control the temperature of a semiconductor substrate with high precision in order to improve and stabilize film formation quality. ing.

[0003] Generally, the sputtering apparatus is provided with a heating mechanism for controlling the substrate temperature. Regarding this heating mechanism, various improvements have been proposed so far, such as increasing the temperature rising / falling rate from the viewpoint of improving productivity, and improving the film quality by improving the accuracy of the temperature distribution.

FIG. 4 shows an example of a sputtering apparatus provided with a substrate heating mechanism which has been often used in the past. This example is disclosed in JP-A-62-35517.

As shown in FIG. 1, the sputtering apparatus 1 generally includes a sputtering chamber 3 and a transfer chamber 7 connected to the sputtering chamber 3 via a gate valve 5. A cathode 9 and a revolving mechanism 11 are provided in the sputtering chamber 3, and a semiconductor substrate 17 is mounted on a substrate holder 15 held by the revolving mechanism 11. In the sputtering chamber 3, a substrate heating mechanism (heating lamp) 21 for heating the substrate 17 and a revolving mechanism 1 are provided.
1 is provided with a cooling mechanism 23 provided inside.

The cooling mechanism 23 moves in the direction of the arrow shown in FIG. 4 to contact the cooling member 23 a constituting the cooling mechanism with the back surface of the semiconductor substrate 17 to absorb heat and cool the substrate 17. . The above-mentioned cooling mechanism 23 is used for the semiconductor substrate 1
7, a film formation at a low temperature can be started in a shorter time when a film formation is performed at a low temperature after a film formation is performed at a high temperature.

The operation of the sputtering apparatus 1 will be described below. First, the semiconductor substrate 17 transported into the sputtering chamber 3 and having a temperature equal to room temperature is rotated from the position A in FIG. 4 to the position B in FIG. For example, it is heated to 400 ° C.

Next, the semiconductor substrate 17 is moved to the position C, and a first film is formed facing the cathode 9. When the first film formation is completed, the semiconductor substrate 17 is positioned at the position D by the rotation of the revolving mechanism 11, the cooling member 23a of the cooling mechanism 23 moves in the direction of the substrate holder 15, and the semiconductor substrate 17 is moved. It is cooled and cooled from 400 ° C. to a temperature required for the second film forming process, for example, 100 ° C. And
It moves so as to face the cathode 9 and performs the second film formation at the position D.

In the sputtering apparatus 1 having such a conventional substrate heating mechanism 21, the substrate heating mechanism 2
Since 1 and the sputtering mechanism are independently provided in different places, sputtering and heating cannot be performed simultaneously. Therefore, during sputtering, the substrate 17
It has been pointed out that the temperature cannot be controlled, the temperature changes during the sputtering process due to the influence of the thermal energy received from the plasma, and the reproducibility of the film quality is poor.

In addition, since the revolving mechanism 11 needs to be provided, there is a problem in terms of space efficiency, which has hindered miniaturization of the apparatus. On the other hand, to solve the problem of reproducibility of film quality, a sputtering apparatus having a gas heating mechanism has been proposed.

FIG. 5 schematically shows a sputtering apparatus having a gas heating mechanism. In this sputtering apparatus, the same parts as those in FIG.
The description is omitted. This apparatus is different from the sputtering apparatus shown in FIG. 4 in that the temperature of the substrate holder 15 is monitored by a built-in thermocouple 31, and the temperature is raised and lowered to a preset temperature. It is a point that is done.

In the case of temperature rise, heating is performed by a heater 33 built in the substrate holder 15, and in the case of temperature decrease, the power of the heater 33 is turned off and introduced from a cooling water pipe 35 built in the substrate holder 15. The cooling water is cooled by heat conduction.

After the substrate holder 15 is stabilized at the set temperature,
The semiconductor substrate 17 is transferred from the transfer chamber 7 to the substrate holder 15 facing the cathode 9 of the sputtering chamber 3. The semiconductor substrate 17 held by the substrate holder 15
The heating gas transferred by the heating gas supply pipe 3 is transferred to the semiconductor substrate 17 by using the heating gas carried by the heating gas introduction pipe 37 as a heat medium. The heating gas introduction pipe 37 is branched at its tip into a plurality of gas pipes 37a, 37a,.

The heat received from the plasma during sputtering is monitored by a thermocouple 31 attached to the substrate holder 15, and the actual temperature of the substrate is controlled by controlling the power of the heater 33 and exchanging heat with a cooling mechanism of cooling water. Controlling. With such a gas heating mechanism, it has become possible to stabilize the substrate temperature during sputtering with high accuracy.

However, in this sputtering apparatus, since the substrate holder 15 and its temperature control mechanism (heater 33, cooling water 35, thermocouple 35, heating gas 37) are integrated, it is necessary to switch the temperature conditions of the sputtering process. In some cases, it takes too much time to switch the temperature of the substrate holder 15 to the temperature of the next process and stabilize it, and there is still a problem that the substrate processing capability of the sputtering apparatus is reduced.

This is because the substrate holder 15 is integrated with the heating mechanism and the cooling mechanism. Therefore, in comparison between the heating / cooling capacity and the heat capacity of the holder, if the heating / cooling capacity is too high, the holder will not work. This is because it is difficult to keep the temperature stable and constant. On the other hand, if the heat capacity of the holder is too large, it takes too long time to reach a certain temperature by performing heating and cooling. Therefore, in this sense, at present, this gas heating mechanism has not been a drastic solution.

[0017]

That is, the example shown in FIG. 4 has a problem that the substrate temperature changes during the sputtering process. Therefore, it has been a problem to stabilize the temperature during sputtering and maintain good quality of the sputtered film. In addition, there is a problem that the space efficiency is deteriorated because the revolving mechanism of the substrate is required.

On the other hand, in the example shown in FIG. 5, it has been a problem to reduce the waiting time until the next sputtering is performed while changing the process conditions (set temperature).

In the sputtering apparatus of the present invention,
In view of the above problems, it is an object of the present invention to provide a sputtering apparatus which has a heating / cooling mechanism capable of stably maintaining a set temperature, and has a good space efficiency, while shortening a holder temperature switching time for each process condition.

[0020]

Means for Solving the Problems In order to solve the above-mentioned problems, the following means have been taken in the sputtering apparatus of the present invention. That is, the sputtering apparatus according to claim 1 includes a substrate holder that holds a semiconductor substrate,
Wherein disposed opposite to the semiconductor substrate held in the substrate holder, and the target to release the material to be deposited on the semiconductor substrate, for regulating the temperature of the substrate holder
An auxiliary block, the substrate holder and the target,
A vacuum processing chamber for accommodating the auxiliary block.
Ri, the substrate holder, and heating means for adjusting the temperature of the semiconductor substrate held by the substrate holder
Temperature for sensing the temperature of the cooling means and the substrate holder
A first heating / cooling mechanism comprising temperature sensing means, and the auxiliary block includes a heating means, a cooling means,
Temperature sensor for sensing the temperature of the auxiliary block
Second heating and cooling mechanism is provided comprising a grayed section, and the substrate holder and the auxiliary block, these relative
To the movable and driving mechanism, the contact and separable
Is a, during sputtering the accessory and the substrate holder
The first heating / cooling mechanism is separated from the auxiliary block.
The temperature of the semiconductor substrate to a predetermined sputtering temperature
Adjust the sputtering temperature of the semiconductor substrate
Before performing the operation, the second heating / cooling mechanism
Adjust the auxiliary block and the substrate holder
Relatively moving and contacting, the substrate holder
After the temperature is reached, the substrate holder and the auxiliary block
It is characterized in that it is separated from the hook .

In such a sputtering apparatus, the substrate holder and the auxiliary block are separated from each other during sputtering, and the first sputtering is performed in this state.

[0022] Then, before moving to the next second sputtering process, the temperature of the pre auxiliary block, previously adjusted to correspond to the substrate temperature in the second sputtering, after the first sputtering process has been completed The auxiliary block is brought into contact with the substrate holder using the driving mechanism. After the temperature adjustment, separate them again,
A second sputtering process is performed. Therefore, a continuous sputtering process performed at different temperatures can be rapidly performed. The sputtering temperature of the semiconductor substrate
When changing the substrate, the auxiliary block
Make sure that the temperature is different from the set temperature of the
Preferably. In particular, the auxiliary block includes the substrate
When raising the temperature of the holder,
Temperature higher than the temperature, the temperature of the substrate holder
Lowering the temperature of the substrate holder.
It is preferable to set each time.

The sputtering apparatus according to claim 4, wherein, in the sputtering apparatus according to claim 1, wherein the auxiliary block, while accommodating to surround the substrate holder when in contact with the substrate holder, the substrate holder is separated It becomes an opening is formed in the direction, when the substrate holder is spaced from said auxiliary block, the substrate holder is positioned opposite to the target, characterized in that it is possible sputtering.

In such a sputtering apparatus, when the substrate holder and the auxiliary block are in contact with each other, the auxiliary block is housed so as to surround the substrate holder, so that heat is smoothly transferred. . An opening is formed in the auxiliary block in a direction in which the substrate holder is separated, so that the two can be easily contacted and separated. Further, in a state in which both are separated, the semiconductor substrate and the target approach each other, and sputtering can be performed on the surface of the semiconductor substrate, and the auxiliary block and the substrate holder are separated from each other. Blocks are less susceptible to heat transfer.

Furthermore, since the auxiliary block is always separated from the said target, the target is less affected by heat conduction from the auxiliary block. Therefore, a change in the quality of the sputtered film due to a change in the temperature of the auxiliary block during sputtering is unlikely to occur. Since the process can be switched only by the reciprocating motion of the substrate holder , the space efficiency of the entire sputtering apparatus including the heating / cooling mechanism is good.

The sputtering apparatus of claim 5, before
Serial in any of the sputtering apparatus, said auxiliary block, the heat of the auxiliary block, characterized in that the gas heat transfer mechanism for transmitting to the substrate holder of the gas as a heat medium is provided.

In such a sputtering apparatus, when the substrate holder and the auxiliary block are in contact with each other, the substrate holder is moved from the auxiliary block to the substrate holder.
To over, it is possible to perform thermal conduction by the heated gas. Therefore, the heat of the auxiliary block is transmitted rapidly to the substrate holder, it is possible to quickly change the substrate holder to a temperature of the second sputtering process.

The sputtering apparatus of claim 6, prior to
In the sputtering apparatus of the serial claim 4, in the scan <br/> sputtering in the apparatus, it will be the second auxiliary block is provided for closing the opening openably to said second auxiliary block the third heating and consisting of a heating means and cooling means for heating and cooling said second auxiliary block
A cooling mechanism, the heat of the second auxiliary block, characterized in that the gas heat transfer mechanism for transmitting to the substrate holder of the gas as a heat medium are provided.

In the above sputtering apparatus, the third heating / cooling mechanism of the second auxiliary block itself provides
Heat transfer becomes smoother. Further, since the opening is closed by the second auxiliary block, the substrate holder is closed.
The heat transfer from the heater is prevented, and the heat transfer efficiency is improved.
Moreover, the second auxiliary block closes the opening.
In state, both before and said first auxiliary block
To form a closed space accommodating the serial substrate holder, by flowing heated gas, the pressure in the closed space is increased,
Heat transfer efficiency is further improved. Further, since the second auxiliary block is provided so as to open and close the opening, the second auxiliary block does not hinder the separation and contact between the substrate holder and the first auxiliary block.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. 1 and 2 show the first embodiment of the present invention.
FIG. 1 is a diagram showing an embodiment of the present invention, wherein reference symbol A is a sputtering device.

In this sputtering apparatus A, as shown in FIG. 1, a sputtering chamber 51 and a transfer chamber 53 are separated by a gate valve 55. In the sputtering chamber 51, a substrate holder 61 for holding a semiconductor substrate 57,
A target 63 that is arranged to face the semiconductor substrate 57 held by the substrate holder 61 and emits atoms or molecules by collision with Ar plasma ions is housed therein, and is usually held in a vacuum state.

The substrate holder 61 is provided with a first heating / cooling mechanism 65 for adjusting the temperature of the semiconductor substrate 57 held by the substrate holder 61. The first heating / cooling mechanism 65 includes a heater 67
A cooling water pipe 71 for passing cooling water, a heating gas pipe 73 for introducing a heating gas, and a substrate holder 61.
And a thermocouple 75 for sensing the temperature. The heating gas pipe 73 is, as shown in FIG.
A plurality of gas pipes 73a, 73a,.
The entire surface of the semiconductor substrate 57 can be heated with good uniformity.

Below the substrate holder 61, a second heating and cooling device for heating and cooling the substrate holder 61 is provided.
An auxiliary block 81 having a built-in cooling mechanism 77 is provided so as to surround the substrate holder 61.

The auxiliary block 81 is also provided with a heater 83, a cooling water pipe 85, a heating gas pipe 87, and a thermocouple 89 as a heating / cooling mechanism 77. The heating gas pipe 87 (similar to the gas pipe 73 and branched into a plurality of gas pipes 87a, 87a,...) Uses the heat of the auxiliary block 81 as a heat medium for the substrate holder 61.
Plays the role of a gas heat transfer mechanism that communicates to the Further, an opening 93 is provided above the auxiliary block 81, and the substrate holder 61 is connected to a vertical driving air cylinder 95.

Next, the operation of the sputtering apparatus A having the above configuration will be described. First, a heating sequence will be described with reference to FIGS. After the temperature of the substrate holder 61 is set by the first heating / cooling mechanism 65, sputtering is performed under the first process condition. In the meantime, the next second process condition is selected, and the control unit (not shown) of the apparatus recognizes that the heating condition is changed.

The controller of the apparatus determines whether to heat or cool the substrate holder 61 under the second process condition . Then, the auxiliary block 81 is heated or cooled to a target temperature. At this time, keep monitored the temperature of the auxiliary block 81 with a thermocouple 89 which is incorporated in the auxiliary block 81. The target temperature in this case is higher in the case of heating and lower in the case of cooling than the set temperature of the second process condition. This is to make the time to reach the set temperature as short as possible.

When the first process is completed, the substrate holder 61 is lowered by the vertical driving air cylinder 95, and is put into the auxiliary block 81 through the opening 93 provided on the auxiliary block 81 to make contact with each other. . After that, the heat of the auxiliary block 81 is transferred to the holder 61 by the heat conduction of the gas using Ar as a heating medium and a heat medium, together with heat conduction by contact.

Thermocouple 75 built in substrate holder 61
The temperature of the holder 61 is measured in advance, and when the temperature reaches the set temperature, the substrate holder 61 is raised to a position facing the target 63 by using the vertical driving air cylinder 95. Then, the holder 61 and the auxiliary block 81
And the holder 61 is moved to a position where sputtering can be performed.

In this case, the substrate holder 61 is separated from the auxiliary block 81 and is not affected by the heat transfer of the auxiliary block 81 during sputtering. Then, after the substrate holder 61 is stabilized by the first heating / cooling mechanism under the temperature condition of the second process, the second process is started. Further, during the second process, the temperature of the auxiliary block 81 is set in advance to the third set temperature in the same manner as described above.

The heating gas is preferably the same kind of gas as the sputtering gas. In the present embodiment, Ar gas is used as a sputtering gas, and accordingly, Ar gas is also used as a heating gas. The sputtering chamber 51 is usually kept in a vacuum state. However, even if the Ar gas for heat transfer slightly leaks into the sputtering chamber, the Ar gas for sputtering is the same as the Ar gas for sputtering. The quality of the sputtered film is not degraded due to the deterioration of the ground. Further, although not shown, when the substrate holder 61 and the auxiliary block 81 come into contact with each other, the heating gas pipes 93a, 93a,... Of the auxiliary block 81 are replaced with the heating gas pipes 73a, 73a,. If connected, the heat of the auxiliary block 81 is more easily transmitted to the substrate holder 61.

Next, a second embodiment of the present invention will be described. The sputtering apparatus A shown in FIG. 3 is substantially the same as the sputtering apparatus shown in FIG. Here, the same portions as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. This sputtering apparatus A is different from the sputtering apparatus shown in FIG. 1 in that an opening 93 is provided on a first auxiliary block 81 provided in the sputtering apparatus A.
Is provided with a second auxiliary block 101 for opening and closing the opening.

Further, the second auxiliary block 101 has a heater 107 as a third heating / cooling mechanism 105 for heating and cooling the second auxiliary block 101.
And a cooling water pipe 111 and a heating gas pipe 113. Further, the heating gas pipe 113 has a plurality of gas pipes 113a,
3a, and so on. The second auxiliary block 101 is rotatably fixed to the sputtering chamber 51 of the sputtering apparatus A by a rotating mechanism 115.

In the sputtering apparatus having such a structure, the substrate holder 61 is lowered by the vertical driving air cylinder 95 and is stored in the first auxiliary block 81, and the second auxiliary block 101 is Rotation mechanism 11
5, the opening 93 of the first auxiliary block 81 is closed. Here, the second auxiliary block 101 has both a holder heating / cooling position and a sputtering escape position by the rotating mechanism 115. Then, when the second auxiliary block 101 is at the holder heating / cooling position,
The opening 93 is closed to form a closed space 121.

The first auxiliary block 81 and the second auxiliary block
The lock 101 includes the above-described second and third heating / cooling mechanisms. The lock 101 accommodates the substrate holder 61 and closes the opening 93. the thermal transfer occurs by gas, the pressure of the closed space 121 is higher than the space of the sputtering chamber by the inflow of the heating Ar gas. Therefore, the efficiency of heat transfer from the first auxiliary block 81 and the second auxiliary block 101 is improved. Further, in this state, the second auxiliary block 101
In order to close the opening 93, the heating Ar gas flows into the closed space 1.
It also plays a role in preventing leakage to outside 21 (sputtering chamber).

When sputtering is performed by moving the substrate holder 61 upward, the second auxiliary block is rotated by a rotating mechanism to open the opening 93, and then the substrate holder 61 is moved up and down. To At this time, Ar
Although the gas leaks, it is the same gas as the sputtering gas, so that it does not adversely affect the sputtering process.

[0046]

As described above, in the sputtering apparatus according to the first aspect, since the substrate holder having the heating / cooling mechanism is provided, the temperature of the semiconductor substrate itself can be stably maintained. Further, since an auxiliary block for assisting the temperature setting of the substrate holder is provided, it is possible to quickly change the temperature to the next sputtering process. Further, since the auxiliary block and the substrate holder can be in contact with or separated from each other, a plurality of continuous sputtering processes performed at different temperatures can be rapidly performed.

In the sputtering apparatus according to the fourth aspect , when the substrate holder and the auxiliary block are in contact with each other, the auxiliary block is arranged so as to surround the substrate holder, so that heat is smoothly transferred.

Further, in a state where both are separated from each other, the substrate holder and the target come close to each other, so that sputtering on the semiconductor substrate becomes possible. At this time, the auxiliary blocks are spaced from the substrate holder and the target
Since that, the semiconductor substrate is not affected by the heat transfer by the auxiliary block, it is possible to maintain the temperature of the semiconductor substrate during sputtering more stably. Also, without requiring a revolution mechanism of the substrate holder, only reciprocating motion of the substrate holder,
Since the sputtering process can be switched at different substrate temperatures, space efficiency is improved.

In the sputtering apparatus according to the fifth aspect , a gas heat transfer mechanism for transferring heat between the substrate holder and the auxiliary block is provided. Therefore, when a plurality of sputtering processes having different setting conditions of the substrate temperature are continuously performed, the switching time of the temperature of the substrate holder can be reduced.

In the sputtering apparatus according to the sixth aspect , the heat transfer is smoothed by the third heating / cooling mechanism provided in the second auxiliary block. Furthermore, the second
Since the opening is closed by the auxiliary block, the heat release is prevented and the heat transfer efficiency is improved, and the gas pressure in the formed closed space is increased, so that the heat transfer is further improved. Therefore, the efficiency of heat transfer to the substrate holder can be improved while maintaining good operability.

As described above, in the sputtering apparatus of the present invention, when performing a continuous sputtering process, it is possible to achieve both the two problems of speeding up the temperature switching for each process and stabilizing the set temperature. In addition, it is possible to improve the productivity of semiconductor devices and the like, make the in-plane distribution of the film thickness of the sputtering film uniform, and improve the film quality. Further, space efficiency is improved, so that the cost and size of the device can be reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view of a sputtering apparatus shown as a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a process flow of a continuous film forming process using the sputtering apparatus shown as the first embodiment of the present invention.

FIG. 3 is a cross-sectional view of a sputtering device shown as a second embodiment of the present invention.

FIG. 4 is a schematic configuration diagram of a conventional sputtering apparatus.

FIG. 5 is a schematic cross-sectional view of a conventional sputtering apparatus using gas heat transfer.

[Explanation of symbols]

A: sputtering apparatus, 51: sputtering chamber, 5
3: transfer chamber, 55: gate valve, 57: semiconductor substrate,
61: substrate holder, 63: target, 65: first heating / cooling mechanism, 67: heater, 71: cooling water pipe,
73 ... heating gas pipe, 75 ... thermocouple, 77 ... second heating / cooling mechanism, 81 ... auxiliary block, 87 ... heating gas pipe, 93 ... opening, 95 ... vertical drive air cylinder, 1
01: second auxiliary block, 105: third heating / cooling mechanism, 115: rotating mechanism

Claims (6)

(57) [Claims] A substrate holder for holding a semiconductor substrate;
Wherein disposed opposite to the semiconductor substrate held in the substrate holder, and the target to release the material to be deposited on the semiconductor substrate, for regulating the temperature of the substrate holder
An auxiliary block, the substrate holder and the target,
In the sputtering apparatus comprising a vacuum processing chamber for accommodating said auxiliary block, the substrate holder, heating and cooling means for adjusting the temperature of the semiconductor substrate held by the substrate holder
Means and a temperature sensor for sensing the temperature of the substrate holder.
First heating and cooling mechanism comprising a Nshingu means are provided, said auxiliary block, said auxiliary heating means and the cooling means
Temperature sensing means for sensing the temperature of the block;
Second heating and cooling mechanism consisting of are provided, and the substrate holder and the auxiliary block, these phases
The driving mechanism for the movable pairs manner, contact and configured to be spaced apart, said auxiliary during sputtering and the substrate holder blocks
The first heating / cooling mechanism.
Adjust the temperature of the semiconductor substrate to the specified sputtering temperature
And changing the sputtering temperature of the semiconductor substrate.
In this case, the temperature is adjusted in advance by the second heating / cooling mechanism.
Relative to the auxiliary block and the substrate holder
The substrate holder to the desired temperature.
After reaching, the substrate holder and the auxiliary block
A sputtering apparatus characterized in that the sputtering apparatus is separated . 2. The sputtering temperature of the semiconductor substrate is
When making changes, the auxiliary block is
Temperature is now set to a different temperature
The sputtering apparatus according to claim 1, wherein
Place. 3. The substrate holder according to claim 2, wherein the auxiliary block is provided with the substrate holder.
When raising the temperature of the substrate holder than the temperature of the substrate holder
Set to high temperature, lower the temperature of the substrate holder
When setting, set the temperature lower than the temperature of the substrate holder
3. The switch according to claim 2, wherein
Putting equipment. 4. The sputtering apparatus according to claim 1 , wherein the auxiliary block is in contact with the substrate holder.
With housing so as to surround the substrate holder, in a direction in which the substrate holder is separated will be openings formed, upon which the substrate holder is spaced from said auxiliary block, wherein the substrate holder is the target and the counter A sputtering apparatus characterized in that the sputtering apparatus is positioned in such a manner as to be capable of sputtering. 5. The sputtering apparatus according to claim 1 , wherein the auxiliary block transmits heat of the auxiliary block to the substrate holder using a gas as a heat medium. A sputtering apparatus comprising a gas heat transfer mechanism. 6. A sputtering apparatus according to claim 4, inside the sputtering apparatus, it will be the second auxiliary block is provided for closing the opening openably to said second auxiliary block, A third heating / cooling mechanism comprising heating means and cooling means for heating / cooling the second auxiliary block, and the heat of the second auxiliary block using gas as a heat medium and And a gas heat transfer mechanism for transmitting the gas heat to the substrate holder.