Patent Application
20230134296
The present embodiments relate to an isolation transformer having shielding structures that improve magnetic power transfer and provide isolation from electrostatic field currents.
Plasma has long been employed to process substrates (e.g., wafers) into semiconductor products, such as integrated circuits. In many modern plasma processing systems, a substrate may be placed onto an RF chuck for plasma processing inside a plasma processing chamber. The RF chuck may be biased with an RF signal, using RF voltages in the range from tens to thousands of volts and RF frequencies in the range from tens of KHz to hundreds of MHz. Since the RF chuck also acts as a substrate support, proper control of the RF chuck temperature is an important consideration to ensure repeatable process results.
Generally speaking, the RF chuck's temperature is maintained by one or more electric heaters, which may be integrated or coupled within the substrate support. Electrical power to the electric heater is typically obtained from line AC voltage via an appropriate control circuit to maintain the substrate support at a desired temperature range. By way of example, the electric heater may be powered by DC, line frequency (e.g., 50/60 Hz AC) or KHz range AC power.
Thus, the substrate support needs to be simultaneously subject to substantial levels of RF power, while also powering the heaters. AC circuitry providing power to these heaters can inadvertently draw RF power from the plasma in the chamber, resulting in loss of etch-rate, reduced power transfer to the heaters and/or damage to the AC circuitry. In an attempt to address these issues, it is common to connect filters to block electrostatic currents. These filters usually employ large LC tank circuits, e.g., using coils wound on cores to provide inductance along with capacitor banks to provide high impedance at select frequencies.
Unfortunately, traditional filters suffer from several disadvantages. One is unit to unit variability of coil windings. This variability introduces repeatability issues in the primary resonance. Also, parasitic resonances of such RF filters introduce further unpredictability.
It is in this context that embodiments of the present disclosure arise.
Broadly speaking, the embodiments described herein provide for an efficient transformer isolator. The transformer isolator implements a unique shielding configuration that is optimized for efficient power transfer from a primary to a secondary, while providing efficient isolation from currents returning from the secondary back to the primary
In one embodiment, an apparatus for a transformer isolator used for transferring power to an element of a substrate support used in a plasma chamber is provided. A primary of the transformer isolator includes a primary base plate configured to electrically couple to ground. A primary ferrite disposed over the primary base plate, and the primary ferrite has a primary circular channel A primary coil is wound within the primary circular channel. A primary shield is disposed over the primary ferrite and the primary coil. The primary shield includes a first plurality of radial segments that extend from a primary center region to outside a periphery of the primary ferrite. An extended region of the primary shield has a curved section to connect the primary shield with the primary base plate. In one example, the secondary of the transformer isolator has similar construction as the primary and are used together as part of the transformer isolator.
In another embodiment, a transformer isolator for transferring power to an element of a substrate support used in a plasma chamber is provided. A primary of the transformer isolator includes a primary base plate configured to electrically couple to a ground. A primary ferrite is disposed over the primary base plate. The primary ferrite has a primary circular channel. A primary coil is wound within the primary circular channel A primary shield is disposed over the primary ferrite and the primary coil. The primary shield includes a first plurality of radial segments that extend from a primary center region to outside a periphery of the primary ferrite and a first curved section to connect the primary shield with the primary base plate. The transformer isolator includes a secondary that has a secondary base plate configured to electrically couple to a radio frequency (RF) ground return of the plasma chamber. A secondary ferrite is disposed over the secondary base plate. The secondary ferrite has a secondary circular channel. A secondary coil is wound within the secondary circular channel of the secondary ferrite. A secondary shield is disposed over the secondary ferrite and the secondary coil. The secondary shield includes a second plurality of radial segments that extend from a secondary center region to outside a periphery of the secondary ferrite and a second curved section to connect the secondary shield with the secondary base plate. The primary shield is oriented to be spaced apart from and face the secondary shield.
In yet another embodiment, a shield structure for use in a transformer isolator is provided. The shield structure includes a dielectric substrate having a center, a substantially flat surface that radially extends from the center to a periphery, and a curved extension that extends from the periphery. A conductive pattern is formed over the dielectric substrate, and the conductive pattern forms a plurality of radial segments. Each radial segment has a plurality of slits extending over the substantially flat surface and the curved extension, and each of the plurality of radial segments includes a segment end located near the center of the dielectric substrate. The conductive pattern includes a center segment aligned with the center, and wherein select ones of the segment ends are connected to the center segment.
Other aspects of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which:
The present invention will now be described in detail with reference to a few embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention.
On etch tools, heating the electro-static chuck is one way to tune and improve process uniformity. The alternating current (AC) circuitry providing power to these heaters can inadvertently draw RF power from a chamber resulting in loss of etch-rate.
Typically, radio frequency (RF) filters are employed to block this RF power from returning to circuitry of the AC circuitry of the heater's AC/DC power supply. These filters are traditionally designed as parallel LC tanks with coils wound on cores (or air cores) to provide inductance along with capacitor banks to provide high impedance at select resonant frequencies. There are inherent disadvantages to this filtering approach as the coil windings and their associated primary and parasitic resonances have repeatability issues. In one embodiment, a transformer approach for RF filtering is believed to assist in solving some of these problems. A transformer provides a capacitive rejection response, and as there are no resonances this approach is more immune to repeatability issues.
In one configuration, the transformer's primary and secondary are separated by a physical gap to avoid RF capacitively coupling from secondary to primary, as the latter is connected to sensitive AC circuitry and/or DC circuitry. RF is inherently present at the secondary of the transformer as it is coupled in common-mode, e.g., RF ground of the chamber hardware. However, simply having a large physical gap between the primary and secondary of the transformer in order to solve the aforementioned issue would drastically reduce the efficiency of power transmission. In accordance with one embodiment, an isolation transformer is configured with RF shielding to block electromagnetic field penetration and/or electrostatic currents back to the primary, but also allow for efficient magnetic power to transfer and penetrate from the primary to the secondary for powering the heaters of the chuck. In one embodiment, to prevent loss of RF power from secondary to ground and primary circuitry, the physical gap may be between about 0.5 mm and about 30 mm and capable of standing off several kilovolts (KVs). For example, the DC voltage between the gap may be between 1 KV and 15 KVs. Thus, one purpose of physical gap between the secondary and primary is to enable RF isolation and also enable DC isolation, yet provide for efficient power transfer.
By way of example, the RF shield should not only block RF frequencies from 400 kHz to 300 MHz at 500 W to 50 kW of RF power but also allow magnetic power (0.5 kW to 50 kW) to transfer through at 100 kHz to 1 MHz switching frequencies. In one embodiment, the RF shield disclosed herein has multiple slits at multiple levels in order to minimize the eddy current dissipation which occurs when a magnetic-field couples through the shield. Generally, by incorporating slits in the RF shields; a primary shield and a secondary shield, current produced from RF power in the chamber will be directed to ground. The slits are further designed to prevent excessive circling of eddy currents that would otherwise reduce magnetic penetration and efficiency of power delivered to the heaters. Increasing the number of slits also constrains the area in which the eddy currents can loop. Reduction in eddy currents will thus increase the coupling efficiency of currents required to be induced in the coil of the secondary. Accordingly, the RF shield configuration of the present embodiments will reduce capacitive coupling to thereby block most, if not all, of the current returning from the RF power in the chamber, but also include slits that are designed to reduce eddy currents that would reduce the efficiency of magnetic penetration from the primary to the secondary.
In one embodiment, the transformer isolator 122 is configured to efficiently transfer power through magnetic penetration over a transformer structure having one or more RF shields, while also substantially blocking current penetration from RF power used to generate plasma in the plasma chamber 101, during processing. The heater 110 is also illustrated as a single heater, but in some embodiments multiple heaters will be incorporated into the substrate support 102. For example, some embodiments will utilize four multi-zone heaters, while other configurations will utilize an array of heaters that are individually controlled to provide strategic micro-controlled heating levels at different regions of the substrate support 102. For purposes of example, some heater arrays can include up to 150 individual heaters or more, depending on the substrate support design.
The square AC signal is therefore provided to a primary coil 230a of the transformer isolator 122. As schematically shown, the primary ferrite 232a is used to contain the primary coil 230a, as will be shown in more detail below. A primary shield 240a is shown disposed over the primary coil 230a and the primary ferrite 232a. The primary shield 240a is coupled to ground 250. A secondary coil 230b, secondary ferrite 232b, and secondary shield 240b are shown oriented opposite the primary shield 240a, while maintaining a separation gap. The secondary shield 240b is shown connected to ground 250 by way of the RF ground return 252 of the plasma chamber 101.
As discussed above, when plasma 104 is generated in the plasma chamber 101, an RF return to ground from the plasma 104 moves through the ground 250, and the secondary shield 240b is connected to ground 250. The transformer isolator 122 will therefore have complementary and opposing shields that are separated by a gap, and the shields will have a slit pattern designed to reduce eddy currents and improve magnetic field transfer of power to the load (e.g., one or more heaters in the substrate support), while substantially blocking currents produce from the RF return in the plasma chamber 101.
In one example, the gap separation between the shields 240a/240b may range between about 0.5 mm and about 30 mm. This gap separation may produce a capacitance of between about 30 pico-Farads (pF) and about 100 pF. The voltage between the gap separation may be between about 0.5 kilo-volts (KV) and about 50 KV. In some embodiments, the voltage between the gap separation may be between about 1 kilo-volt (KV) and about 15 KV.
The secondary coil 230b of
As mentioned above, the number of heaters will depend on the heater's configuration within the substrate support. Some substrate supports will be multi-zone substrate supports that are provided specific levels of power. Some substrate supports include arrays of heaters, which are controlled and fine-tuned depending on the process and needs for temperature variation to improve uniformity in etch operations. Lam Research Corporation, the assignee of this application implements these types of heater arrangements and are referred to as “Hydra heaters” or as a “Hydra-ESC,” and examples of such heaters can be found in U.S. Publication 2014/0220709A1, which is incorporated by reference.
In one configuration, the coils 230a, 230b are made from Litz wires. Litz wires are multi-strand wires or cables used to carry alternating current (AC) at radio frequencies. Therefore, although the primary and secondary coils 230a/b are illustrated as blocks in the graphical illustration, the coils are actually wrapped around multiple times in the channels defined in each of the primary ferrite 232a and the secondary ferrite 232b. The number of turns in each of the coils 230a, 230b will vary, depending on the voltages and ratios being transferred across the transformer.
The secondary shield 402b has a similar construction, whereby the shield 402b includes a plurality of slits defining radial segments that extend from the center of the secondary shield 402b and out to the periphery before extending upward toward the secondary base plate 302b. As mentioned above, the primary base plate 302a is connected to ground, and the secondary base plate 302b is connected to ground of the plasma chamber, when the transformer isolator 122 is implemented in a configuration similar to that of
Therefore, by segmenting the shields 402a, 402b with slits 560 to form radial segments 502, it is possible to reduce the circulation of the eddy currents that are generated when the H-field 340 passes through of the shields 402a, 402b. Further, because each shield 402a, 402b is extended beyond the periphery 504e that faces the opposite shield 402a, 402b, and is extended away and downward or upward toward the respective grounded base plates 302a, 302b, the power consuming effects of eddy currents can be reduced. More specifically, by creating the radial segments 502 and the extended region 504d outside of the periphery 504e that faces the respective shield 402a, 402b, the resistance of the path that the eddy currents must traverse in each radial segment 502 will increase.
As is known, power is equal to current squared times resistance. In the configurations shown in
This combination of features will allow for the maximum amount of magnetic flux to be transferred between the primary and the secondary in regions where the ferrites 232a, 232b face each other. This construction also provides for reduction in capacitive coupling, to thereby substantially block the currents flowing from the plasma to penetrate from the secondary to the primary of the transformer isolator 122. Collectively, this construction provides for efficient transfer of magnetic power between the primary to the secondary for powering the heaters of the substrate support of the plasma chamber while at the same time reducing the currents from penetrating back to the primary.
The number of patterns, shapes and configurations can be chosen to fine tune and control the flow of eddy currents in the respective radial segments 502, in order to increase power transfer efficiency between the primary and secondary. That is, by reducing the eddy current flows in the shield 402a, 402b, it is possible to increase the coupling efficiency of the currents that are intended to be induced in the secondary via the primary
The curved section that transitions the gap-facing surface of the primary shield 402a to the primary side 402a′ is shown to connect to the primary base plate 302a by way of a primary ring 702a. The primary ring 702a connects electrically the primary shield 402a to AC ground 250. Similarly, the curved section of the secondary shield 402b connects the gap-facing surface of the secondary shield 402b to the secondary side 402b′ which is then connected to the secondary base plate 302b via a secondary ring 702b. By incorporating the curved sections in the transition at the periphery 504e of the respective shields 402, a positive effect of reducing eddy current power dissipation is achieved. That is, the eddy currents will be allowed to efficiently traverse along the radial segments 502 from the gap-facing surfaces that face each other and gradually to the extended region of the shields 402 without causing heat buildup that would have been produced if the edges were sharp. The extended regions are shown respectively as the primary side 402a′ and secondary side 402b′.
Additionally, the extended regions of the shields 402 that include the curved sections and the sides, i.e., the primary side 402a′ and secondary side 402b′, will assist to effectively extend the length over which the eddy currents must traverse thereby increasing the resistance and reducing power dissipation. By way of example, in one eddy current simulation run at 80 kHz, it was observed that patterned radial segments 502 having the curved sections were effective to achieve an eddy current power dissipation of less than 50 watts, even considering the higher dissipation regions that align with the ferrite areas. In some areas over the slotted shield 402, the eddy current power dissipation was substantially lower, e.g., in the range of 2-20 watts. The curved sections also provide for a significantly reduced risk of arc over events and provide better stand-off to high voltages.
In some embodiments, it is possible to extend the shield 402 radially outward without including the curved sections. However, by including the curved sections, it is possible to reduce the overall diameter of the shields 402 of the transformer isolator 122 thereby reducing the capacitive coupling. Collectively, these features act to increase the efficiency of the magnetic flux transfer of power between the ferrites of the primary to the secondary, while blocking current penetration returning from the plasma chamber back to the primary.
The extended region 504d goes beyond the periphery 504e of the flat portion of the primary shield 402a. In one embodiment, as shown in
In one embodiment, it is also desired that the thickness of the conductive patterns 820 should not be too much thicker than the skin depth, so that efficient magnetic penetration can be achieved from the primary to the secondary. Accordingly, there is a trade-off being made in selecting the thickness of the conductive patterns 820. On the one hand, the thickness should be enough to block current penetration returning from the plasma, while at the same time also allowing efficient magnetic penetration from the primary to the secondary to power the heaters in the substrate support of the plasma chamber. It should be understood that the skin depth may vary depending on the frequency of operation and the plasma chamber in which the transformer isolator 122 is used.
In some embodiments, the thickness of the conductive patterns 820 will be optimized in cases where there are multiple frequencies being used. By way of example, it is possible that higher frequencies, e.g., 60 MHz or higher may be used as well as lower frequencies, e.g., 400 kHz or less. In such cases, the skin depth and material being used for the conductive patterns 820 will be taken into consideration to define the appropriate conductive patterns 820 thickness that will achieve a balance of isolation from electromagnetic fields penetrating back from the plasma versus the efficiency of magnetic penetration to be transferred from the primary to the secondary. That is, it is possible to have a thickness for the conductive pattern 820 to be less than the skin depth, yet still provide for efficient isolation and efficient power transfer. In various implementations, the operating frequencies may range between 400 kHz or less to about 100 MHz.
In one embodiment, a shield structure itself is disclosed. The shield may be used on one side of a transformer (e.g., either the primary or the secondary) or on both sides as shown in the example transformer isolator 122. The shield structure includes a dielectric substrate having a circular shape that extends from a center of the circular shape to an outside diameter. In another example, the top part or gap-facing surface of the shield structure could also be square or rectangle or an n-sided polygon. The substrate has a flat surface that extends from the center to a periphery and a curved extension that extends from the periphery to the outside diameter. A conductive pattern is formed over the dielectric substrate. The conductive pattern includes a plurality of radial segments that extend over the flat surface, over the curved extension, and to the outside diameter. Each radial segment includes a plurality of slits. Each of the plurality of radial segments includes segment ends located near the center of the dielectric substrate. The conductive pattern includes a center segment, and wherein select ones of the segment ends connect to the center segment.
In some embodiments, the shield structure may be a consumable part. Over time, the shield may wear down and it may need to be replaced to maintain the transformer isolator.
Embodiments may be practiced with various computer system configurations including hand-held devices, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers and the like. The embodiments can also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a network.
With the above embodiments in mind, it should be understood that the embodiments can employ various computer-implemented operations involving data stored in computer systems. These operations are those requiring physical manipulation of physical quantities. Any of the operations described herein that form part of the embodiments are useful machine operations. The embodiments also relate to a device or an apparatus for performing these operations. The apparatus may be specially constructed for the required purpose, such as a special purpose computer. When defined as a special purpose computer, the computer can also perform other processing, program execution or routines that are not part of the special purpose, while still being capable of operating for the special purpose. Alternatively, the operations may be processed by a general purpose computer selectively activated or configured by one or more computer programs stored in the computer memory, cache, or obtained over a network. When data is obtained over a network the data may be processed by other computers on the network, e.g., a cloud of computing resources.
One or more embodiments can also be fabricated as computer readable code on a computer readable medium. The computer readable medium is any data storage device that can store data, which can thereafter be read by a computer system. Examples of the computer readable medium include hard drives, network attached storage (NAS), read-only memory, random-access memory, CD-ROMs, CD-Rs, CD-RWs, magnetic tapes and other optical and non-optical data storage devices. The computer readable medium can include computer readable tangible medium distributed over a network-coupled computer system so that the computer readable code is stored and executed in a distributed fashion.
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.
Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications can be practiced within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the embodiments are not to be limited to the details given herein, but may be modified within their scope and equivalents of the claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US21/23670 | 3/23/2021 | WO |
