Abstract:
Apparatus, systems and methods for plasma etching substrates are provided that achieve dissipation of charge build-up on a substrate being plasma etched to avoid notching or twisting in high aspect ratio contents and similar features. Charge build-up on a substrate being etched by plasma etching can be dissipated by a method for etching a substrate, the method comprising: providing a plasma processing chamber comprising a chamber enclosure and a substrate support adapted to support a substrate within the chamber enclosure; supporting a substrate on the substrate support; forming a plasma within the chamber enclosure such that a surface of the substrate is in contact with the plasma; etching the substrate by generating a negative bias on the substrate surface relative to the plasma; and intermittently changing the bias on the substrate surface to positive relative to the plasma. The present method can be integrated into known plasma processing systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 11/362,409, filed Feb. 23, 2006, now U.S. Pat. No. 7,713,430, issued May 11, 2010. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to etching processes used in the semiconductor industry and, more specifically, to methods and apparatus for controlling charge build-up during plasma etching. 
     BACKGROUND 
     Higher performance, lower cost, increased miniaturization of electronic components, and greater density of integrated circuits are ongoing goals of the computer industry. One commonly used technique to increase density of integrated circuits involves stacking of multiple layers of active and passive components one atop another to allow for multi-level electrical interconnection between devices formed on each of these layers. This multi-level electrical interconnection is generally achieved with a plurality of metal-filled vias (“contacts”) extending through dielectric layers that separate the component layers from one another. These vias are generally formed by etching through each dielectric layer by etching methods known in the industry, such as plasma etching. Plasma etching is also used in the forming of a variety of features for the electronic components of integrated circuits. 
     As described in U.S. Pat. No. 6,544,895, incorporated herein by reference, in plasma etching, a glow discharge is used to produce reactive species, such as atoms, radicals, and/or ions, from relatively inert gas molecules in a bulk gas, such as a fluorinated gas, such as CF 4 , CHF 3 , C 2 F 6 , CH 2 F 2 , SF 6 , or other freons, and mixtures thereof, in combination with a carrier gas, such as Ar, He, Ne, Kr, O 2 , or mixtures thereof. Essentially, a plasma etching process comprises: 1) reactive species being generated in a plasma from the bulk gas, 2) the reactive species diffusing to a surface of a material being etched, 3) the reactive species absorbing on the surface of the material being etched, 4) a chemical reaction occurring that results in the formation of a volatile by-product, 5) the by-product being desorbed from the surface of the material being etched, and 6) the desorbed by-product diffusing into the bulk gas. 
     In a plasma etching chamber for semiconductor manufacturing, a plasma is maintained by coupling energy from a power source into the plasma, which comprises mobile positively and negatively charged particles. An electric field, or bias voltage, develops in a sheath layer around the plasma, accelerating the ions toward the semiconductor substrate by electrostatic coupling. Applying an oscillating bias power can modulate the potential difference between the plasma and the semiconductor substrate. The difference between an instantaneous plasma potential and a surface potential defines a sheath potential drop. During the positive voltage phase, the substrate collects an electron current from electrons that have enough energy to cross the sheath layer while during the negative voltage phase, positive ions are accelerated by the sheath voltage drop, strike the substrate, and are collected by the substrate. 
     However, plasma etching processes (as well as ion implantation and other charge beam processes) may damage the semiconductor substrate and the devices and circuits formed therein or thereon. In particular, electrical charging is a well-known problem that can occur during the plasma processing of semiconductor devices, leading to the degradation of the device performance. 
       FIG. 6  depicts the conventional phenomenon of electrical charging on a semiconductor device  240  in the process of a plasma etch. A material layer  244  to be etched is shown layered over a semiconductor substrate  242 . A patterned photoresist layer  246  is provided on the material layer  244  for the etching of a via. During the plasma etching process, the patterned photoresist layer  246  and material layer  244  are bombarded with positively charged ions  248  and negatively charged electrons  252  (i.e., the reactive species). This bombardment results in a charge distribution being developed on the patterned photoresist layer  246  and/or the semiconductor substrate  242 . This charge distribution is commonly called “feature charging.” 
     In order for feature charging to occur, the positively charged ions  248  and the negatively charged electrons  252  must become separated from one another. The positively charged ions  248  and negatively charged electrons  252  become separated by virtue of the structure being etched. As the structure (in this example a via  254 ) is formed by etching, the aspect ratio (height-to-width ratio) becomes greater and greater. During plasma etching, the positively charged ions  248  are accelerated (e.g., as a result of a DC bias at the semiconductor substrate  242 ) toward the patterned photoresist layer  246  and the material layer  244  in a relatively perpendicular manner, as illustrated by the arrows adjacent positively charged ions  248 . The negatively charged electrons  252 , however, are less affected by the DC bias at the semiconductor substrate  242  and, thus, move in a more random isotropic manner, as depicted by the arrows adjacent negatively charged electrons  252 . This results in an accumulation of a positive charge at a bottom  256  of the via  254  because, on average, positively charged ions  248  are more likely to travel vertically toward the semiconductor substrate  242  than are negatively charged electrons  252 . Thus, any structure with a high enough aspect ratio tends to charge more negatively at photoresist layer  246  and an upper portion of the material layer  244  to a distance A (i.e., illustrated with “−” indicia) and more positively at the via bottom  256  and the sidewalls  258  of the via  254  proximate the via bottom  256  (i.e., illustrated with “+” indicia). 
     As shown in drawing  FIG. 7 , the positively charged via bottom  256  deflects the positively charged ions  248  away from the via bottom  256  and toward the sidewalls  258  of the via  254 , as a result of charge repulsion. The deflection results in an etching of the sidewalls  258  proximate the via bottom  256 , which is known as “notching” or “twisting.” Furthermore, the presence of the positively charged via bottom  256  slows the positively charged ions  248  as they approach the positively charged via bottom  256 , thereby reducing etching efficiency. 
     Current High Aspect Ratio Contacts (HARCs) are known to twist at aspect ratios greater than about 20:1. Twisting is the deviation of the bottom of a contact from the centerline of the etch front. Twisting is caused by asymmetric charge build-up in and around the contact, which causes a lateral deviation of the ion projectory. The twisting may be so serious that the etch processes actually generate corkscrew-shaped contacts. The twisting of the contact is a concern for shorting contacts to other structures or to each other. Certain tools and chemistries help reduce the twisting, but all known tools show this phenomenon. 
     Twisting is a current failure mechanism for contacts on 95 nm parts and has required extensive process developments to overcome. HARC etches are probably the most difficult etch needed on DRAM parts and have very tight constraints on profiles, film selectivities, and Critical Dimensions (CDs). The twisting behavior of contacts will limit the aspect ratio of contacts that can be etched in the near future and there is little research to understand this phenomena and no known solution to eliminate it. 
     In a standard plasma etch system the ion angular distribution is very anisotropic whereas the electron angular distribution is very isotropic. For HARC features, the electrons will mainly strike the contact near the top of the feature; while ions will reach the bottom of the feature. This is what causes the top of the contacts to charge negative, while the bottom of the contacts charge positive. Small asymmetries in the top of the contacts due to photolithography or polymer loading will cause asymmetric charging at the top of the contact leading to bending of the incident ions. This will then cause the contact to etch faster on one side of the contact due to increased ion flux to this area. 
     BRIEF SUMMARY 
     A hallmark of the method of this invention is the use of DC power to reverse the sheath voltage in a plasma etch process, thereby dissipating a charge build-up on the substrate being etched. 
     In one aspect, the invention includes a plasma etching apparatus. In one embodiment, the etching apparatus comprises a plasma processing chamber with a chamber enclosure; a support adapted to hold or support a substrate within the chamber enclosure; a radio frequency (RF) power source configured to impose a negative bias across a substrate supported by the substrate support; a DC power supply configured to generate a positive voltage with respect to the plasma within the chamber and in intermittent electrical communication with the substrate support; and a control switch to select between connected and unconnected electrical communication between the DC power supply and the substrate support. 
     In another embodiment of the etching apparatus, the DC power supply comprises a function generator power supply configured to generate a varying positive DC voltage with respect to plasma within the chamber enclosure, wherein the function generator power supply is in electrical communication with the substrate support. 
     In yet another embodiment of the etching apparatus, the DC power supply comprises a function generator power source configured to generate a variable voltage signal, and the etching apparatus further includes a signal amplifier, wherein the alternating voltage signal imposes a bias relative to a plasma across a surface of a substrate supported by the substrate support. 
     In another aspect, the invention provides methods for etching a substrate. In one embodiment, the method comprises forming a plasma within a plasma processing chamber containing a substrate to be etched, etching the substrate by generating a negative bias on the surface of the substrate relative to the plasma, and intermittently changing the bias on the surface of the substrate to positive relative to the plasma. Exemplary substrates that can be etched by the methods of the invention include insulating materials (e.g., oxides, nitrides, polymers) utilizing a plasma comprising ionizable gases. 
     In another embodiment, the method comprises applying an RF signal to generate a negative bias on the surface of the substrate relative to the plasma to cause ions in the plasma to etch the substrate, and applying a varying DC voltage to the substrate support to intermittently change the bias on the surface of the substrate to positive relative to the plasma. 
     In a further embodiment, the method comprises applying a varying RF signal to generate a bias on the surface of the substrate, the bias varying from negative to positive relative to the plasma such that, when the bias is negative, ions in the plasma etch the substrate. 
     In yet another embodiment, the method comprises applying an RF signal to generate a negative bias on the surface of the substrate relative to the plasma to cause ions in the plasma to etch the substrate, generating a constant DC voltage signal that is positive relative to the plasma, and selectively operating a control switch between an open position that precludes electrical communication between the constant DC voltage signal and the substrate support and a closed position that establishes electrical communication between the constant DC voltage signal and the substrate support, such that when the switch is in a closed position, the constant DC voltage signal changes the bias of the surface of the substrate to positive relative to the plasma. 
     The invention provides devices and methods to etch high aspect ratio features. Features such as vias and contact openings that etched according to the invention advantageously are substantially free of feature charging, notching and/or twisting. The present invention can be readily integrated into conventional plasma processing systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described below with reference to the following accompanying drawings, which are for illustrative purposes only. Throughout the following views, reference numerals will be used in the drawings, and the same reference numerals will be used throughout the several views and in the description to indicate same or like parts or steps. 
         FIG. 1  is a flow diagram illustrating steps in etching a substrate according to an embodiment of a method of the present invention. 
         FIG. 2  is a schematic diagram showing one embodiment of an apparatus of the invention. 
         FIG. 3  is a schematic diagram showing another embodiment of an apparatus of the invention. 
         FIG. 4  is a schematic diagram of another embodiment of an apparatus of the invention. 
         FIGS. 5A and 5B  are cross-sectional views of a contact being etched according to an embodiment of a method of the invention as depicted in  FIG. 1 . 
         FIG. 6  is a cross-sectional view of a via during a prior art etching process that results in the phenomenon of feature charging. 
         FIG. 7  is a cross-sectional view of a via during a prior art etching process wherein feature charging results in deflection of positively charged ions away from a bottom of the via and toward sidewalls of the via. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, references are made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. Although the examples presented are directed to the formation of an opening or via, it is understood that the present invention may be utilized in a variety of feature formations and plasma processes. 
     In the context of the current application, the term “semiconductor substrate” or “semiconductive substrate” or “semiconductive wafer fragment” or “wafer fragment” or “wafer” will be understood to mean any construction comprising semiconductor material including, but not limited to, bulk semiconductive materials such as a semiconductor wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure including, but not limited to, the semiconductive substrates, wafer fragments or wafers described above. Furthermore, when references are made to a wafer or substrate in the following description, previous process steps may have been used to form regions or junctions in the base semiconductor structure or foundation. 
       FIG. 1  shows a flowchart of an embodiment of an etching process  101  according to an embodiment of the invention for plasma etching a substrate while  FIGS. 5A and 5B , which will be discussed concurrently with  FIG. 1 , show schematic representations of a substrate being etched by plasma in accordance with the embodiment of  FIG. 1 .  FIGS. 5A and 5B  illustrate a wafer fragment  340 , as referred to herein as wafer  340 , with a substrate  344  to be etched overlying a material layer  342 , which can comprise an unprocessed semiconductor wafer or other substrate, the wafer  340  with various process layers formed thereon including one or more semiconductor layers or other formations, and active or operable portions of semiconductor devices. Exemplary materials for substrate  344  include oxides, nitrides, polymers, silicon, and other insulating and semiconducting materials. 
     In a first Step  103 , wafer  340  with the substrate  344  to be etched, bearing a patterned mask layer  346  (e.g., photoresist over an oxide layer), is loaded into a plasma processing chamber (not shown) onto a support. Etch gases are fed into the plasma processing chamber, and a plasma is then generated within the chamber in Step  105 . 
     The plasma generation is decoupled from wafer bias. Decoupling can be carried out with a variety of plasma reactors. Examples of sources used to create a plasma include an electron cyclotron resonance (ECR) source, an inductively coupled plasma (ICP), capacitive discharges, microwave discharges (e.g., applied by a surface wave process, etc.), or a helicon wave source, etc., all of which are well known in the art. The plasma may be generated utilizing a conventional etching device, such as an etcher available from Applied Materials, Inc., under the trade designation of ENABLER® etcher, a Kiyo TCP Oxide Etcher available from Lam Research Corporation, or other plasma etcher. It should be readily apparent to one skilled in the art that, depending upon the particular etching apparatus utilized to generate the plasma, various parameters provided herein can be modified to accomplish the objectives of the invention. 
     By way of example and not by way of limitation, an inductively coupled plasma (ICP) source of a dry or plasma processing apparatus, such as an ENABLER® etcher (Applied Materials, Inc., Santa Clara, Calif.) with a top and a bottom electrode (dual power chamber), can be used for the inventive plasma etching process. In the ENABLER® etcher, the top and bottom electrodes are not coupled. The top electrode is configured to provide a plasma source (“top power”), and the bottom electrode is configured to provide a bias voltage (“bottom power”) and a wafer chuck. In the Enabler etcher, the bias power is provided as a RF signal to the bottom electrode. By increasing bottom RF power the bias voltage is increased across the substrate, which is positioned on the bottom electrode. 
     In Step  107 , an RF signal is supplied to the plasma processing chamber. Self-rectification of the RF signal produces a negative bias on the surface of the wafer  340  with respect to the plasma. The RF power, and resultant negative substrate bias, provides the energy to drive the plasma ions to the surface of the wafer  340 . Thereby, in Step  109 , the plasma ions etch a portion of the substrate  344  (e.g., an oxide layer) exposed through openings of a patterned mask  346  to form a feature  354  (e.g., a via or contact). As shown in  FIG. 5A , positive ions  348  will travel anisotropically toward the wafer  340  while negative ions  352  and electrons  353  travel more isotropically. The result is the typical charge separation with the photoresist layer  346  and an upper portion of the substrate  344  being more negatively charged and a bottom  356  of the etched feature  354  being more positively charged. 
     In Step  111 , power is supplied to the wafer  340  (normally through a substrate support pedestal (not shown)) to intermittently reverse the sheath voltage from negative to positive. The reversal of the sheath voltage allows charge build-ups on the upper portion of the substrate  344  and at the bottom  356  of the etched feature  354  to dissipate. As shown in  FIG. 5B , when the sheath voltage is positive, negative ions  352  and electrons  353  travel anisotropically toward the wafer  340 , which facilitates negatively charged particles reaching the bottom  356  of feature  354  to offset the positive charge buildup from Step  109 . Likewise, the positive ions  348  travel more isotropically when the sheath voltage is positive, which results in the positive ions  348  striking mainly the surface of the wafer  340  and upper portions of the etched feature  354  to offset the negative charge buildup at these portions of the wafer  340 . 
     The intermittent reversal of the substrate bias can be accomplished by alternatively switching back and forth from a constant DC power supply, by providing a pulsed signal using a function generator and signal amplifier, or by using a pulsed DC power supply. These methods of reversing the bias and an apparatus for accomplishing the result are discussed in detail below. 
     In Step  113 , if further etching is required, Steps  107 ,  109 ,  111  are repeated as necessary. If no further etching is required, the process can be terminated. 
     An embodiment of an apparatus for carrying out the method of invention is illustrated in  FIG. 2 . As shown, a plasma processing chamber  201  includes a chamber enclosure  203 . The chamber enclosure  203  is electrically connected to ground  209 . A plasma source  205  generates plasma  207  within the chamber enclosure  203 . A wafer chuck  211  supports a substrate  213  (e.g., wafer) within chamber enclosure  203 . Typically, the wafer chuck  211  is an electrostatic chuck (ESC). Wafer  213  has a surface  215  that is exposed to the plasma  207  for etching. 
     Attached to the chuck  211  is a function generator  217 . The purpose of the function generator  217  is to generate an alternating electric signal whereby the voltage changes over time, which can be accomplished with conventional apparatus and processes. For this embodiment, the function generator  217  provides pulses of positive voltage with respect to ground  219 , which alternate with pulses of less positive voltage (including zero voltage and negative voltage, if desired). The shape, frequency, amplitude and duration of such pulses can be adjusted depending on the etching performance. 
     A function generator is a device that can produce various waveform patterns of voltage at a variety of frequencies and amplitudes. Function generators are well known in the art and are frequently used as test equipment. Most function generators allow the user to choose the shape of the output from a number of options. Typical waveforms include a square wave where the signal goes directly from high to low voltage, a sine wave where the signal curves like a sinusoid from high to low voltage, and a triangle wave where the signal goes from high to low voltage at a fixed rate. These waveforms can be either repetitive or single-shot (once only) in which case some kind of triggering source is required (internal or external). 
     The amplitude control on a function generator varies the voltage difference between the high and low voltage of the output signal. The direct current (DC) offset control on a function generator varies the average voltage of a signal relative to the ground. The frequency control of a function generator controls the rate at which output signal oscillates. On some function generators, the frequency control is a combination of different controls. One set of controls chooses the broad frequency range (order of magnitude) and the other selects the precise frequency. This allows the function generator to handle the enormous variation in frequency scale needed for signals. 
     Function generators, like most signal generators, may also contain an attenuator, various means of modulating the output waveform, and often the ability to automatically and repetitively “sweep” the frequency of the output waveform (by means of a voltage-controlled oscillator) between two operator-determined limits. This capability makes it very easy to evaluate the frequency response of a given electronic circuit. 
     The signal from the function generator  217  is then amplified by amplifier  221  and fed into the wafer chuck  211 . An RF filter  223  may be used between the amplifier  221  and the wafer chuck  211  to prevent feedback communication of the RF signal to the function generator  217 . The net result is that the sheath voltage is forced to predetermined negative values. The change in sheath voltage accelerates negatively charged species (e.g., electrons and negative ions) to the surface of the substrate  213  and dissipates the positive charge build-up at the bottom  356  of the features  354 , as depicted in  FIGS. 5A and 5B  and discussed earlier. 
       FIG. 3  illustrates another embodiment of a plasma etch apparatus according to the invention. The parts and functions of the labeled components of  FIG. 3  are identical to the same numbered components as in  FIG. 2 . The process of the plasma etch apparatus of  FIG. 3  differs in that an RF Power is supplied to the wafer  213  in a conventional manner, as depicted by dashed box  225 . The RF power supply  227  is connected to a ground  229 . The signal from the RF power supply  227  is fed to chuck  211  through matchbox  231 . The net result is that the sheath voltage is forced to predetermined negative values for a short period of time, thereby accelerating negatively charged species to the surface  215  of the wafer  213  and dissipating the positive charge build-up at the bottom of the contacts ( FIGS. 5A and 5B ). 
     It is common to use tuning components to help match the impedance of the plasma to the output impedance of the RF power supply. These components, usually a shunt capacitor to ground and a series 3-4-turn inductor, are located along with the series capacitor in the “matchbox,” which is physically located adjacent to the cathode position. The inductor is fixed, and both of the capacitors (shunt and series) are variable. A control circuit within the matchbox controller senses the reflected power (from the matchbox and plasma back to the power supply) and adjusts the variable capacitors to minimize the reflected power. Usually this is done automatically by means of reversible motor drives on the capacitors, but occasionally laboratory-based systems will have manual controls for the tuning network. 
       FIG. 4  illustrates another embodiment of a process and a plasma etch apparatus according to the invention. Plasma processing chamber  201 , chamber enclosure  203 , plasma source  205 , plasma  207 , ground  209 , wafer chuck  211 , substrate  213 , and surface  215  of substrate  213  are as described with respect to the plasma etch apparatus of  FIG. 2 . The RF power supply as shown by dashed box  225  is as described with respect to the plasma etch apparatus of  FIG. 3 . A DC power supply  233 , which generates a constant positive voltage with respect to the plasma  207 , is additionally attached to the wafer chuck  211 . The DC power supply  233  is connected to a ground  235 . The positive voltage signal from DC power supply  233  is fed through a switch  237  that can alternate back and forth between open (unconnected) and closed (connected) positions. When connected, the DC power supply  233  changes the sheath voltage to determined positive values for the duration of the connection. Opening the connection allows the negative bias to reestablish on surface  215  of substrate  213 . 
     In the present example of plasma etching an oxide layer, a preferred source for plasma  207  is a fluorocarbon or hydrofluorocarbon feed gas. When utilized to generate a plasma, those feed gases dissociate resulting in fragments for use in an oxide etching process, for example, C x H 3 F 2   +  ions or C x F 2   +  ions. Exemplary fluorocarbon or hydrofluorocarbon gases include CF 4 , CHF 3 , C 2 F 6 , C 3 F 6 , CH 2 F 2 , C 2 HF 5 , among others, and can be used alone or in combination with another gas such as hydrogen (H 2 ) or oxygen (O 2 ) to adjust the nature of the carbon- and fluorine-containing ions within the plasma. 
     Thus, the present invention is capable of providing a simple and controllable method of affecting the quality and efficiency of plasma etching and is easily implemented on most existing plasma reactors. The present invention is useful in etching apertures having a length-to-diameter ratio of 20:1 or greater in insulating materials deposited by chemical vapor deposition techniques. Such insulating materials include oxides, nitrides, polymers, combinations thereof, etc. Furthermore, although the examples presented are directed to the formation of an opening or via, it is understood that the present invention may be utilized in a variety of feature formation and plasma processes. 
     The reduction of charge build-up could also be applied to line and space structures or any other feature where the build-up of charge is causing negative effects. Other negative effects of charge build-up (besides twisting) are notching, aspect ratio dependent etch rate, profile distortion of the etched feature (bow and tapered profiles, for example), and etch stop to name a few. One application that is significantly different would be the reduction of charge build-up that leads to micro-arcing during the etch process. Micro-arcing can occur at the edge of the wafer where the lithography pattern is disturbed while forming discontinuous films. We believe this may be caused by charge build-up of the feature that then discharge when another conducting layer is contacted. This causes a micro-arc and damages the wafer. Micro-arcs can also cause a chain reaction leading to larger arcing events damaging the wafer and the chamber. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.