Patent Application
20110048929
This invention relates generally to focused ion beam etching of copper and copper materials, and more particularly to the chemically-assisted etching of copper over dielectric materials.
Copper (Cu) is the primary material used in integrated circuits (ICs) to create electrically conductive interconnects, and the etching of copper in ICs using focused ion beam (FIB) techniques is important in the field of circuit editing (CE) for failure verification and debugging of the ICs. Circuit editing of ICs with a focused-ion-beam (FIB) system requires that copper planes and traces be milled (cut) uniformly and cleanly so as to electrically isolate circuit elements across the separation created.
Achieving clean uniform FIB etching of copper for CE is difficult. In spite of significant past efforts to improve the quality of FIB copper etching processes, problems persist and improvements are needed in two main areas. These two areas are reducing electrically conductive re-deposited copper in the FIB etching area, and improving the etching selectivity and removal of copper while protecting the adjacent and underlying dielectric from unwanted etching and removal. Both of these problems arise from the properties of copper.
In contrast to aluminum (Al) that that may be removed quickly and cleanly in a FIB operation using an agent such as iodine or an iodine-containing etchant, copper does not create simple volatile compounds with iodine as does aluminum (e.g., AlI3) that can survive under ion bombardment and are easily removed. Rather, copper etching in a FIB process is due substantially exclusively to ion beam sputtering. This can inevitably lead to re-deposition of conductive copper which causes problems as discussed below.
In addition, copper has a crystalline structure comprising crystal grains which have different crystallographic orientations. Grains with different orientations exhibit significantly different FIB etching rates, and, therefore, FIB etching of copper is very non-uniform and uneven. It results in a very rough surface on the etched copper, and may perforate the copper layer down to the underlying dielectric. This may lead to significant damage of the underlying dielectric, and may result in electrical short circuiting of IC circuits from re-deposited electrically conductive copper. Past efforts to improving selectivity have focused principally on protecting the dielectric rather than enhancing the removal of copper, since the required selectivity that is achievable for copper etching is low relative to other materials, e.g., aluminum, where volatile etching byproducts are created.
The ion beam sputtering process causes copper re-deposition all about the area exposed by the ion beam. This creates significant problems since the milled copper material that is intended to be removed re-deposits in this area, making electrical isolation of a signal difficult or impossible. In addition to affording protection of the adjacent and underlying dielectric against unwanted damage or removal, copper etch-assisting chemical agents must also actively assist in either limiting re-deposition of conductive copper material or converting re-deposited conductive copper to a non-conductive state to prevent unwanted electrical short circuits and ensure disconnection of copper conductors intended to be disconnected.
In the top example A of
However, not all oxidizing agents will work with copper. For example, halogens should not be used as etch-assisting agents. Halogens, with the exception of fluorine, spontaneously react with and corrode copper without any activation by an ion beam, and seriously degrade the conductivity of the copper. Even if halogen agents such as chlorine, bromine and iodine are carefully controlled, they can remain in the FIB vacuum chamber for a long time and continue to corrode any exposed copper. Furthermore, all halogens (including fluorine) are very aggressive to both high-k and low-k dielectrics.
When etching copper, there should be reasonably small and controlled over-etching of the dielectric so that performing CE on one metallization layer does not break through to an adjacent or underlying layer and create electrical leakage. Moreover, if cutting a copper line on a plane is not the last operation in the CE process, steps should be taken to ensure that the dielectric floor is flat after copper removal. Otherwise, subsequent operations can be seriously affected. As noted previously, because of its crystalline structure, the sputtering rate of copper is highly dependent on its grain orientation (and the ion beam current density, as well), and the average etching rate can vary significantly, as by a factor of four or more for a given set of FIB operating parameters. This is illustrated in
Since copper removal is due to ion beam sputtering (not volatilization), copper removal occurs relatively slowly in comparison to the dielectric. Thus, any copper etch assisting agent must provide protection of the adjacent or underlying dielectric to prevent unwanted etching of the dielectric. Once an area of dielectric is exposed, the etch assisting agent should either halt or significantly slow down the dielectric sputtering.
For silicon dioxide, SiO2, dielectric, it has been found that oxygen, water or a mixture of vapors of water and ammonia can slow down dielectric sputtering by up to a factor of ten if the ion beam current density and vapor pressure are adjusted properly. Water and ammonia are good oxidizers and have been found to afford reasonably good protection for conventional dielectrics, and oxygen, water and a mixture of water and ammonia have been used as copper etch assisting chemicals for etching copper over conventional SiO2 dielectric. However, these compounds have been found to be useless for protecting the new low-k dielectrics being increasingly used in ICs. The main problem is that many low-k dielectrics contain carbon as one of the main components of the dielectric structure, which is why low-k dielectrics are sometimes called “organic” dielectrics. Both water and oxygen easily oxidize carbon in the dielectric structure to produce carbon monoxide (CO) or carbon dioxide (CO2), both of which are gases and are volatilized. Therefore, rather than being protective agents for the dielectric, they accelerate dielectric etching by volatilizing one of the main components of the dielectric structure.
There has been and is an increasing tendency in the IC industry to employ dielectrics with even lower k numbers by increasing of the proportion of carbon in their structures. This has led to a demand for new copper etch assisting chemistries that are capable of protecting the dielectric. U.S. Pat. No. 7,060,196 discloses and claims a number of chemicals, mainly nitro-compounds such as Nitro-methane, Nitro-ethane, Nitro-propane, Nitro-ethanol and others, for use as etch assisting agents to protect dielectrics in FIB copper etching applications. Nitro-ethanol presently is one of the most widely used chemical agents for etching copper over organic dielectrics in FIB operations. While Nitro-ethanol has been effective in limiting dielectric etching, it has not been very effective addressing the issue of re-deposition of sputtered conductive copper material on surfaces adjacent to the IC work area. This is shown in
Thus, there is a need for etching methods and etch assisting agents for FIB etching of copper that addresses the foregoing and other problems with known methods and etch assisting agents. In particular, there is a need for etching methods and etch assisting agents for FIB etching of copper that both protect the adjacent and underlying dielectric from unwanted etching, and that avoid the problems caused by re-deposition of conductive copper and copper materials. It is to these ends that the present invention is directed.
The invention affords FIB copper etching processes and agents that address the foregoing and other known problems of FIB copper etching processes and agents for CE of ICs. In particular, the invention affords FIB copper etch assisting agents that address the two principal problems with known etch assisting agents, i.e., protection of the adjacent dielectric when etching copper, and rendering sputtered and re-deposited conductive copper non-conductive to prevent electrical short circuiting. The etch assisting agents of the invention protect the adjacent dielectric during copper etching by having low volatility which affords high stickiness and long resident time when absorbed on the dielectric surface, and by being formed of compounds that form oxides or nitrides and contribute to the replenishment of dielectric lost by ion beam sputtering. The etch assisting agents additionally afford efficient oxidation of re-deposited copper to convert the sputtered and re-deposited conductive copper to a non-conductive oxide.
In one aspect the invention affords a FIB process and etch assisting agent for etching copper in the presence of a dielectric by a focused ion beam, in which the copper and the dielectric are exposed to the etch assisting agent that contains an N—N bonding in its molecules, where N is nitrogen.
In a more specific aspect, the agent is hydrazine or a derivative thereof, having a boiling point between about 70° C. and 200° C. And even more particularly, the agent is Hydrazine monohydrate or Nitrosodimethylamine.
In yet another aspect the invention affords a FIB process and etch assisting agent for etching copper in the presence of a dielectric by a focused ion beam, in which the copper and dielectric are exposed to the etch assisting agent comprising, in combination, a first compound containing an N—N bonding in its molecules, where N is nitrogen, that protects the dielectric, and a second compound that is a strong oxidizer that renders re-deposited conductive copper non-conductive.
More particularly, the first compound is Hydrazine monohydrate or Nitrosodimethylamine, and said second compound comprises Nitrogen Tetroxide.
In still a further aspect, the invention affords a FIB process for etching copper in the presence of a dielectric by a focused ion beam, in which the copper and dielectric are cooled to a temperature between about −15° C. to +10° C., and the copper and dielectric are exposed to an etch assisting agent comprising Nitrogen Tetroxide.
The invention is particularly well adapted to the FIB etching of copper interconnects, power and ground planes, and the like in ICs for circuit editing (CE), and will be described in that context. However it will be appreciated that this is illustrative of only one utility of the invention, and that the invention has greater applicability.
Conventional focused ion beam (FIB) apparatus and operations are well known and will not be described in detail herein, and may be used in practicing the invention as explained below. For instance, U.S. Pat. No. 7,060,196, which is hereby incorporated by reference herein, discloses FIB apparatus for milling copper over organic dielectrics in a context and environment similar to that of the present invention, and such apparatus may be used to practice the invention.
As explained above, the invention affords a gaseous etch assisting agent that protects dielectric adjacent to copper against over etching in a FIB etching process, and oxidizes sputtered conductive copper re-deposited in the etching work area to convert it to a non-conductive state.
As may be appreciated from
Where it is necessary to cut a copper line 60 on the bottom of a trench 62 where other copper lines 82, 84 from other metal layers may be exposed at the trench walls 64, 66, as illustrated in
To address this contradiction, the invention affords more volatile and more aggressive oxidizing copper etch assisting agents with less carbon atoms per molecule and lower working ion beam current densities in order to maintain reasonable dielectric protection. Additionally, the invention provides a combination of different compounds as copper etch assisting agents, one compound being sticky for good protection of dielectrics and the other compound being more volatile and a more aggressive oxidizer to render the sputtered and re-deposited conductive copper non-conductive.
The invention provides etch assisting agents that satisfy both of the two primary requirements of copper etch assisting agents. First, they protect the adjacent dielectric, both conventional and low-k dielectrics, during copper etching. This requires that the etch assisting agent have low volatility, which implies high boiling point, low vapor pressure, high stickiness, and long residence time when adsorbed on a surface, because efficient protection requires collecting significant amount of the agent on the protected surface. Secondly, the etch assisting agents afford efficient and continuous oxidation during sputtering and re-deposition of conductive copper onto surfaces adjacent to the work spot. Since it is difficult and impractical to clean re-deposited copper following etching that is more that a couple of mono-layers deep, the agents oxidize copper both during sputtering and during or immediately following re-deposition as a mono-layer onto adjacent surfaces. This requires that an etch assisting agent be more volatile and be an aggressive and efficient oxidizer, and that the agent be delivered to all adjacent surfaces in sufficient amounts (flux) to oxidize the re-deposited copper.
If the etch assisting oxidizing gas is too sticky, i.e., has a low volatility due to a high boiling point and low vapor pressure, it has a long residence time when absorbed on a surface. The gas cannot reach re-deposited copper in the shadowed areas of the trench and will not be effective as an oxidizing agent. An etch assisting agent that is too sticky cannot provide efficient oxidation of the re-deposited copper in holes with a high enough aspect ratio. Yet, if the etch assisting agent is too volatile (low stickiness), it does not remain on the dielectric surface long enough to prevent dielectric etching.
In accordance with the invention, it has been found that certain compounds protect all known dielectrics from sputtering by Gallium (Ga+) ions. The distinguishing feature of these compounds is that they contain an N—N bonding (N being Nitrogen) in their molecules. Due to the presence of N—N fragments in the molecules, when exposed to an ion beam and to the flux of the etch assisting agent, dielectric sputtering is significantly compensated through growing of replacement dielectric by Nitrogen (N) from the etching agent combining with Silicon (Si) from the dielectric or Gallium (Ga+) from the ion beam to produce Si3N4 or GaN, respectively. Preferred compounds comprise hydrazine or hydrazine derivatives with boiling points between about 70° C. and 200° C.
More particularly, preferred compounds that have been found that have reasonably good stickiness for dielectric protection as well as good oxidation efficiency are Nitrosodimethylamine (“NDMA”), C2H6N2O, and Hydrazine monohydrate (“HMH”), N2H4*H2O. In addition, for high aspect ratio areas where the oxidizing compound should be very mobile, a combination of HDMA or HMH with Nitrogen Tetroxide, N2O4, may be used. Nitrogen Tetroxide is an extremely powerful oxidizer with a very high vapor pressure (boiling point of 21° C.) which makes it an ideal oxidizing agent for deep holes or other high aspect ratio (“HAR”) areas.
All the above compounds can be used at reasonably low temperatures of the target IC. Since their volatility decreases with decreasing temperature, their stickiness to the target surface increases, and the temperature of the work area may be controlled to control the etching process, as explained below. In fact, Nitrogen Tetroxide (N2O4) has particularly advantageous utility at temperatures in the range, for example, of −20° C. to +10° C., and more particularly in the range −15° C. to +10° C., that are reasonably close to its melting point of −11.2° C. Since N2O4 has a solid phase at this temperature, in a temperature range around this temperature increased amounts of N2O4 can be collected on surfaces of the IC. Therefore, N2O4 can work both for protection of the dielectric (it has an N—N group and is efficiently collected on surface at low temperature) and as an oxidizer for re-deposited copper.
The temperature of the IC may be lowered to a desired temperature for FIB copper etching by mounting the IC on a surface of a thermoelectric cooler (Peltier) module in the FIB vacuum chamber to cool the IC. The electrical inputs to the thermoelectric cooler can be controlled to manipulate the temperature of the IC through the Peltier effect to cool the IC to the desired temperature. A thermocouple in the chamber may be used to measure the temperature of the IC. U.S. Patent Publication US 2006/0065853, Mar. 30, 2006, to Chad Rue, which is hereby incorporated by reference herein, discloses such an apparatus and method for manipulating the temperature of a sample in a FIB that may be used for controlling the temperature of the IC during etching.
The following Table 1 gives a comparison of the properties of NDMA, HMH and N2O4 as copper etch assisting agents, along with NitroEthanol for reference purposes.
The following tables present operating parameters and working examples of FIB copper etching processes employing the foregoing gaseous etch assisting agents in accordance with the invention.
The gallium, Ga+, ion beam potentials of the FIB are preferably adjusted to be about 30 kV out of the ion column, and the ion beam current is preferably adjusted to a value within the range of the order of about 1 picoAmps (pA) or below to several tens of nanoAmps (nA) so that ion beam current density is in the range of the preferred values for each etch assisting agent, as specified below. The preferred gas pressures and current densities are listed in the following Table 2, which gives specific preferred FIB process parameters for each of the etching agents of the invention.
The values of the preferred parameters in Table 1 may vary and assume values within a range of ±30% of the preferred values shown. Time is a parameter that will depend upon the thicknesses of the etched layers.
The following Table 3 gives preferred working parameters when each of the two main copper etch assisting agents, NDMA and HMH is used in combination with the oxidizing agent N2O4 for both conventional and organic (low-k) dielectrics.
The following Tables 4-5 give working examples and representative operating parameters for NDMA and HMH, respectively, when used alone. The sets of parameters in the Tables have been found to produce good results for the different chemistries of the invention when used with both conventional SiO2 and organic dielectrics.
Since the sputtering rate of copper is proportional to the ion current density, the ratio of the current density to the flux of a particular gaseous etch assisting agent influences both the oxidation of the sputtered and re-deposited copper as well as the protection afforded to the dielectric. If the current density is too high, the gas does not have sufficient time to adequately oxidize the sputtered and re-deposited copper, and if the gas flux is too low, it may not adequately protect the dielectric. Generally, it is preferable to select a gas flux, and then adjust the current density to give a desired ratio. The values in the Tables have been found to afford good results. As may be appreciated, these values are representative values and may be adjusted within ranges about the values given for the specifics of a particular etching operation.
While the foregoing has been with reference to preferred embodiments of the invention, it may be appreciated that changes may be made in these embodiments without departing from the principles and the spirit of the invention, the scope of which is defined in the appended claims.
