Abstract:
A process system and a deposition method for depositing a highly controlled layered film on a workpiece is disclosed. The basic component of the apparatus is a pulsing plasma source that is capable of either exciting or not-exciting a first precursor. The pulsing plasma source includes an energy source to generate a plasma, and a plasma adjusting system to cause the plasma to either excite or not-excite a precursor. The precursor could flow continuously (an aspect totally new to ALD), or intermittently (or pulsing, standard ALD operation process). The deposition method includes the steps of pulsing the plasma to excite/not-excite the precursors and the ambient to deposit and modify the deposited layers. This procedure then can be repeated until the film reaches the desired thickness.

Description:
[0001]     This application is a continuation of U.S. patent application Ser. No. 10/744,889, filed Dec. 23, 2003 (TEGL-01182US1) which is a continuation of U.S. patent application Ser. No. 09/721,162, filed Nov. 22, 2000 (TEGL-01182US0), now U.S. Pat. No. 6,689,220, issued on Feb. 10, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to an apparatus for processing of semiconductor wafers, and more particularly to a system and method for deposition of thin films.  
       BACKGROUND OF THE INVENTION  
       [0003]     A fundamental process in IC fabrication is chemical vapor deposition (CVD), which uses vapor precursors to deposit thin films on a semiconductor substrate. The reactor used for CVD processes includes a precursor delivery system, a substrate and an energy source to decompose the precursor vapor to reactive species to allow a thin film to form on the substrate (CVD process). Effective power sources are heat and plasma energy such as radio frequency (RF) power, microwave energy (MW), low frequency (10 KHz-1 MHz) power, and optical energy (e.g. a laser or ultraviolet light) which decompose the introduced precursors. Plasma energy power is below 6000W. The amount of power required in each process is determined by the process reaction and a typical power level is between 500-1000W. Also, the substrate could be biased or heated (to 100° C.-1200° C.) to promote the reaction of the decomposed atoms or molecules and to control the physical properties of the formed films.  
         [0004]     Traditionally, precursors used in semiconductor CVD processes are gaseous. An example of a CVD process to deposit silicon dioxide (SiO 2 ) is to use gaseous precursors such as silane gas (SiH 4 ) and oxygen gas (O 2 ): 
 
SiH 4  (gas)+O 2 (gas)−(heat)→SiO 2  (solid)+2H 2  (gas) 
 
         [0005]     The basic requirements of a precursor are that the desired product (in this example, SiO 2 ) is solid and that all of the other products are gases (in this example, H 2 ) which can be exhausted away. The energy required for the reaction to take place is the thermal energy which is about 400-800° C.  
         [0006]     To broaden the processes, more and more liquid and solid precursors have been used, especially in the area of metal-organic chemical vapor deposition (MOCVD). To perform this task, a liquid precursor is typically first turned into vapor which decomposes and reacts on the substrate. A solid precursor must often be dissolved into a solvent to form a liquid precursor. The liquid precursor then must be converted into the vapor phase before being introduced into the deposition zone. An example of a CVD process to deposit copper (Cu) uses liquid precursor vapor copper HexaFluoroACetylacetone TriMethylVinylSilane (hfac-copper-tmvs, C 5 HO 2 F 6 —Cu—C 5 H 12 Si): 
 
2Cu-hfac-tmvs (vapor)−(heat)→Cu (solid)+hfac-Cu-hfac (gas)+2tmvs (gas) 
 
         [0007]     Another deposition technique is the atomic layer epitaxy (ALE) process. In ALE, the precursors are pulsed sequentially into the ALE process chamber. Each precursor sequentially generates a chemical surface reaction at the substrate surface to grow about an atomic layer of the material on the surface. The growth of one atomic layer in ALE is controlled by a saturating surface reaction between the substrate and each of the precursors. Sometimes a reduction sequence activated with extra energy such as heat or photon is used to re-establish the surface for a new atomic layer. The fundamental criterion of ALE is to have a minimum of two different chemical reactions at the surface with each reaction being carefully controlled to generate only one atomic layer. An example of ALE is the growth of ZnS at ˜470° C. using sequential flow of elemental zinc and sulfur as precursors as disclosed in U.S. Pat. No. 4,058,430 to Suntola et al. Another example of ALE is the growth of germanium (Ge) on a silicon substrate at ˜260-270° C. by first pulsing GeH 4  vapor to generate an atomic cover layer of GeH 4  and pulsing Xe lamp radiation to decompose the surface GeH 4  as disclosed by Sakuraba et al, J. Cryst. Growth, 115(1-4) (1991) page 79.  
         [0008]     The ALE process is a special case of atomic layer deposition (ALD). The focus of ALE is the deposition of epitaxial layers, which means forming perfect crystal structures. In contrast, the ALD process seeks to deposit one layer at a time with the focus on forming film uniformity, and not on creating single crystal structures.  
         [0009]     The major drawbacks of CVD and ALD processes are the high temperatures needed for the chemical reactions and the limited number of available precursors. CVD and ALD processes always start with an extensive evaluation of various potential precursors and their chemical reactions to determine see if there is a suitable process reaction.  
         [0010]     To lower the temperature needed for the chemical reaction, and to further promote possible reactions, plasma energy can be used to excite the precursors before the reaction takes place in CVD processes. Such processes are called plasma enhanced CVD (PECVD). An energy source using radio frequency (RF) power or microwave (MV) power can be used to generate a plasma, which is a mixture of excited gaseous species, to supply energy to the precursors to promote chemical reactions.  
         [0011]     However, there is no plasma enhanced ALD process or equipment. The main advantages of a plasma enhanced ALD would be the low temperatures required for the reactions to occur, and the increased number of precursors made available by the addition of plasma energy to excite the precursors. Furthermore, plasma treatment could modify the surface condition which also leads to a wider range of available precursors.  
         [0012]     It would be advantageous to develop a plasma enhanced ALD system.  
         [0013]     It would be advantageous if plasma treatment could be incorporated in an ALD process.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, a plasma enhanced atomic layer deposition (PEALD) apparatus that offers atomic layer deposition capabilities using a plasma source to excite precursors is provided. In addition to the prior art surface reactions using non plasma-excited precursor, the present invention also offers surface reactions using plasma-excited precursors. With plasma-excited precursors, the surface reaction could cause either a deposition reaction or a material modification by plasma bombardment.  
         [0015]     The basic component of the present invention apparatus is a pulsing plasma source that is capable of either exciting or not-exciting a first precursor. The pulsing plasma source includes an energy source that generates a plasma, and a plasma adjusting system that causes the plasma to either excite or not-excite a precursor. The precursor could flow continuously (an aspect totally new to ALD), or intermittently (or pulsing, which is a standard ALD operation process).  
         [0016]     The plasma power source is preferably an inductive coupled plasma (ICP) source, but any plasma source, such as a capacitance plasma source, microwave guide plasma source, electron cyclotron resonance plasma source, magnetron plasma source, DC power plasma source, etc., works equally well.  
         [0017]     In the simplest design, the plasma adjusting system is a power switch, that causes the plasma to be either ON or OFF. When the plasma is OFF, the precursor is not excited by the plasma because there is no plasma. When the plasma is ON, the precursor is excited by the plasma. Typical plasma power when ON is between 15 to 6000W. The low power is used for sensitive precursors such as those containing organic components. The timing for this design is long, in the order of many seconds, because of the time needed for the plasma to stabilize.  
         [0018]     To shorten the plasma stabilizing time, the plasma adjusting system comprises a two-level plasma power switch: a low power first level and a high power second level. The first level plasma power generates a plasma that does not excite the precursor, either because the power is low enough or the precursor is far away from the plasma. The second level plasma power generates a large enough plasma to excite the precursor. By using the first level plasma, the stabilizing time is much shorter because the plasma is already present, and powering up from the first level to the second level power takes a shorter time. The first power level is typically from 15 to 300W and the second power level is from 100 to 6000W.  
         [0019]     Another way to block the plasma is to apply an electric field. The plasma adjusting system then comprises an electrode that has a potential. By varying the potential, the plasma could either pass through or be confined. At ground potential, the electrode will terminate the plasma which allows no plasma to pass through. At a positive potential, the electrode will repel all positive charges in the plasma field which allows only negative charges such as electrons to pass through. At a floating potential, which means that the electrode is not connected, the electrode will obtain a self-potential but the plasma will pass through. The electrode could be the workpiece support or a wire mesh above the workpiece.  
         [0020]     The present invention apparatus further comprises a heater source to raise the workpiece to a process temperature.  
         [0021]     The present invention apparatus further comprises a second precursor that is positioned in a way such that it is always not excited by the plasma, and a third precursor that is positioned in a way such that it is always excited by the plasma. These precursors complement the pulsing action of the plasma source on the first precursor for developing a wider selection of process conditions.  
         [0022]     The present invention apparatus further comprises pulsing systems for the first, second and third precursors. The pulsing systems further allow the sequential deposition of these precursors. Together with the pulsed plasma, using the pulsed precursors offer more control of the timing of the process. The first, second and third precursors could comprise a plurality of precursors with different pulsing systems. As such, the precursors could pulsed together at the same time, pulsed at different times, or pulsed in a synchronized fashion such that when one of the pulsed precursors is on, the others are off.  
         [0023]     The present invention further provides a method to deposit atomic layer using plasma enhanced ALD system. The method comprises the steps of: 
        (a) the plasma does not excite the first precursor flow; and     (b) the plasma does excite the first precursor flow;     with the stepping sequence being interchangeable, meaning that either step (a) before step (b) or step (b) before step (a). This procedure can then be repeated alternately until the film reaches the desired thickness.        
 
         [0027]     Generally, step (a) deposits a layer of material from the un-excited precursor to the workpiece surface. Step (b) could deposit a different layer from the excited precursor to the workpiece surface, or step (b) could modify the previously deposited layer with the excited precursor. Therefore, with step (b) deposition, the method grows a thin film on a substrate by: 
        (a) subjecting the substrate to the vapor of the un-excited precursor to form a layer of material on the substrate; and     (b) subjecting the thus-formed surface to the vapor of the excited precursor to form a different layer of material on the thus-formed surface.        
 
         [0030]     With the step (b) modification, the method grows a thin film by: 
        (a) subjecting the substrate to the vapor of the un-excited precursor to form a layer of material on the substrate; and     (b) subjecting the thus-formed surface to the vapor of the excited precursor to modify the material of the deposited layer on the thus-formed surface.        
 
         [0033]     The present invention method also provides for the presence of a second precursor that is always not-excited and a third precursor that is always excited. Steps (a) and (b) then have the second and third precursors together with the first precursor. The addition of the second and the third precursors broadens the process parameters, which allows for the development of many advanced processes.  
         [0034]     The precursor flows in steps (a) and (b) above can be continuous or intermittent (pulsing). An example of continuous precursor flow is TetraDiMethylAminoTitanium (TDMAT) in a process used to produce TiN film. With a low enough substrate temperature, TDMAT precursor does not react at the substrate. With the plasma on, TDMAT is decomposed and forms a thin TiN layer. With a pulsing plasma, a highly controlled layered TiN film is formed. Adding a continuous flow of a non-excited second precursor such as nitrogen gas, for example, could reduce the TDMAT partial pressure for controlling the deposition rate. Adding a continuous flow of a third plasma-excited precursor, such as nitrogen and hydrogen, for example, could change the film composition such as by reducing the amount of carbon.  
         [0035]     The first, second and third precursors could each comprise a plurality of precursors. With pulsing precursors, which means that the precursor flow is either on or off, using a plurality of precursors offers sequential flow. For example, the first of the first precursors could flow, and then stop, then the second of the first precursors could flow, and then stop, and so on, until the last of the first precursors, before returning to the first of the first precursors.  
         [0036]     Also, with pulsing precursors, a method in accordance with the present invention includes two more steps: 
        (c) when the precursor flow is off, the plasma does not excite the process chamber ambient residue enough to have an effect on the workpiece; and     (d) when the precursor flow is off, the plasma does excite the process chamber ambient residue enough to have an effect on the workpiece,     with the stepping sequence being interchangeable, which means that either step (c) before step (d) or step (d) before step (c). The stepping sequence between step (a), step (b), step (c) and step (d) is also interchangeable depending on the process development. The difference between steps (c), (d) and steps (a), (b) is the absence of the precursor flow. Steps (a), (b) occur when there is a precursor flow and steps (c, d) occur when there is no precursor flow. Without precursor flow, the ambient still has enough residual gaseous particles to sustain a plasma. This plasma, though without active precursor, still has enough energy to have an effect on the deposited film. With four steps, there are 4×3×2×1=24 possible sequences. This procedure can then be repeated alternately until the film reaches the desired thickness.        
 
         [0040]     An example is the sequence: a c (a c a c . . . ). This is the prior art ALD process where one of the first precursors flows without being excited by the plasma to form a layer on the workpiece, and then stops, then another of the first precursors flows without being excited by the plasma to form another layer on the previous layer, and then stops, and the sequence continues until the film reaches the desired thickness. A variation of this example is the sequence b c (b c b c . . . ). In this sequence, one of the first precursors flows while being excited by the plasma to form a layer on the workpiece, and then stops, then another of the first precursors flows while being excited by the plasma to form another layer on the previous layer or to modify the material of the previous layer, and then stops, and the sequence continues. A specific example of this sequence is the use of TetraDiMethylAminoTitanium (TDMAT) in a process to produce TiN film. With the plasma on, TDMAT decomposes and forms a thin TiN layer (step b), then stops (step c). Then plasma-excited nitrogen and hydrogen flow modifies this thin TiN layer (step b) and then stop (step c). The sequence continues until the film reaches the desired thickness.  
         [0041]     Another example is the combination of the first two example a c b c (a c b c . . . ). In this sequence, one of the first precursors flows without being excited by the plasma to form a layer on the workpiece, and then stops, then another of the first precursors flows while being excited by the plasma to form another layer on the previous layer or to modify the material of the previous layer, and then stops, and the sequence continues. A specific example of this sequence is the use of TetraDiMethylAminoTitanium (TDMAT) in a process to produce TiN film. With plasma off, TDMAT is not quite decomposed and a thin layer of TDMAT coats the substrate (step a), then stops (step c). Then plasma-excited nitrogen and hydrogen flow modifies this thin TDMAT layer (step b) and then stop (step c). The sequence continues until the film reaches a desired thickness. A variation of this combination is the sequence b c a c (b c a c . . . ).  
         [0042]     Another example is the sequence: b c d c (b c d c . . . ). In this sequence, one of the first precursors flows while being excited by the plasma to form a layer on the workpiece, and then stops, then the ambient is excited by the plasma to modify the newly deposited layer, and then stops, and the sequence continues. A specific example of this sequence is the use of TetraDiMethylAminoTitanium (TDMAT) in a process to produce TiN film. With plasma on, TDMAT decomposes and forms a thin TiN layer (step b), then stops (step c). Then plasma-excited nitrogen and hydrogen flow modifies this thin TiN layer (step b) and then stops (step c). Then the plasma-excited ambient also modifies this thin TiN layer (step d) and then stops (step c). The sequence continues until the film reaches the desired thickness. A variation of this example is the sequence: a c d c (a c d c . . . ). In this sequence, one of the first precursors flows without being excited by the plasma to form a layer on the workpiece, and then stops, then the ambient is excited by the plasma to modify the newly deposited layer, and then stops, and the sequence continues.  
         [0043]     Another example is the sequence: a b c (a b c . . . ). In this sequence, one of the first precursors flows without being excited by the plasma to form a layer on the workpiece, then this precursor flows while being excited by the plasma to form another layer on the workpiece or to modify the material of the previous layer, and then stops. A variation of this sequence is: a b d c (a b d c . . . ). In this sequence, the ambient is excited by the plasma to modify the newly deposited layer before stopping.  
         [0044]     Another example is the sequence: b a c (b a c . . . ). In this sequence, one of the first precursors flows while being excited by the plasma to form a layer on the workpiece, then this precursor flows without being excited by the plasma to form another layer on the workpiece or to modify the material of the previous layer, and then stops. A variation of this sequence is: d b a c (d b a c . . . ). In this sequence, the ambient is excited by the plasma to clean the surface before the precursor flows.  
         [0045]     Although a few of the sequences for practicing the method of the invention have been disclosed, it will be appreciated that there are many more sequences and further modifications and variations thereto that may be made while keeping within the scope of the invention as defined in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]      FIG. 1  shows the present invention plasma enhanced atomic layer deposition system.  
         [0047]      FIG. 2  shows another embodiment of the present invention plasma enhanced atomic layer deposition system.  
         [0048]      FIG. 3  shows the pulsing manifold of precursor source.  
         [0049]      FIG. 4  shows the characteristic table of step a and step b of the present invention method of deposition.  
         [0050]      FIG. 5  shows the characteristic table of step a, step b, step c, and step d of another embodiment of the present invention method of deposition.  
         [0051]      FIG. 6  shows the deposited layers of step a.  
         [0052]      FIG. 7  shows the deposited layers of step b.  
         [0053]      FIG. 8  shows the deposited layers of step c.  
         [0054]      FIG. 9  shows the deposited layers of step d.  
         [0055]      FIG. 10  shows the deposited layers of sequence b a b.  
         [0056]      FIG. 11  shows the deposited layers of sequence a c a c.  
         [0057]      FIG. 12  shows the deposited layers of sequence b c b c.  
         [0058]      FIG. 13  shows the deposited layers of sequence b c d c.  
         [0059]      FIG. 14  shows the deposited layers of sequence a b c.  
         [0060]      FIG. 15  shows the deposited layers of sequence a b d c. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]      FIG. 1  shows the present invention plasma enhanced atomic layer deposition system. The system includes a process chamber  7  with a workpiece  6  for deposition of layered films. Heater  17  controls the temperature of the workpiece  6  at the process temperature. An energy source  10  generates a plasma inside the process chamber  7 . The plasma has two stages:  4  and  5 , which are controlled by a plasma adjusting system (not shown) to increase the plasma from position  4  to position  5  (via direction  16 ) and vice versa. The plasma adjusting system in  FIG. 1  controls the plasma power, with plasma  4  containing less power than plasma  5 . By increasing the plasma power, more gas ionizes and the plasma volume increases. The process chamber includes two first precursor inlets  12  and  15  having precursor flows  1  and  11 , respectively. The first precursors inlets and the plasma source are positioned in such a way that plasma  4  does not excite the precursors  1  and  11 . With plasma  5 , the precursors  1  and  11  are excited, thus the system uses plasma pulsing to pulse the energy of the precursors  1  and  11 . The system further includes a second precursor inlet  13  having precursor flow  2 . This precursor  2  is not excited by the plasma because of its position beneath the plasma area. The system further includes a third precursor inlet  14  having precursor flow  3 . This precursor  3  is always excited by the plasma because it always passes through the plasma area. An exhaust  9  maintains an exhaust flow  8  to keep the chamber at the desired pressure.  
         [0062]      FIG. 2  shows another embodiment of the present invention plasma enhanced atomic layer deposition system. This system uses an electrode  22  that is controlled by a voltage source  21  to adjust the plasma. By grounding the electrode  22 , the plasma terminates at this electrode  22 . By allowing the electrode  22  to float, the plasma ignores this electrode and resumes a large area.  
         [0063]      FIG. 3  shows the pulsing manifold of the precursor source. Precursor inlet is  31 . Chamber inlet is  32  and vacuum pump exhaust is  33 . By switching the valves  34  and  35 , the precursor flow  31  can be effectively pulsed. When valve  34  is opened and is valve  35  closed, the precursor flows into the chamber. When valve  34  is closed and valve  35  is opened, the precursor flows to the exhaust. This pulsing manifold allows fast switching of the precursor flow without the need for gas flow stabilization.  
         [0064]      FIG. 4  shows the characteristic table of step (a) and step (b) of the present invention method of deposition. The first precursor gas  1  is controlled by the plasma position. When the plasma is on, gas  1  is excited. When the plasma is off, gas  1  is not excited. The second precursor gas  2  is always not excited by the plasma and the third precursor gas  3  is always excited by the plasma.  
         [0065]      FIG. 5  shows the characteristic table of step (a), step (b), step (c) and step (d) of another embodiment the present invention method of deposition. Steps (a) and (b) are the same as in  FIG. 4  which are, applicable when gas  1  is flowing. When gas  1  is not flowing, the ambient, which is composed of residual gas, is controlled by the plasma power. The ambient gas is excited by the plasma when the plasma is on. When gas  1  is flowing and the amount of plasma is on, the ambient gas is small compared to that of gas  1 , thus its effect is negligible.  
         [0066]      FIG. 6  shows the deposited layers of step (a): not-excited gas  1  flows. With certain temperatures, no film is deposited on the substrate  40  (path  44 ). With the right temperature, a layer  43  is deposited on the substrate  40  (path  45 ).  
         [0067]      FIG. 7  shows the deposited layers of step (b): excited gas  1  flow. The excited gas  1  could react with the present layer  49  and convert layer  49  to another layer  51  that has different properties (path  53 ). The excited gas  1  could deposit a layer  52  on the existing layer  49  on top of the substrate  46  (path  54 ).  
         [0068]      FIG. 8  shows the deposited layers of step (c): no gas  1  flow and no excited ambient. Nothing happens so that layer  56  on substrate  55  remains layer  56  on substrate  55  (path  57 ).  
         [0069]      FIG. 9  shows the deposited layers of step (d): no gas  1  flow and the ambient is excited. The plasma excited ambient reacts with the layer  61  on substrate  60 . Layer  61  undergoes reaction to become layer  62  that has different properties (path  63 ).  
         [0070]      FIG. 10  shows the deposited layers of sequence b a b. The workpiece starts with layer  71  on substrate  70 . After step (b) (path  75 ), a layer  72  is deposited on layer  71 . The temperature is chosen such that no reaction occurs during step (a), thus nothing happens to the workpiece (path  76 ). After another step (b), layer  72  could undergo plasma reaction to become layer  74  that has different properties (path  77 ), or could a layer  73  can be deposited on layer  72  (path  78 ).  
         [0071]      FIG. 11  shows the deposited layers of sequence a c a c. The workpiece starts with layer  81  on substrate  80 . After step (a), a layer  82  is deposited on layer  81  (path  84 ). Step (c) cleans out the off precursors, thus nothing happens to the workpiece (path  85 ). Another step (a) deposits layer  83  on layer  82  (path  86 ). Step (c) cleans out the off precursors, thus nothing happens to the workpiece (path  87 ).  
         [0072]      FIG. 12  shows the deposited layers of sequence b c b c. The workpiece starts with layer  91  on substrate  90 . After step (b), a layer  92  is deposited on layer  91  (path  94 ). Step (c) cleans out the off precursors, thus nothing happens to the workpiece (path  95 ). Another step (b) could promote a reaction with layer  92  to create layer  99  that has different properties (path  96 ) or could deposit layer  93  on layer  92  (path  97 ). Step (c) cleans out the off precursors, thus nothing happens to the workpiece (path  98 ).  
         [0073]      FIG. 13  shows the deposited layers of sequence b c d c. The workpiece starts with layer  101  on substrate  100 . After step (b), a layer  102  is deposited on layer  101  (path  104 ). Step (c) cleans out the off precursors, thus nothing happens to the workpiece (path  105 ). Step (d) promotes a reaction with layer  102  to create layer  103  that has different properties (path  106 ). Step (c) cleans out the off precursors, thus nothing happens to the workpiece (path  107 ).  
         [0074]      FIG. 14  shows the deposited layers of sequence a b c. The workpiece starts with layer  111  on substrate  110 . After step (a), a layer  112  is deposited on layer  111  (path  114 ). A step (b) could promote a reaction with layer  112  to create layer  118  that has different properties (path  115 ) or could deposit layer  113  on layer  112  (path  116 ). Step (c) cleans out the off precursors, thus nothing happens to the workpiece (path  117 ).  
         [0075]      FIG. 15  shows the deposited layers of sequence a b d c. The workpiece starts with layer  121  on substrate  120 . After step (a), a layer  122  is deposited on layer  121  (path  134 ). A step (b) could promote a reaction with layer  122  to create layer  124  that has different properties (path  135 ) or could deposit layer  123  on layer  122  (path  136 ). Step (d) does not have an effect on layer  124  (path  137 ) or promotes a reaction with layer  123  to create layer  125  that has different properties (path  138 ). Step (c) cleaning out off precursors, thus nothing happens to the workpiece (path  139 ).  
         [0076]     Although preferred embodiments of practicing the method of the invention have been disclosed, it will be appreciated that further modifications and variations thereto may be made while keeping within the scope of the invention as defined in the appended claims.