Abstract:
An electromagnetic field generator and method of operation for ion beam deposition of magnetic thin-film materials is presented. A combination of open frame electromagnetic field generator elements provides precise control of magnetic field directionality. This control enables deposition of oriented magnetic films with minimal directionality error. The magnetic field direction may be oriented to enable the deposition of alternating layers of directionally oriented magnetic films. An open frame element reduces the weight of the electromagnetic field generator while truncated corners reduce diagonal clearance that may be required in a vacuum chamber. An open frame design also enables the electromagnetic field generator to surround and thus remain clear of the active deposition area; the electromagnetic field generator can thus be shielded from accumulation of sputtered material. Shielding from accumulation of sputter material reduces maintenance requirements.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to deposition of magnetic materials. More particularly, the invention relates to an electromagnetic field generator for improved deposition of magnetic materials and a method of operation.  
         BACKGROUND OF THE INVENTION  
         [0002]    Deposition systems (e.g. ion beam, physical vapor, and evaporation deposition) are capable of depositing high-quality thin films of a wide variety of materials on many different types of substrates. Applications for ferromagnetic thin films, such as deposition of magnetoresistive (MR) and giant magnetoresistive (GMR) films for thin-film magnetic heads, usually require orientation of the magnetic moments in a specific direction by application of an external magnetic field. The required field strength is usually 20-100 Gauss.  
           [0003]    It has been found, however, that DC magnetic fields are troublesome for plasma deposition systems due to interactions between the magnetic field and the plasma. Such interactions can, for example, decrease the thickness uniformity of the deposited magnetic film as a result of decreased plasma uniformity. We have also found that magnetic fields in the deposition chamber can adversely affect ion beam deposition processes even at levels as low as 10-15 Gauss due to interactions of the magnetic fields with the ion beam. During ion beam deposition, magnetic field disturbances can cause broadening or displacement of the ion beam which can result in film contamination. Further, the electrons in the ion beam chamber, which normally act to neutralize any charge build-up on the substrate, are very easily trapped by magnetic fields. Such trapping can cause loss of neutralization and subsequent arc damage on any exposed insulating surfaces on the substrate or any electrostatic discharge-sensitive device structures embedded in the substrate wafer, such as magnetoresistive sensors. This is particularly important if an ion-assisted deposition process is used because the assisting ion beam is aimed at the substrate directly.  
           [0004]    What is needed is a method of depositing magnetic materials in a deposition system in the presence of an assisting magnetic field without disrupting the source or creating charge build-up on the sample or in the chamber.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the present invention, there is described an electromagnetic field generator and method of operation for deposition of magnetic materials. The electromagnetic field generator comprises an open frame electromagnetic assembly having a first pair of spaced apart magnetic members for generating on a substrate a magnetic field in a first direction and a second pair of magnetic members for generating a magnetic field on the substrate in a second direction. Preferably the electromagnetic field generator includes a second electromagnetic assembly spaced apart from the first assembly and magnetically linked to the first assembly to enhance field uniformity on the substrate. The method comprises the steps of: placing a sample within an electromagnetic field generator with at least two selectable magnetic field orientations; operating a source, for example an ion source or plasma source, to deposit material onto the sample influenced by the electromagnetic field generator; and creating a field pattern around the sample with the electromagnetic field generator to control the deposition of the magnetic material.  
           [0006]    There are also described various methods for control of deposited materials using different current signals applied to the electromagnetic field generator, with resultant different magnetic fields. The currents include alternating current; positive/negative pulsed direct current; exponentially decaying alternating current; half-wave rectified alternating current; positive-pulsed direct current; positive direct current bias; pulsed direct current with positive direct current bias; and time phased magnetic field and ion generator operation.  
           [0007]    An objective is that a combination open frame/base plate electromagnetic field generator provides measured magnetic field directionality within a tolerance of 0.5 degrees over a six inch square substrate or an eight inch circular substrate. This enables deposition of oriented magnetic films with minimal error in directionality.  
           [0008]    Another objective is the ability to orient the magnetic field in any arbitrary direction which enables the deposition of alternating layers of differently oriented magnetic films.  
           [0009]    Another objective is the ability to continuously change the orientation of the magnetic field in a rotating manner, so as to demagnetize the deposited film.  
           [0010]    Another objective is that it provides a uniform magnetic field region over a sample with field uniformity of +/−5%.  
           [0011]    Another objective is the reduction in weight afforded by the open frame top plate with its truncated corners. The truncated corners also reduce the diagonal clearance required in the chamber to accommodate the field generator. The open frame design also enables the electromagnetic field generator to surround and thus remain clear of the active deposition area; the electromagnetic field generator can thus be shielded from accumulation of sputtered material. This reduces the level of maintenance required to keep the field generator in peak operating condition.  
           [0012]    It is not intended that the invention be summarized here in its entirety. Rather, further features, aspects, and advantages of the invention are set forth in or are apparent from the following description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a top view of a single frame electromagnetic field generator;  
         [0014]    [0014]FIG. 2 is a perspective view of a four-coil two-open frame electromagnetic field generator with pole pieces;  
         [0015]    [0015]FIG. 3 is a perspective view of an eight-coil two-open frame electromagnetic field generator with pole pieces;  
         [0016]    [0016]FIG. 4 is a perspective view of an electromagnetic field generator;  
         [0017]    [0017]FIG. 5A is a detailed schematic perspective view of the electromagnetic field generator and wafer chuck;  
         [0018]    [0018]FIG. 5B is a detailed schematic perspective view of the wafer chuck and electromagnetic field generator base plate;  
         [0019]    [0019]FIG. 6A is a plot of electromagnetic field generator current vs. time using alternating current (AC);  
         [0020]    [0020]FIG. 6B is a plot of electromagnetic field generator current vs. time using positive and negative (+/−) pulsed direct current (DC);  
         [0021]    [0021]FIG. 6C is a plot of electromagnetic field generator current vs. time using alternating current (AC) with an exponentially decaying amplitude;  
         [0022]    [0022]FIG. 7A is a plot of electromagnetic field generator current vs. time using half-wave rectified alternating current;  
         [0023]    [0023]FIG. 7B is a plot of electromagnetic field generator current vs. time using positive (+) pulsed direct current;  
         [0024]    [0024]FIG. 7C is a plot of electromagnetic field generator current vs. time using a positive direct current bias;  
         [0025]    [0025]FIG. 8A is a plot of electromagnetic field generator current vs. time using alternating current with a positive direct current bias;  
         [0026]    [0026]FIG. 8B is a plot of electromagnetic field generator current vs. time using pulsed direct current with a positive direct current bias;  
         [0027]    [0027]FIG. 9 is a plot of electromagnetic field generator current vs. time using positive (+) pulsed direct current operation with a phase difference between the electromagnetic field generator and the ion beam currents;  
         [0028]    [0028]FIG. 10A illustrates the phase relationship of the current in two orthogonal coil, producing a continuously changing orientation of the magnetic field;  
         [0029]    [0029]FIG. 10B illustrates the field orientation, as related to FIG. 10A.  
         [0030]    [0030]FIG. 11 A is a top view of a single frame electromagnetic field generator with a first pair of coils energized to produce a field in a first direction;  
         [0031]    [0031]FIG. 11B is a top view of a single frame electromagnetic field generator with a second pair of coils energized to produce a field in a second direction; and  
         [0032]    [0032]FIG. 12 illustrates a sliding contact to change the effective number of turns in a coil. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0033]    The invention describes an apparatus with various alternative embodiments and methods of operation to control material deposition.  
         [0034]    Apparatus  
         [0035]    The terms substrate, wafer and sample are to be used interchangeably throughout this specification. The term open-frame member is intended to be descriptive and within the specification and claims is intended to describe a member that has a larger open area. Thus, an open-frame member is contrasted with a substantially solid member, the substantially solid member being understood to have a smaller open area than the open-frame member. The term substantial or substantially is also used in the specification and the claims, and is intended to be a term of description allowing for some variation, without any particular defining quantitative measure.  
         [0036]    Referring first to FIG. 1, a first embodiment of an electromagnetic field generator in accordance with the invention is shown. Specifically, the generator comprises an open frame member  110  having a first pair of spaced apart magnetic members  120  for generating on a substrate  115  a magnetic field in a first direction and a second pair of spaced apart magnetic members  130  for generating a magnetic field on the substrate in a second direction. Preferably, magnetic members  120  are disposed perpendicular to magnetic members  130  so that the first and second magnetic fields are orthogonal. Open frame  110  may be constructed, for example, of any of the variety of soft steels.  
         [0037]    Referring to FIGS. 11A and 11B, the orthogonal pairs of coils  120  and  130  may be selectively activated to enable deposition of orthogonally oriented magnetic films. In FIG. 11A, the magnetic field is formed in a first direction when coils  120  are energized. In FIG. 11B, the magnetic field is formed in a second direction when coils  130  are energized. Energizing both coils simultaneously can yield any arbitrary field orientations, with the orienting direction being proportional to the ratio of the currents through the two coils.  
         [0038]    [0038]FIG. 2 is a perspective view of a four-coil two-open frame electromagnetic field generator according to a second embodiment of the invention. The pairs of coils  210  and  215  are wrapped around open frame members  220  and  230  to induce a magnetic field in central opening  240 . Orthogonally oriented magnetic fields are applied using opposing coil pairs  210  and  215 . The induced magnetic fields are most uniform within the central opening  240 , but also extend above the opening, where a wafer or substrate (not illustrated) would most preferentially be positioned. Four pole pieces  250  at each corner help shape the electromagnetic field and increase the field uniformity, especially above central opening  240 . Pole pieces  250  may be constructed, for example, from soft steel. To accommodate packaging with other components, open frame members  220  and  230  may have truncated or rounded corners.  
         [0039]    [0039]FIG. 3 is a perspective view of an eight-coil two-open frame electromagnetic field generator with pole pieces  250  according to a third embodiment of the invention. This embodiment differs from the second embodiment, illustrated in FIG. 2, in that an open frame  310  is wrapped with coil pairs  330  and  335  and open frame  320  is wrapped with coil pairs  340  and  345 . Using individual coils for each open frame, this embodiment allows individual control of the current in each of the coils and thereby provides greater control over the magnetic field generated by each coil. To accommodate packaging with other components, open frames  310  and  320  may have truncated or rounded corners.  
         [0040]    In an alternative embodiment for the embodiments illustrated in FIGS. 2 and 3, electrical windings  210 ,  215 ,  330 ,  335 ,  340  and  345  may be replaced by embedded windings or patterned electrodes.  
         [0041]    [0041]FIG. 4 is a perspective view of an eight-coil two-open-frame electromagnetic field generator. In this configuration, electromagnetic field generator  400  includes base plate  410  which may include a central opening  430 . In this configuration, central opening  430  accommodates a wafer chuck (not shown) and other mechanical and electrical feed-through. Central opening  430  may also be surrounded by a winding-free zone  420  for mechanical connections. The remainder of base plate  410  is preferably wrapped with coil pairs  480  and  485  to induce the orthogonal magnetic fields as previously described. In another configuration, a substrate or wafer chuck and required feed-through may be supported from the side of electromagnetic field generator  400 , thus eliminating the need for central opening  430 .  
         [0042]    In FIG. 4, an open frame member  450  has a first pair of spaced apart magnetic members  440  for generating on a substrate a magnetic field in a first direction and a second pair of spaced apart magnetic members  445  for generating a magnetic field on the substrate in a second direction. Open frame member  450  has central opening  460 . To accommodate packaging with other components, top plate  450  may have truncated or rounded corners. Top plate  450  and base plate  410  are spaced apart to accommodate a substrate or wafer chuck between the plates and other mechanical and electrical components. Base plate  410  has magnetic field lines oriented in such a way that they reinforce the top plate  450  field lines in the desired direction and partially cancel undesired components in the other direction in order to create a more uniform field on the sample. Said differently, base plate  410  straightens and strengthens the field provided by top frame  450 .  
         [0043]    Though FIG. 4 is not illustrated with pole pieces, they may be used and oriented as illustrated in FIGS. 2 and 3. If used with the apparatus of FIG. 4, pole pieces help shape the electromagnetic field and increase field uniformity of the apparatus.  
         [0044]    The wafer chuck (not shown) is preferably positioned within central opening  460  such that the wafer rests just above the center  470  of the magnetic field created by top plate  450 . With respect to field uniformity, this is the preferred location. However, considering other factors, including cleanliness (keeping sputtered material off of the electromagnetic field generator) and shadowing, the substrate may be located above the center  470  of the magnetic field without substantially compromising magnetic field uniformity.  
         [0045]    [0045]FIG. 5A is a perspective view of the electromagnetic field generator and wafer chuck unit  500  according to one embodiment of the invention. Open frame apparatus  510  houses the magnetic field generator top plate, which could be in the form of an open frame  220  (FIG. 2) or  310  (FIG. 3) or  450  (FIG. 4). In this configuration, wafer  530  preferably rests slightly above the center of the induced magnetic field and slightly below the mid-plane of open frame apparatus  510 . The wafer  530  is held in place by clips  520  which are attached to the wafer chuck  560 .  
         [0046]    Open frame apparatus  510  may be water-cooled and differentially pumped to a pressure between chamber base pressure and atmosphere, for example 10 −3  Torr. Open frame apparatus  510  is supported above cover plate  540  by supports  570 . Preferably, one of the supports  570  is hollow and accommodates the electrical and mechanical feed-through for open frame apparatus  510 . Cover plate  540  encloses the open frame  230  (FIG. 2) or  320  (FIG. 3) or base plate  410  (FIG. 4) of the electromagnetic field generator. Regardless of the type of lower element, the base portion of the electromagnetic field generator lies below cover plate  540 , and may be isolated from the surrounding vacuum chamber, or maintained at atmospheric pressure because the insulating materials associated with the electrical windings ( 480  &amp;  485  in FIG. 4) are potential contaminants to the vacuum chamber.  
         [0047]    Cover plate  540  may also have a center opening, hidden from view in FIG. 5A. As shown in FIG. 5B, the shaft  555  of wafer chuck  560 , may pass through that opening in the center of cover plate  540 . The remaining support structure  550  shown in FIG. 5A preferably houses the required mechanical and electrical feed-through for unit  500  which are standard components of wafer chuck assemblies.  
         [0048]    [0048]FIG. 5B is a perspective view of the wafer chuck  560  and electromagnetic field generator top plate of FIG. 5A with open frame apparatus  510  exposed for clarity. Wafer chuck  560  is shown supported by wafer seat  565  and shaft  555  passing through opening  545  in base plate  535 . Base plate  535  may have the configuration of open frame  210  (FIG. 2),  320  (FIG. 3) or  410  (FIG. 4). As an example, the base plate configuration  410  (FIG. 4) is illustrated, though electrical winding pairs  440  and  445  (FIG. 4) are omitted from the illustration. Wafer seat  565  may include heating and/or cooling elements for maintaining temperature control of the wafer  530 .  
         [0049]    In FIG. 5A, the portions of the electromagnetic field generator/wafer chuck unit  500  which are not responsible for generating a magnetic field are preferably constructed of non-magnetic materials. The structures immediately surrounding the wafer, for example within 3 or 4 inches of the wafer, including wafer chuck  560 , clips  520 , wafer seat ( 565  in FIG. 5B), shaft ( 555  in FIG. 5B), cover plate  540 , and open frame apparatus  510  are preferably constructed from non-magnetic stainless steel alloys  321 ,  316  or  310 , aluminum, or copper. The remaining structures of wafer chuck  560  may also be constructed from these non-magnetic materials  321 ,  316  or  310 , or may be constructed from stainless steel alloy  304 . Stainless steel alloy  304  becomes magnetized over time when exposed to high magnetic fields and so should not be used in close proximity to the wafer  530 .  
         [0050]    The examples provided have used a two-magnetic-element design, with each magnet element having a different number of turns “n” of magnetic wire, and each energized by a separate power supply to provide the optimum current “I” through the element. In this and other embodiments where the magnetic flux is proportional to the current, it is assumed that the soft magnetic core is not magnetically saturated at the maximum current, otherwise it is not possible to vary the magnetic flux with a proportional variation in the current.  
         [0051]    In another of the preferred embodiments, a single power supply is used to energize each magnetic element. The magnetic flux from any magnetic element is directly proportional to either “n” or “I” and thus the magnetic flux at any point from magnetic element # 1  is proportional to n 1  and I 1  and from magnetic element # 2  is proportional to n 2  and I 2 . Thus with any combination of n 1 , n 2 , I 1  and I 2 , the number of turns in an element can be adjusted from n 1  to n 1 ′ so that the magnetic element will produce the same magnetic flux when energized with current I 2  as it did with n 1  and current I 1 . In this manner, the same power supply can be used for both magnetic elements.  
         [0052]    The required number of new turns can be calculated as: 
           n   1 ′= n   1 * I   1 / I   2   
         [0053]    In an example of this embodiment, where I 1  is 6 amps, n 1  is 185 turns, I 2  is 8 amps and n 2  is 288 turns, to achieve the same result using the same power supply (I 1 =I 2 =8 amps), the number of turns (n 1 ) is changed to 6/8* n 1  or about 139 turns.  
         [0054]    It should be noted that while the example has used just two magnetic element, the technique applies equally to multiple magnetic elements.  
         [0055]    Referring to FIG. 12, a simple method to adjust the effective number of turns on a coil is illustrated. A sliding contact  600  makes adjustable contact along the coil  601  to change the effective number of turns. Using a similar method to that illustrated in FIG. 12, the effective number of turns on any coil illustrated in FIGS. 1, 2,  3 ,  4 ,  5 A,  5 B,  11 A or  11 B may be adjusted. In this manner, the effective number of turns is widely variable and therefor readily supports use of a single power supply to provide variable magnetic flux.  
         [0056]    Method of Operation  
         [0057]    According to one aspect of the present invention, the electromagnetic field generator is operated in a cyclic or pulsed manner during deposition; the field applied to the electromagnetic field generator is cycled on and off in a periodic pattern. Several embodiments of cycling methods of the invention are illustrated in FIGS. 6 through 9. These different methods are applicable to generation of a single magnetic field in one direction, or they may be used to generate magnetic fields in different directions by controlling the phase between the two energizing currents.  
         [0058]    [0058]FIG. 6A shows the field generator current as alternating current (AC); in FIG. 6B, the electromagnetic field generator current is positive and negative (+/−) pulsed direct current (DC). FIG. 6C shows the alternating electromagnetic field generator current of FIG. 6A with an exponentially decaying amplitude for demagnetization procedures. FIG. 7A shows half-wave rectified alternating current; FIG. 7B shows positive (+) pulsed direct current. FIG. 7C shows a positive direct current bias; a negative direct current bias could alternatively be used. FIG. 8A shows alternating current operation with a positive direct current bias; FIG. 8B shows pulsed direct current operation with a positive direct current bias. FIG. 9 shows positive (+) pulsed direct current operation with a phase difference between the electromagnetic field generator and the ion beam currents.  
         [0059]    The alternating current of FIG. 6A and the positive and negative (+/−) pulsed direct current of FIG. 6B provide a net zero average field effect on the ion beam, and are most advantageously utilized with soft adjacent layer (SAL) films, such as NiFe-based films, for example NiFeCr, NiFeRh, NiFeTa, or amorphous Co-based alloy materials. FIG. 6C shows alternating current with an exponentially decaying amplitude which may advantageously be used with or without substrates to demagnetize susceptible parts in the fixture weldment magnetized by previous operation of the magnetic fixture.  
         [0060]    The half-wave rectified alternating current of FIG. 7A and positive (+) pulsed direct current of FIG. 7B provide field-free deposition during a portion of the total process time, which reduces the total applied-field effect and allows surfaces which may have become charged to neutralize. This technique is particularly applicable to unidirectional antiferromagnetic materials, for example the antiferromagnetic films such as NiO, FeMn or NiCoO used in MR heads or also PtMn, PtPdMn, IrMn, and PtIrMn. Note that while the cycling methods of FIGS. 6A and 6B provide uniaxial magnetic film deposition (one axis but both “up” and “down” orientations), the cycling methods of FIGS. 7A and 7B provide unidirectional magnetic film deposition (one orientation only), as does the direct current bias of FIG. 7C.  
         [0061]    For some materials, application of a directional field (DC bias) during the entire deposition process may be required. The alternating current operation with a positive direct current bias of FIG. 8A and pulsed direct current operation with a positive direct current bias of FIG. 8B maintain some level of orienting field during the entire deposition process. By adding alternating current or pulsed components, the required direct current bias can be minimized, thus minimizing any distortion of the plasma or ion beam. The current embodiments of FIGS. 8A and 8B are therefore compatible with the widest range of different types of magnetic films.  
         [0062]    [0062]FIG. 9 shows positive (+) pulsed direct current operation with a phase difference between the electromagnetic field generator and the ion beam or plasma source currents. According to an aspect of the present invention, the ion source and electromagnetic fields can be cycled and the phase difference between the two cycles can be adjusted to a specific value. When the ion beam or plasma source is on, the magnetic target material is sputtered; when the ion beam or plasma source is off, no sputtering will occur. By adjusting the relative phase of the two cycles, the portion of the deposition time during which the magnetic field is applied can be controlled.  
         [0063]    For example, in order to minimize the magnetic field effect on the ion beam or plasma source, a ninety degree phase difference can be used, as shown in FIG. 9. In this case, there is no applied magnetic field during the sputtering, so the ion beam is unaffected by the magnetic field. Preferably, to ensure proper film orientation, the frequency range of operation is limited such that the frequency is at least high enough so that the delay time from peak to peak field condition is no more than the time required to deposit one or two monolayers of magnetic film but low enough to prevent eddy current heating of the magnetic structure. These requirements lead to a desired frequency less than about 10 Hz but greater than 1/t, where t is deposition time in seconds. This range of frequencies, (between 1/(deposition time in seconds) Hz and 10 Hz) applies to any of the variable current techniques illustrated in FIGS. 6A through 9.  
         [0064]    [0064]FIG. 10A illustrates a phase relationship for the current in two orthogonal coils, such as  120  and  130  in FIG. 1, that will produce a continuously rotating field orientation on the substrate. When the sinusoidal current of the  0  coil in FIG. 10A is applied to coil  120 , and the sinusoidal current of the  90  coil in FIG. 10A is applied to coil  130 , the resulting magnetic field will rotate over time, as illustrated in FIG. 10B. One effect of this continuously rotating magnetic field orientation is that the applied film is demagnetized.  
         [0065]    As illustrated in FIG. 10A, the current in the  0  coil is described by the formula A * sin(omega * t) and the current in the  90  coil is described by the formula A * cos(omega * t). A is a current amplitude, omega is angular velocity and t is time. The combination of these two sinusoidal orthogonal magnetic fields produces a magnetic field that rotates with angular velocity omega. This rotational velocity is depicted in FIG. 10B.  
         [0066]    Although different embodiments of the present invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments. It is further understood that the figures provide illustrative examples and do not describe or illustrate the only embodiments contemplated in the invention. Various changes and further modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.