Oxide plasma etching process with a controlled wineglass shape

[0002] The invention relates generally to a plasma etching process. In particular, it relates to a process for etching into silicon oxide an aperture having a complex shape.

[0003] The continuing development of silicon-based integrated circuits has integrated an ever increasing number of semiconductor devices on a single chip. The number is approaching tens of millions, and is still growing. This level of integration has been accomplished in part by ever more complex structures and processes.

[0004] One such structure is the inter-level via or contact. To electrically interconnect the tens of millions of devices requires a multi-layer wiring structure. In somewhat regularly arranged memories, two or more wiring layers are needed, while in the more irregularly arranged logic devices five or more wiring layers are currently needed. As illustrated in the cross-sectional view of FIG. 1, each wiring level includes an inter-level insulating layer 10 interposed between a lower layer 12 and a metallic upper layer. Typically, the insulating layer 10 is formed of silicon dioxide or related silica glasses, both hereinafter referred to as an oxide. The lower layer 12 may be the silicon substrate in which is already formed various types of semiconductor devices that need to be contacted. Alternatively, the lower layer 12 may be a lower wiring layer which is already formed into a lower interconnect pattern. The upper metal layer 14 is eventually formed into its own interconnect wiring pattern. The interconnect metal is usually aluminum or an aluminum alloy although its composition is not directly related to the present invention.

[0005] Usually, the deposition of the upper metal layer 14 includes deposition of the same metal into an aperture 16 preformed in the oxide layer 10. This invention is directed to the etching of that aperture 16. If the underlying layer is silicon or polysilicon, the aperture 16 is referred to as a contact hole, and extra care must be exercised to not degrade the semiconducting characteristics of the underlying layer 12. If the underlying layer is a metal or polysilicon interconnect, the aperture 16 is referred to as a via hole. As the level of integration has increased, the via or contact holes 16 have been required to become narrower and more vertically anisotropic, that is, to have a high aspect ratio of depth to width. Methods for forming highly anisotropic contact and via holes 16 in an oxide have been developed for use in a plasma reactor. A typical method uses a fluorocarbon or hydrofluorocarbon etching gas in an argon carrier gas and applies an RF bias to the pedestal supporting the wafer. The RF bias creates a DC electrical self-bias in the plasma adjacent to the wafer, and the DC field accelerates the etching ions or an inactive carrier gas ions towards the wafer in a vertical flux pattern. The resulting etching, if properly controlled, is highly anisotropic with oxide holes 16 having aspect ratios of five or even more being attainable.

[0006] However, this anisotropic inter-level etch has at least two problems. First, very highly anisotropic etching often requires the use of high-density plasma reactors, often using inductive coupling of RF energy into plasma source region of the etch reactor as well as the capacitive coupling of RF energy onto the pedestal to create the DC self-bias. The recently developed high-density plasma reactors are expensive. Secondly, the filling of the metal layer 14 into a narrow and deep hole 16 becomes problematic. Sputter deposition of the metal tends to bridge the top of a rectangular hole 16 before it is filled, thus creating a void in the contact or via. Methods are available to fill such a narrow and deep hole, but again these methods are complex and often require expensive metal deposition equipment.

[0007] In some structures, the contact or via needs to be narrow at its bottom but the spacing is more relaxed at its top. Typically, the resolution required of wiring patterns decreases in the upper wiring layers. To take advantage of these differing requirements, a wine-glass etch pattern, as illustrated in FIG. 1, has been developed. The hole includes a highly anisotropic lower portion 18 (referred to as the stem) and a wider upper portion 20 (referred to as the bowl).

[0008] One way of forming the wine glass, as partially illustrated in the cross-sectional view of FIG. 2, is to cover the oxide layer 10 with a patterned mask layer 22 having an mask aperture 24 generally conforming to the area of the stem 18 and the desired area of the contact to the substrate 12. A first etching step uses an isotropic etch which not only etches downwardly in the area beneath the mask aperture 24 but also etches sidewardly to undercut the mask layer. The generally isotropic etch can be performed in a plasma reactor without significant RF biasing of the pedestal or with a remote plasma source (RPS). As the figure shows, the isotropic etch with RPS actually etches somewhat more laterally than vertically. Typically, the lateral-to-vertical ratio (L/V) depends on the density and dopant level of the material being etched. Less dense, highly doped materials etch with L/V ratios near or below 1.0 while dense, undoped films etch with L/V ratios ranging from 1.3 to 2.0. After the desired depth of the bowl 20 has been etched in the oxide 10, the structure is anisotropically etched through the oxide layer 10 to the underlying layer 12, as described above, to form the stem 18 underlying the mask aperture. The metal layer 14 is then sputtered to fill the wineglass-shaped hole 16.

[0009] The aspect ratio of the stem portion 18 of the hole 16 is significantly less than a substantially vertical hole 16 extending all the way from the surface of the oxide 10, thus not requiring complex and expensive etch equipment or alternatively an etching chemistry requiring precise control in a commercial environment. Also, metal filling of the wine-glass hole 16 is also more easily accomplished, thus simplifying that step as well.

[0010] Nonetheless, standard wine-glass oxide etching has its problems. For a given size of mask aperture 24, there is only a limited range with the described isotropic etch to control the ratio of the vertical and horizontal dimensions of the bowl 20. The L/V ratio can be controlled with the RPS chamber by varying control parameters such as cathode temperature, the ratio of O₂/CF₄(or other fluorine containing gas), and pressure. However, a typically attainable L/V range is limited to about ±20% with these control parameters. Furthermore, the parameters needed to reduce the L/V ratio substantially reduce the etch rate. Generally, in the conventional processes, the lateral dimension of the bowl 20 tends to be too large, particularly as the spacing between contacts continues to decrease. Nonetheless, the depth of the bowl 20 should be maintained relatively large so as to promote metal filling of the narrow stem 18. Thus, it is desired to reduce the ratio of lateral to vertical etching in the bowl etch. Furthermore, to optimize an integrated process of etching and filling, it is desired to be able to control the lateral-to-vertical ratio as well as to more finely control the shape of the bowl. Such control is not directly available in the processes of the prior art.

[0011] The invention may be summarized as a three-step wine-glass etch process with a common mask. In the first step, an anisotropic etch is performed to a depth determining the vertical dimension of the bowl of the wine glass. In the second step, an isotropic etch is performed to achieve the desired lateral extent. The isotropic etch will further increase the depth of the bowl. In the third step, another anisotropic etch is performed to etch the stem of the wineglass down to the underlying layer.

[0012] FIG. 1 is a cross-sectional view of a wineglass contact or via hole through an oxide layer after the metal filling step.

[0013] FIG. 2 is a cross-sectional view of the process conventionally used in etching the bowl of the wineglass.

[0014] FIG. 3 is a process flow diagram for practicing one embodiment of the invention.

[0015] FIGS. 4, 5, 6, and 7 are cross-sectional view illustrating the steps in processing the wineglass hole of the invention.

[0016] FIG. 8 is a cross-sectional view of a remote plasma-source (RPS) etch reactor usable with the invention.

[0017] The invention allows the independent control of the lateral and vertical dimensions of the etching of the bowl in a wineglass etch. The process, summarized in the flow chart of FIG. 3, creates the structure sequentially developed in the cross-sectional views of FIGS. 4 through 7.

[0018] In a photomasking step 26 of FIG. 3, the photoresist layer 22 illustrated in the cross-sectional view of FIG. 4 is deposited on the oxide layer 10 and is photographically patterned to have an aperture 24 extending to the underlying oxide 10 in the area of the intended contact hole. The size of the aperture 24 is generally of the same cross section as the wineglass stem that will extend to the substrate 12.

[0019] A first, anisotropic etch step 28 of FIG. 3 is used to form a shallow hole 30 in the oxide layer 10, illustrated in FIG. 5, not extending downwardly as far as the intended bottom of the bowl. This is not an aggressive etch, and a capacitively coupled, magnetically enhanced etch reactor, such as the MXP⁺available from Applied Materials, Inc. of Santa Clara, Calif. can be used for this etch. In the MXP⁺reactor, the wafer rests on a cathode pedestal connected to an RF power supply, and the counter-electrode is grounded. Additionally, electromagnetic coils induce a nearly static horizontal magnetic field in the volume between the electrode. This type of reactor is referred to as a magnetically enhanced reactive ion etcher (MERIE) and can be effectively used for anisotropic etching.

[0020] In one example of the invention, the process parameters presented in TABLE 1 were used in an MXP⁺reactor with the aperture 24 of the mask having a width of about 0.9 μm and with the thickness of the oxide layer 10 being about 0.4 μm, but in different applications the thickness may range up to 1.2 μm.

[0021] A second, isotropic etch step 32 of FIG. 3 then enlarges the shallow hole 30 into a wider and deeper hole 36 shown in FIG. 6. The isotropic etch forms undercuts 38 beneath the photomask 22 and also forms curved bottom corners 40 in the oxide layer 10. Bird's beaks 42 are likely to form at the interface between the oxide 10 and the photomask 22. The enlarged hole 36 forms the bowl of the wineglass shape.

[0022] In the example of the invention, the process parameters used in an RPS etch chamber, to be described later, are listed in TABLE 2.

[0023] The isotropic etch completes the formation of the bowl of the wineglass. The extent of the isotropic etch, particularly as determined by the isotropic etching period, determines the lateral dimension of the wineglass etch. The isotropic etch also deepens the bowl by an amount determined by the period of the isotropic etch. That isotropically etched depth needs to be added to the anisotropically etched depth of the first step in determining the total depth. Nonetheless, the extents of the isotropic and anisotropic etches can be varied in combination for a desired lateral-to-vertical ratio.

[0024] A third, anisotropic etch 46 of FIG. 3 etches through the oxide layer 10, as illustrated in FIG. 7, in an area mostly defined by the aperture 24 in the photomask 22. The etch forms an aperture 48 extending down to the underlying substrate 12, and the etched volume corresponds to the stem of the wineglass shape. The third etch may be the most demanding one since it may require the etching of a hole with a high aspect ratio, and further it is desirous that the etch be selective to the underlying material. However, the large lateral extent of the bowl area 36 lessens the severity of the geometry of deep hole etching. Such etches are well known. For example, the third etch 46 can have the same conditions as the first etch 28, as listed in TABLE 1. The same conditions apply to both etching a contact hole over silicon or etching a via hole over a metal. Using the same reactor for the first and third etch steps and an RPS reactor for the second etch step improves throughput since the two reactors are easily included on the same platform.

[0025] A series of experiments were performed varying, the times of the anisotropic and isotropic etch times for forming the bowl. The resultant bowl shape was then measured using scanning electron micrographs. The total etch depth is the total from the first two etching, steps, that is, the depth of the bowl. The shape is characterized by a mean L/V ratio for many etched holes, where L is the maximum lateral extent of the undercut on one side of the bowl (averaged over several holes) and V is the vertical extent. The results are presented in TABLE 3

[0026] These results show that varying the relative times of the anisotropic and isotropic etches allows substantial control of the wineglass shape. If the etch depth data is calculated as a ratio of the anisotropic etch depth to the total etch depth, it is found that this ratio varies nearly linearly with the observed L/V ratio.

[0027] The isotropic etch can be accomplished by a variety of methods, even if the etching is restricted to the preferred plasma dry etching. The MXP⁺chamber can be used for a nearly isotropic etching by using a non-polymer-former chemistry, such as the CF₄+NF₃+O₂chemistry of TABLE 2. A similar chemistry uses SF₆in place of the NF₃. In these two chemistries, any carbon left from the etching is oxidized by the O₂to form CO₂, which is then vented from the system. Chamber pressures can range from 300 mTorr to 3 Torr. For a more isotropic etch, typically no bias is applied to the pedestal, and no magnetic field is applied to the plasma. It is noted that the MXP⁺would produce a tapered etch rather than a curved isotropic etch. Similarly, recently developed inductively coupled high-density plasma (HDP) reactors can be used for the plasma etch by emphasizing the source plasma. An advantage of using the MXP⁺or HDP reactors, is that the same reactor can be used for two or all three of the etching steps.

[0028] However, we have found that an effective isotropic etcher is a remote plasma-source (RPS) etcher, which is used prior to the anisotropic etch in the MXP⁺ reactor. An RPS etch chamber is illustrated in the schematic cross section of FIG. 8. A vacuum chamber 60 contains a pedestal 62 having an electrostatic chuck on its upper surface for selectively clamping a wafer 64. An unillustrated vacuum pumping system pumps the chamber 60 through a throttle valve 70. Processing gas is admitted to an upper cavity 72 in the chamber 60 through a microwave applicator 74. The processing gas in the upper cavity is uniformly distributed to the processing area over the wafer 64 through a gas distribution plate 78 having a plurality of narrow holes 80 through it for passing the processing gas.

[0029] A magnetron 84 supplies microwave power in the gigahertz range through a microwave waveguide 86 to the applicator 74. An autotuner 88 on the waveguide 86 adjusts the microwave impedance for varying plasma conditions. The microwave power applied to the applicator 74 excites into a plasma the processing gas flowing through the applicator 74, and the excited gas flows through the gas distribution plate 78 to the processing area. Because of the distances involved, the plasma is mostly in the form of neutral radicals. In this chamber, there is no additional plasma generating equipment in the vacuum chamber 60, and no bias is applied to the pedestal 62. As a result, the excited gas plasma etching the wafer 64 does so without any directional acceleration across a plasma sheath, and the resulting etch is both soft and isotropic.

[0030] An RPS etcher is relatively inexpensive and is easy to operate and maintain so that a three-step, two-chamber etch is not that much more expensive than a single-chamber etch. A two-step, single-chamber etch for the latter two steps according to the invention can be performed in an MXP⁺chamber by first setting the chamber process conditions to conditions favoring an isotropic etch by increasing the pressure, reducing the bias, and lowering the power. Then the chamber process conditions are set to conditions favoring anisotropic etching, such as using a chemistry similar to that of TABLE 1. It is also possible to add a remote plasma source to a capacitively or inductively coupled plasma reactor, e.g., the MXP⁺. The combination chamber can then be operated in either the isotropic RPS mode or the anisotropic local plasma mode or a combination of the two.

[0031] It is also possible to perform the etching of a tailored bowl shape by a single etch step that is intermediate an isotropic and a strongly anisotropic etch. For example, the pedestal bias and argon flow of can be reduced from those values listed in TABLE 1, but still have finite values. The flow polymer former CHF₃can be increased. This approach is particularly applicable to L/V values of less than unity. For many etch and via etches, the etch conditions are optimized to produce side wall angles of greater than 85°, and preferably close to 90°, the ultimate in anisotropy. It is known that other conditions produce an etch of less anisotropy, for example, producing side walls angles of significantly less than 90°, say 60°. While such an etch is not isotropic, it combines the characteristic of a strongly anisotropic etch and an isotropic etch. A strongly anisotropic etching profile may be defined as one producing a side wall angle of greater than 80°.

[0032] Furthermore, more than two processing conditions producing differing anisotropy may be used in forming the bowl, for example, three or more steps producing decreasing anisotropy in etching the oxide, so as to tailor the curve of the bowl as well as the overall L/V ratio.

[0033] Although the invention has been applied to an oxide layer, the invention can also be applied to other dielectric layers, such as carbon-based dielectrics which may be used for their low dielectric constants.

[0034] It is thus seen that the invention provides additional control over the shape of a hole etched into an oxide layer, particularly in a wineglass etch for a contact or via. Nonetheless, the additional control is achieved with either the use of an additional low-cost chamber or by using the same plasma etch reactor under a number of different conditions.