Etching multi-shaped openings in silicon

[0001] This invention relates to a method of forming devices having varying profiles in situ in a single chamber. More particularly, this invention relates to a method of alternately etching openings in a silicon substrate and depositing a protective polymer coating on the sidewall openings to protect the silicon features from attack during a subsequent silicon etch step.

[0002] Micro-machining and micro-electromechanics (MEMS) technologies have been advancing steadily. These technologies use semiconductor processing techniques to form electrical and mechanical structures in a substrate, particularly in silicon. By using known silicon etch techniques, devices having varying profiles can be formed in a silicon-containing substrate, and then released, as by overetching, to remove the devices from the underlying silicon or silicon oxide. As examples, an anisotropic etch can be carried out to form an opening with straight, parallel sidewalls; or with straight tapered sidewalls, tapered outwardly or inwardly; or an isotropic etch can be carried out which changes the geometry or which can lift up and separate the feature from the substrate. When very small features are to be made, the etch processes must be carefully controlled, particularly for re-entrant profiles.

[0003] Further, the sidewalls of the etched openings must be protected from further etching if the shape of the opening is to be changed sequentially, as, when an anisotropic etch is to be followed with an isotropic etch.

[0004] It would be highly desirable to be able to carry out different sequential etch steps in situ, in a single reaction chamber, that changes the profile of the formed features while protecting the already etched openings from additional etching.

[0005] We have found that after etching an opening, or part of an opening, in a silicon substrate, the sidewalls can be protected with a fluorocarbon-type polymer. The protective polymer layer is deposited in a highly conformal manner, forming a thick, etch impervious polymer layer that is able to protect the already-formed opening sidewalls. Manipulation of bias power during polymer deposition can prevent polymer deposition on the bottom of an opening during sequential etch steps. Further, the conformal polymer layer must be able to be removed readily, as with a dry or wet etch step.

[0006] The fluorocarbon forms and deposits a polytetrafluoroethylene-type polymer coating that is non-porous and conformal, and that is thick enough to protect the etched sidewalls from further etching.

[0007] In order to form high aspect ratio openings, the etch and deposition steps are alternated in a controlled manner so that the lengthening sidewalls are protected from further etching with a protective, conformal polymer coating just after the sidewalls are formed.

[0008] Another use of this technique is to change the etchant from an anisotropic one, that etches straight walled openings, whether parallel or tapered, to an isotropic etchant. The isotropic etchant can change the shape or geometry of the etched feature, as to a vase-like or nozzle-shaped opening, or to provide release of a feature from the substrate for MEMS processing.

[0009] Multi-shaped cavities can be made using a time multi-plexed, gas modulation technique. When the etch rate of a particular etchant and a polymer deposition rate is determined, the etch and deposition steps can be alternated at a predetermined but changeable rate to provide irregular shapes and final release from the substrate.

[0010] We have found that when t2 (deposition rate) over t1 (etch rate) is less than 0.7, i.e., $\frac{\mathrm{t1}}{\mathrm{t2}},<0.7$
openings with a positive taper are obtained. To obtain a negatively tapered opening, t1 should be greater than t2, i.e., $\frac{\mathrm{t1}}{\mathrm{t2}}.>0.7$
To obtain vertical openings having parallel sidewalls, t1 and t2 are related as $\frac{\mathrm{t2}}{\mathrm{t1}}=\frac{7}{10}.$

[0011] By combining time multiplexed loops of alternating etch and deposition steps, many different shapes can be made controllably in situ in a single etch chamber.

[0020] The present etch/deposition process can be used to etch conductors, such as single crystal silicon, epitaxial silicon, polysilicon, silicon-on-insulator and metal gate or other conductor materials. The present method can be used to form deep or shallow trenches, gate structures, patterning of conductor/oxide/silicon and the like to form semiconductor devices including pressure sensors, accelerometers, micro-mirrors, optical switches, medical and other microstructures such as micro-nozzles and the like.

[0021] The present alternating etch/deposition process can be carried out in a decoupled plasma source (DPS) chamber as shown in FIG. 1.

[0022] Referring to FIG. 1, an inductively coupled RF plasma reactor includes a reactor chamber 100 having a grounded conductive cylindrical sidewall 110 and a shaped, dielectric ceiling 112, e.g., dome-like. The reactor includes a substrate support electrode 114 for supporting a substrate 116 to be processed in the chamber 100; a cylindrical inductor coil 118 surrounding an upper portion of the chamber beginning near the plane of the top of the substrate 116, or substrate support electrode 114, and extending upwardly therefrom toward the top of the chamber 100; process gas sources 122 and a gas inlet 124 which can be a plurality of inlets spaced about the interior of the chamber; and a pump 126 for controlling the chamber pressure. The coil inductor 118 is energized by a plasma source power supply, or RF generator 128, through a conventional active RF match network 130, the top winding of the inductor coil 118 being “hot” and the bottom winding being grounded. The substrate support electrode 114 includes an interior conductive portion 132 connected to a bias RF power supply or generator 134, and an exterior grounded conductor 136 which is insulated from the interior conductive portion 132. A conductive grounded RF shield 120 surrounds the coil inductor 118.

[0023] In accordance with one aspect of the chamber 100, uniformity of the plasma density spatial distribution across the substrate is improved by shaping the ceiling 112 as a multi-radial dome and individually determining or adjusting each one of the multiple radii of the ceiling 112. The multiple-radius dome shape in the embodiment of FIG. 1 somewhat flattens the curvature of the dome ceiling 112 about the center portion of the dome, the peripheral portion of the dome having a steeper curvature.

[0024] To carry out the present process, during the deposition step, the source power is turned on and a fluorocarbon or hydrofluorocarbon processing gas is passed into the chamber 100 from the desired gas containers (not shown). The power to the chamber 100 from the inductive RF power source 128 is suitably from about 200 up to about 3000 watts, and is preferably from about 400 to 2000 watts. The RF source can be a 12.56 MHz power source. No bias power is used during the deposition step. The pressure during this step is maintained suitably at from about 5-300 millitorr, preferably about 40 millitorr. The addition of hydrogen bromide (HBr) in an amount of about 5-25% by volume to the fluorocarbon improves the conformality of the polymer deposit.

[0025] Suitable fluorocarbon gases include polymer-generating gases such as dihydrodifluoromethane (CH₂F₂), C₄F₆, C₄F₈ and the like. Such gases form a polytetrafluoroethylene-like, etch-resistant coating on the feature sidewalls, protecting them during a subsequent etching step. The deposition step is generally carried out for about 1 to 10 seconds. Suitably the deposition step is carried out for about 5 seconds at 18 millitorr pressure using about 700 watts of power and a gas flow of about 140 sccm.

[0026] During the etch step, silicon, or a silicon-containing substrate, is etched using a high gas flow of a fluorine-containing etch gas, such as sulfur hexafluoride, nitrogen trifluoride or carbon tetrafluoride, at a pressure of between about 77 millitorr up to about 100 Torr, and suitably at about 250 millitorr. The gas flow rate in the chamber of FIG. 1 is about 250 sccm, but other etch chambers may permit higher gas flow rates. During etching, a high source power of over 500 watts is used.

[0027] At a pressure of 7 millitorr up to 100 Torr, using a source power of about 500 to 1500 watts, an etch rate of up to about 6 microns per minute can be obtained using the above chamber. Still higher etch rates can be achieved using a higher source power, depending on the chamber used for etching. A bias source power to the substrate support electrode is also turned on, conveniently at a low power of from 34 to about 1000 watts. The chamber of FIG. 1 is limited to a bias power of about 3 to 500 watts, but different etch chambers may permit higher bias power to be used.

[0028] A frequency of about 400 kHz can be generated in the present chamber, but other frequencies can be used.

[0029] By varying the timing of sequential deposition and etch steps, an opening in silicon can have perpendicular sidewalls, resulting in an anisotropic opening, or the sidewalls can be tapered, positively or negatively, by varying the timing of the deposition and etch steps. As examples, when a negatively tapered opening in silicon is to be made, deposition is carried out for 5 seconds, and etch is carried out for 10-20 seconds; thus the etch time is longer than the deposition time. For a positive taper, deposition of polymer is carried out for 5 seconds and etch is carried out for 5 seconds; thus the deposition time and etch time is about the same. Straight, perpendicular sidewalls are obtained when deposition is carried out for 5 seconds and etch carried out for 10 seconds; thus the etch time is about double the deposition time.

[0030] The substrate can be cooled during etch processing, generally by means of a coolant passed to a channel in the substrate support electrode 114 in known manner. A coolant gas such as helium can also be passed between the substrate 116 and the substrate support 114 to enhance cooling and maintain the temperature of the substrate within a desired range, generally from about 10 to 100° C.

[0031] To further protect the sidewalls of etched patterns, tetrafluorosilane can also be added to the fluorocarbon deposition gas, generally at a flow rate of from about 20 to 300 sccm.

[0032] Inert gases such as argon and nitrogen, as well as oxygen, can also be added to one or both of the etch and deposition gases if desired.

[0033] The above alternating deposition/etch process has a high selectivity of up to 200 to either a photoresist or a silicon oxide or hard mask.

[0034] A time multiplexed gas modulation method has been developed so that multi-shaped cavities can be formed in silicon, particularly for MEMS/MST applications.

[0035] Depending on the etchant gas used, and the time of etch and deposition, one can obtain openings having straight walls and vertical or tapered sidewalls, or re-entrant profiles or openings that are isotropic. A known isotropic etch is HBr and helium-oxygen, and optionally sulfur hexafluoride. A good isotropic etchant includes HBr to which 5-10 sccm of helium containing about 30% of oxygen and 5-10 sccm of SF₆ is added.

[0036] FIG. 2 illustrates a conformal polymer coating on the vertical, parallel sidewalls of a trench made in accordance with the present alternating two-step etch/deposition method. The trenches were formed using SF₆ as an etchant and C₄F₈ as a polymer deposition gas.

[0037] FIG. 2A illustrates a smooth, conformal polymer deposition on the sidewalls and on the bottom of a trench.

[0038] FIG. 2B illustrates a smooth, conformal polymer deposition only on the sidewalls of a trench. By preventing polymer deposition on the bottom of the trench, as by applying bias power to the substrate support during deposition, the sidewalls are protected but the trench can be deepened, or an isotropic etch can be used for lift-off purposes.

[0039] FIGS. 3A, 3B, 3C and 3D illustrate a four step method for forming a micro-nozzle etched in silicon using the above multi-step process.

[0040] FIG. 3A illustrates a straight walled opening 10 made in a photoresist layer 12, and a protective conformal polymer layer 14 deposited thereover, which greatly improves the selectivity between the photoresist and the underlying silicon substrate.

[0041] As shown in FIG. 3B, a tapered re-entrant opening 16 is then made in the silicon substrate 18 using a suitable deposition/etch ratio, as explained hereinabove. A polymer layer 20 is then deposited in the tapered opening 18, as shown in FIG. 3C. The effect of the polymer deposition is to make the diameter of the opening 16 somewhat narrower than the mask opening 10. An isotropic etchant, such as SF₆, is then used to isotropically etch a bowl-shaped opening 22 as the reservoir of the micro-nozzle. The final shape of the nozzle is shown in FIG. 3D.

[0042] FIG. 4A illustrates a pattern of very high aspect ratio vertical trenches 16 microns in diameter etched in the polysilicon of an SOI substrate. Very high aspect ratio, straight walled openings are made down to the insulator, by changing the etch/deposition gases alternately until the underlying substrate is reached. It can be seen that no notching or lateral etch occurs at the silicon-oxide interface. No silicon residues remain at the bottom of the trench, as shown in FIG. 4B, with only a very slight loss, if any, of bottom oxide. Very complex patterns can be obtained, as shown in FIG. 4C.

[0043] FIG. 5A illustrates stages in the manufacture of microsensors. Seven micron diameter trenches were etched in polysilicon using the present two-step etch/deposition method. Very fine features are obtained.

[0044] FIG. 5B illustrates lift-off of some of the features. This is due to too much etching of a negative taper, thus lifting off some of the features. This shows why very precise control of the deposition-etch steps is needed, particularly when etching very small features.

[0045] The polymer deposit can be deposited on the sidewalls only, but avoiding a polymer deposit on the bottom of the trench, as can be done by manipulating the bias power during the deposition step. The sidewalls are still protected, but the unprotected bottom is able to be further etched. An isotropic etch can be used to etch features laterally, forming various shapes. An anisotropic etch can be used to further deepen the trench.

[0046] FIGS. 6A and 6B illustrate straight walled openings having vertical openings and parallel sidewalls, as shown in FIG. 6A, and openings having a re-entrant profile, with non-parallel sidewalls, as shown in FIG. 6B. This shows the excellent profile control obtainable with the present process.

[0047] FIG. 7 illustrates a sawtoothed-type opening made using alternate deposition and etch steps. Polymer is deposited to protect the etched sidewalls, and an etch sequence is begun that removes the polymer from the bottom of the opening and etches isotropically. Polymer is then deposited and the deposition-etch cycle repeated. This deposition-etch sequence is continued until the desired depth of the opening is obtained. The openings shown are 5 microns in diameter and have a depth of 14.5 microns. The etch rate was 1.79 microns/min and the selectivity to the photoresist was 37.8.

[0048] Openings can be made by etching anisotropically for a time period, depositing a polymer thereover, then a different etch-deposition sequence can be used to produce a different shape, as to produce a re-entrant profile and the like. By proper choice of deposition time and etch time, varied, complex and highly controlled shapes can be made in silicon.

[0049] The thick polymer coating holds all the elements together until it is removed, another advantage of the present etch and deposition method.

[0050] Although the present invention is described in terms of specific embodiments, one skilled in the art will know that other embodiments of etchant gases, deposition gases, substrates and processing chamber can be substituted. The invention is only meant to be limited by the scope of the appended claims.