Method of plasma etching

Abstract

A structure comprising a semiconductor substrate and a layer of PZT (lead zirconate titanate) is etched by performing a first plasma etch step with a first etch process gas mixture. The first etch process gas mixture comprises at least one fluorine containing species. The first plasma etch step is performed so that involatile metal etch products are deposited onto interior surfaces of the chamber. The structure is further etched by performing a second plasma etch step with a second etch process gas mixture. The second etch process gas mixture comprises at least one fluorocarbon species. The second plasma etch step is performed so that a fluorocarbon polymer layer is deposited onto interior surfaces of the chamber to overlay involatile metal etch products deposited in the first plasma etch step and to provide a substrate on which further involatile metal etch products can be deposited.

Claims

1. A method of plasma etching a structure comprising a substrate and a layer of PZT (lead zirconate titanate), the method comprising the steps of: providing a structure comprising a substrate and a layer of PZT; positioning the structure on a support within a chamber; etching the layer of PZT by performing a first plasma etch step in which a first etch process gas mixture is supplied to the chamber, wherein the first etch process gas mixture comprises at least one fluorine containing species and the first plasma etch step is performed so that involatile metal etch products are deposited onto interior surfaces of the chamber; and further etching the layer of PZT by performing a second plasma etch step in which a second etch process gas mixture is supplied to the chamber, wherein the second etch process gas mixture is different from the first etch process gas mixture and comprises at least one fluorocarbon species, and wherein the second plasma etch step is performed so that a fluorocarbon polymer layer is deposited onto the interior surfaces of the chamber to overlay the involatile metal etch products deposited in the first plasma etch step and such that further involatile metal etch products are deposited on the overlay.

2. The method according to claim 1, wherein the at least one fluorocarbon species of the second etch process gas mixture comprises C.sub.4F.sub.8.

3. The method according to claim 2, wherein the C.sub.4F.sub.8 is introduced into the chamber at a flow rate of 5-10 sccm.

4. The method according to claim 2, wherein the at least one fluorocarbon species of the second etch process gas mixture further comprises CF.sub.4 and the second etch process gas mixture further comprises H.sub.2, wherein the ratio of CF.sub.4 to H.sub.2, expressed as a ratio of flow rates in sccm, is less than 1.0:1.

5. The method according to claim 1, wherein the at least one fluorocarbon species of the second etch process gas mixture comprises C.sub.3F.sub.8.

6. The method according to claim 1, wherein the at least one fluorine containing species of the first etch process gas mixture comprises one or more of CF.sub.4, CHF.sub.3 or SF.sub.6.

7. The method according to claim 6, wherein the first etch process gas mixture further comprises H.sub.2.

8. The method according to claim 7, wherein the first etch process gas mixture comprises CF.sub.4 and H.sub.2, wherein the ratio of CF.sub.4 to H.sub.2, expressed as a ratio of flow rates in sccm, is 1.5:1 or greater.

9. The method according to claim 1, wherein the first etch process gas mixture consists essentially of CF.sub.4, H.sub.2 and, optionally, one or more inert diluents.

10. The method according to claim 1, wherein the second etch process gas mixture consists essentially of C.sub.4F.sub.8, CF.sub.4, H.sub.2 and, optionally, one or more inert diluents.

11. The method according to claim 1, wherein an electrical bias power is applied to the structure during the first plasma etch step, and a reduced or zero electrical bias power is applied to the structure during the second plasma etch step.

12. The method according to claim 11, wherein an electrical bias power of 500-1000 W is applied to the structure during the first plasma etch step.

13. The method according to claim 11, wherein an electrical bias power of 0-500 W is applied to the structure during the second plasma etch step.

14. The method according to claim 1, wherein the substrate is a semiconductor substrate.

15. The method according to claim 14, wherein the semiconductor substrate is a silicon substrate.

16. The method according to claim 1, wherein: the chamber has a first gas inlet arrangement comprising one or more gas inlets and a second gas inlet arrangement comprising one or more gas inlets; during the first plasma etch step, the first etch process gas mixture is only supplied to the chamber through the first gas inlet arrangement; and during the second plasma etch step, the second etch process gas mixture is only supplied to the chamber through the second gas inlet arrangement.

Description

DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIGS. 1 and 2 are schematic diagrams of a plasma etching apparatus of the invention;

(3) FIG. 3 shows deposition on the RF source window, and indicates the areas which FIGS. 4 and 5 correspond to;

(4) FIG. 4 is an SEM micrograph of the well-adhered section indicated in FIG. 3;

(5) FIG. 5 is an SEM micrograph of the poorly-adhered section indicated in FIG. 3;

(6) FIG. 6 shows EDX compositional analysis of deposited material layers.

DETAILED DESCRIPTION OF THE DISCLOSURE

(7) A plasma processing apparatus according to the invention is illustrated in FIGS. 1 and 2. The invention may be performed in an adapted version of the Applicant's Omega® Synapse™ etch process module. Well known features such as the exhaust gas pumping system are not shown in FIGS. 1 and 2 but will be well understood by the skilled reader.

(8) The apparatus comprises a plasma etch chamber 11 having a plurality of internal surfaces. The apparatus comprises a first gas inlet arrangement 10, a second gas inlet arrangement 12, a ceramic annular housing 18, an RF antenna 14, a platen RF electrode 16 and a support 20 for supporting a structure 28 undergoing etching. In the embodiment shown in FIGS. 1 and 2, the support 20 is an electrostatic chuck and the platen RF electrode 16 is used to control the directionality of the etch ions. This in turn controls the extent of physical etching achieved during processing. Higher platen powers will increase the substrate etch rate.

(9) The plasma etch chamber 11 has an upper wall or lid. The annular housing 18 is immersed within the chamber 11 and depends downwardly from the upper wall. The annular housing 18 defines a circular region on the interior of the upper wall.

(10) In the embodiment shown in FIGS. 1 and 2, the first gas inlet arrangement 10 is an inner gas plenum and the second gas inlet arrangement 12 is an outer gas plenum. Each gas inlet arrangement comprises a plurality of gas inlets, each gas inlet terminating in an opening through which process gases enter the interior of the chamber 11. The inner plenum 10 sits within the circular region defined by the annular housing 18. The gas inlets of the inner gas plenum 10 are positioned inwardly of the annular housing 18 as a plurality of openings disposed in a circular pattern. The outer plenum 12 is positioned outside the circular region defined by the annular housing 18. The gas inlets of the outer gas plenum 10 are positioned outwardly of the annular housing 18 as a plurality of openings disposed in a circular pattern. The inner gas plenum can have eight gas inlets whereas the outer gas plenum may have about 10 times as many gas inlets. However, it will be appreciated that the first and second gas inlet arrangements can have any suitable number of gas inlets.

(11) The etch chamber is further illustrated in FIG. 2, which illustrates processing of a structure 28 present in the chamber. The chamber comprises a chamber walls 24 within which the structure 28 is placed on the support 20. The plasma 26 is ignited and sustained by RF power coupled into the chamber from an RF power source (not shown) via the RF antenna 14 contained within the annular housing 18. The annular housing 18 acts as a window which allows RF power to be coupled into the chamber. Etch process gases enter the chamber through the gas inlets of either the inner gas plenum 10 or the outer gas plenum 12. A controller 30 is used to control the two step etch process. As part of its operation, the controller 30 controls the flow of the etch process gases into the chamber. In some embodiments, the controller switches the gas entry points from the first gas plenum to the second gas plenum between the first and second etch steps.

(12) A typical wafer structure is a silicon substrate base layer followed by an SiO.sub.2 layer, a platinum layer, a PZT layer, a second platinum layer and finally a photoresist mask on the upper surface of the wafer. The photoresist mask protects the wafer from plasma etching. The mask is patterned according to the desired etch product. Typically, the platinum electrode layers will have a thickness of 50-250 nm and the PZT layer will have a thickness of 500-2500 nm.

(13) During the first step, a higher platen power is used to etch the PZT at a high rate, with low selectivity, to a stop layer (PZT/Pt). The stop layer is typically a platinum electrode. The platen power is reduced in the second step and may be switched off entirely. The reduced platen power leads to a reduced etch rate of PZT but improves selectivity with Pt. This means that during the second step the plasma will continue to etch any remaining PZT but will not etch the stop layer or remove Pt at a substantially reduced rate.

(14) The internal surfaces of the chamber were textured to improve the adhesion of the first layer of deposited material. The metal shielding in the chamber was coated with arc-spray Al to achieve a surface roughness of approximately 20-35 μm while the ceramic window 18 was coated with an yttria coating to achieve a surface roughness of approximately 6 μm. Trials were conducted by etching high open area (80% OA) patterned wafers having photoresist mask (4.5 μm thickness)/Pt (100 nm thickness)/PZT (2 μm thickness)/Pt (100 nm thickness) layers formed thereon, using the process conditions shown in Table 1 with either the inner or outer gas plenum used to deliver the etch process gases.

(15) Table 1 shows typical process parameters for each step. PZT etching is typically performed at a chamber temperature of 55° C. and a pressure of 5-50 mTorr.

(16) TABLE-US-00001 TABLE 1 Step 1 Step 2 C.sub.4F.sub.8 flow (sccm) 0  5-10 CF.sub.4:H.sub.2 ratio >1.5:1 <1:1 Antenna RF power (W) 1000-1500 1500-1900 Platen RF power (W)  500-1000  0-500 Time (min)  5-10 10-15

(17) It was found that when the outer gas plenum was used to perform an etch step, deposition was evident on the chamber surface in the vicinity of the outer gas plenum. When the inner gas plenum was used to perform an etch step, deposition was visible on the annular housing and in the vicinity of the inner gas plenum. In excess of 214 microns of PZT was successfully etched using the inner plenum before material de-laminated from the ceramic window of the annular housing.

(18) FIG. 3 shows material deposited on the annular housing 18. It can be seen that the material is well-adhered in some areas while it is flaking off in others.

(19) The SEM micrograph shown in FIG. 4 corresponds to the well-adhered area indicated in FIG. 3. Thick layers of fluorocarbon polymer are present between each layer of metallic etch products. As the number of wafers processed in the chamber increases, a laminar structure of etch products and fluorocarbon polymer builds up on the chamber interior.

(20) In contrast, the SEM micrograph in FIG. 5 is taken from the poorly-adhered area indicated in FIG. 3. In this area, there is a thin or non-existent fluorocarbon layer between the etch product layers. The SEM images indicate that regions where there is a thick fluorocarbon polymer layer between each redeposited etch product layer adhere better than regions where the fluorocarbon layer is thin or absent.

(21) FIG. 6 shows EDX compositional analysis of redeposited material taken from the well-adhered area. The composition of the material is consistent with the presence of thick fluorocarbon layers sandwiched between layers of metal etch product as seen in FIG. 4.

(22) Without wishing to be limited by any particular theory or conjecture, it is believed that there are two main mechanisms of deposition on the chamber interior. The first mechanism is direct deposition of the process gas on the chamber interior. The second is redeposition of etch products from the wafer during etching. It is believed that material deposition from the etch process gas mixture varies throughout the chamber due to local variation in the precursor gas concentration as a function of distance from the gas inlet. This leads to higher deposition in the vicinity of the gas inlets. By switching between two gas inlet arrangements a more uniform deposition can be achieved, and the time between chamber cleans is extended accordingly. Deposition from the wafer etch products is believed to act in a line-of-sight manner from the wafer, and is therefore not affected by the gas inlets. The invention described herein is primarily concerned with deposition of the fluorocarbon polymer layer which acts as an excellent substrate for the deposition of a subsequent layer of the involatile metal products. This can be performed in conjunction with switching the gas inlets between the first and second plasma etch steps to achieve optimal results. However, it is not necessary that the switching of gas inlets is performed. Instead, it will be appreciated that the advantageous laminar structure comprising alternating layers of involatile metal etch product and fluorocarbon polymer can be achieved without switching of the gas inlets.

(23) It is believed that the thickness of the fluorocarbon film can be modified by variation of process parameters such as RF coupling, the temperature of the surface being deposited onto and local variation in the concentration of the fluorocarbon precursor or precursors used to provide the fluorocarbon polymer layer. Such variation is within the ambit of the skilled reader.