Abstract:
A new method is provided for the creation of spacers for the CMOS gate electrode. A layer of a spacer material is deposited over the gate structure; a layer of photoresist is deposited over the layer of spacer material. The layer of photoresist of the invention is partially stripped removing the photoresist from above the gate structure and providing a thinner layer of photoresist over the surrounding layer of spacer material. The layer of spacer material is partially etched whereby the layer of photoresist serves as a partial etch stop layer. The remainder of the photoresist is removed, the spacer material is further etched using a dry etch whereby a thin layer of spacer material (oxide) remains deposited over the surface of the substrate. As a final step the thin layer of spacer material (oxide) is removed from the surface of the substrate using a wet etch.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to the fabrication of integrated circuit devices and, more particularly, to a method of etching the spacers of the CMOS gate electrode that is used in image sensor devices. 
     (2) Description of the Prior Art 
     The manufacturing of semiconductor devices requires the creation of a variety of components that collectively perform functions of data manipulation (logic functions) and of data retention (storage functions). The vast majority of these functions operates in a digital or one-off mode and as such recognize zero and one conditions within the operational levels of the circuits. There are, in addition, applications that make use of analog levels of voltage where the voltage may have a spectrum of values between a high limit and a low limit. There are furthermore applications where both the digital and the analog methods of signal processing reside side by side in the same semiconductor device. 
     This mixture of functions and processing capabilities brings with it a mixture of components that can coexist within one semiconductor device. Where the vast majority of device components is made up of transistors, gate electrodes and a variety of switching components that address logic processing functions, it is not uncommon to also see resistors and capacitors that form part of a semiconductor device. It is for instance known that capacitors form a basic component of many analog circuits that are used for analog applications such as switched capacitor filters. It is also well known in the art that capacitors are widely applied in digital applications such as the storage node for Dynamic Random Access Memory (DRAM) circuits. This ability of capacitors to function in either the digital or the analog mode is referred to as the mixed mode application of the capacitor. 
     The DRAM technology is widely used for data storage where one transistor and one capacitor form one DRAM cell. For the capacitor a stacked capacitor is frequently used since this structure has good data storage performance characteristics. To fabricate this device, a modified CMOS process is typically used. One other application in which the CMOS structure has been successfully applied is in the creation of image sensors; it is this latter application that is addressed by the invention. 
     An image sensor is, in its broadest terms, used to convert an optical image that is focused on the sensor into electrical signals. The image sensor typically includes an array of light detecting elements, where each element produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or may otherwise be used to provide information about the optical image. 
     One very common type of image sensor is a charge-coupled device (CCD). Integrated circuit chips containing CCD image sensors have a relatively low yield and are expensive due to the specialized processing involved. The CCD&#39;s also consume a relatively large amount of power. 
     A much less expensive type of image sensor can be formed as an integrated circuit by using a CMOS process. In such a CMOS type image sensor, a photodiode or phototransistor (or any other suitable device) is used as the light-detecting element, where the conductivity of the element corresponds to the intensity of light impinging on the element. The variable signal thus generated by the light detecting element is an analog signal whose magnitude is approximately proportional (within a certain range) to the amount of light that impinges on the element. 
     The light-detecting elements are typically formed in a two-dimensional core array, which is addressable by row and column. Once a row of elements has been addressed, the analog signals from each of the light detecting elements in the row are coupled to the respective columns in the array. An analog-to-digital converter may then be used to convert the analog signals of the columns to digital signals so as to provide only digital signals at the output of the image sensor chip. 
     What is needed is an inexpensive, but highly efficient, image sensor, which produces reliable images. Implied in this is that leakage current in the spacer regions, that is the source/drain regions of the CMOS gate electrode of the image sensor, is reduced to a minimum. Potential plasma damage that can be caused during the spacer etching must therefore be kept at a minimum. 
     FIG. 1 shows a Prior Art gate electrode with the etching of the spacer, as follows: 
     FIG. 1 a  shows the poly silicon gate electrode  10 , a layer  12  of tetra-ethyl-ortho-silicate (TEOS) has been deposited over the gate electrode  10  and the top surface of substrate  14 . FIG. 1 a  shows that only one layer of the dielectric TEOS is deposited in order to form the gate electrode spacers. 
     FIG. 1 b  shows the results of the spacer etch, gate electrode spacers  16  are formed after etching has been completed. It is apparent from FIG. 1 b  that lack in control of the etching or over-etching can readily cause damage the surface areas  18  of substrate  14 . 
     Using current fabrication technology, it is well known that defects in the substrate cause leakage current between the gate electrodes of the image sensor, especially where the substrate defects are caused by plasma damage. It is therefore of key importance to produce a substrate surface that is free of damage and, more particularly, to be able to perform spacer etching without causing damage to the substrate surface. Current practice uses a single layer of dielectric above the spacer between the gate electrodes of the image sensor. With only a single layer of dielectric, it is difficult to sense and control the etch stop above the substrate. This difficulty in controlling the etching process results in substrate surface damage; this in turn results in leakage current between the gate electrodes of the CMOS image sensor device. 
     An additional problem is that, during the growth of field oxide, a phenomenon occurs that causes defects when the gate oxide is grown. This problem is referred to as white ribbon or white pixels. A thin layer of silicon nitride can form on the silicon surface (i.e., the pad-oxide/silicon surface interface) as a result of the reaction of NH 3  and silicon at that interface. When the gate oxide is grown, the growth rate becomes impeded at the locations where the silicon nitride has been formed. The gate oxide is thus thinner at these locations than elsewhere, causing low-voltage breakdown of the gate oxide. 
     The invention teaches a new method of etching the spacers for gate electrode structures. Typical silicon oxide gate spacers are formed via anisotropic RIE of a deposited layer of silicon oxide layer, using CHF 3  or CF 4 —O 2 —He as an etchant. Typical silicon nitride spacers are formed via anisotropic RIE of a deposited layer of silicon nitride layer, using CHF 3  or SF 6 —O 2  as an etchant. Typical gate spacers can also be formed using thermal S i N or using CVD S i N or using thermal SiO x N y  or using CVD SiO x N y  to a thickness within the range between 250 and 1500 Angstrom. The invention provides a new sequence of steps for the formation of gate spacers. 
     The invention addresses the above-indicated problems of white pixel formation while at the same time providing a good method for endpoint detection during the etching of the gate spacers. The invention further prevents the occurrence of plasma damage to the surface of the substrate thereby reducing leakage current along the surface of the substrate. 
     U.S. Pat. No. 5,899,722 (Huang) discloses a process using a double spacer. 
     U.S. Pat. No. 5,863,824 (Gardner) shows a two step spacer process that protects the substrate surface during the etch. 
     U.S. Pat. No. 5,811,342 (Wu) shows a sloping spacer  22  process for a graded LDD. However, this reference differs from the invention. 
     U.S. Pat. No. 5,837,563 (Hynecek) and U.S. Pat. No. 5,385,849 (Nakashiba) show CMOS Tx image sensor processes. 
     SUMMARY OF THE INVENTION 
     A principle objective of the invention is to reduce the formation of white pixels during the process of creating gate spacers for a CMOS gate electrode. 
     Another objective of the invention is to provide a good method of end-point detection for the etching of gate spacers for a CMOS gate electrode. 
     Yet another objective of the invention is to prevent plasma damage to the surface of the substrate during the etching of gate spacers for a CMOS gate electrode. 
     In accordance with the objectives of the invention a new method is provided for the creation of spacers for the CMOS gate electrode. A layer of a spacer material is deposited over the gate structure; a layer of photoresist is deposited over the layer of spacer material. The layer of photoresist of the invention is partially stripped removing the photoresist from above the gate structure and providing a thinner layer of photoresist over the surrounding layer of spacer material. The layer of spacer material is partially etched whereby the layer of photoresist serves as a partial etch stop layer. The remainder of the photoresist is removed, the spacer material is further etched using a dry etch whereby a thin layer of spacer material (oxide) remains deposited over the surface of the substrate. As a final step the thin layer of spacer material (oxide) is removed from the surface of the substrate using a wet etch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross section of a Prior Art gate electrode, as follows: 
     FIG. 1 a  shows a cross section of a Prior Art gate electrode with a layer of gate material deposited over the gate structure. 
     FIG. 1 b  shows a cross section of a gate electrode after the gate spacers have been formed. 
     FIGS. 2 through 7 show the process of the invention, as follows: 
     FIG. 2 shows a cross section of a gate structure over which a layer of spacer material and a layer of photoresist have been deposited. 
     FIG. 3 shows a cross section of a gate electrode after the layer of photoresist has been partially stripped. 
     FIG. 4 shows a cross section of a gate electrode after the layer of spacer material has been partially etched. 
     FIG. 5 shows a cross section of a gate electrode after the layer of photoresist has been removed. 
     FIG. 6 shows a cross section of a gate electrode after the layer of spacer material has been partially etched back. 
     FIG. 7 shows a cross section after the remainder of the spacer material has been removed from the surface of the substrate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now specifically to FIG. 2, there is shown a cross section of a gate electrode  22  formed on the surface of a substrate  20 . A layer  24  of spacer material has been deposited over the gate structure  22  and over the exposed surface of the substrate  20 . Over the layer  24  of spacer material in turn has been deposited a layer  26  of photoresist. 
     A number of materials can be used for the gate dielectric such as oxides (RTO oxide, JVD oxide), nitrides (RTP S i N, RTP SiO x N y ) and polysilicon. The preferred material for the gate electrode of the invention is polysilicon. 
     The gate electrode  22  is typically created by first depositing an in-situ doped polysilicon layer using LPCVD processing at a temperature between about 550 and 850 degrees C. to a thickness between about 2000 and 4000 Angstrom using SiH 4  and PH 3 . The polysilicon structure is created by standard photolithographic masking techniques followed by RIE processing which uses HBr and Cl 2  etch ambient to define the desired pattern in the layer of polysilicon. 
     A variety of materials can be used in the formation of spacers for a gate electrode structure. Gate spacer materials that are known in the art can contain silicon nitride, silicon oxide, BSG, PSG, polysilicon and other materials preferably of a dielectric nature, CVD oxide formed from a TEOS source. Often amorphous materials are used that inhibit the deposition of epitaxial silicon thereupon. 
     The preferred spacer material to be used for the process of the invention is TEOS. The layer  24  of spacer material can be formed by thermal deposition or by using CVD techniques and is typically deposited to a thickness between about 700 and 3000 Angstrom. 
     The layer  26  of photoresist can be formed using thermal deposition (spin-coating and baking) or by using CVD techniques and is typically deposited to a thickness between about 250 and 1000 Angstrom. 
     It must be emphasized that, as a result of the deposition of the layer  26  of photoresist as shown in FIG. 2, a thinner layer of photoresist  26  is deposited over that surface of the layer  24  of spacer material that is above the structure  22  of the gate electrode. 
     FIG. 3 shows a cross section of the gate electrode  22  after the layer  26  of photoresist has been partially stripped and thereby significantly reduced in thickness. The photoresist layer  26  can be partially removed using plasma oxygen ashing and careful wet clean. The oxygen plasma ashing is heating the photoresist in a highly oxidized environment, such as an oxygen plasma, thereby converting the photoresist to an easily removed ash. The oxygen plasma ashing can be followed by a native oxide dip for 90 seconds in a 200:1 diluted solution of hydrofluoric acid. 
     FIG. 3 shows that the layer of photoresist is essentially removed from above the poly gate structure  22  while that layer  26  of photoresist that overlays the gate material  24  has been reduced in thickness to between about 400 and 800 Angstrom. 
     The purpose of the remaining layer  26  of photoresist is to function as a partial etch stop layer during the subsequent etch of the layer  24  of spacer material. This will become apparent during the following discussion. 
     FIG. 4 shows a cross section after a dry etch has been applied to the exposed surface of the spacer material  24  thereby reducing the spacer material that is overlying the gate structure to a thickness between about 250 and 1000 Angstrom, the preferred thickness of this layer is 400 Angstrom. The process of dry etching can be performed in a chlorine-based plasma or a sequence of chlorine-based and fluorine based dry etches. Typically, one or more halogenated compounds are used as an etchant gas. For example CF 4 , CHF 3  (freon 23), SF 6 , NF 3 , can be used. Added can be gases such as O 2 , Ar, N 2 . The etch can be performed in an etcher such as a parallel plate RIE apparatus or an electron cyclotron resonance (ECR) plasma reactor. 
     The preferred etching conditions for the TEOS etch are as follows: etchant gas: CF 4  or CHF 3  at a flow rate of about 15 sccm, gas pressure about 800 mTorr, rf power density about 400 Watts, no magnetic field applied, wafer temperature about −17 degrees C. (which is the chuck temperature), time of the etch about 10 seconds. 
     FIG. 5 shows a cross section after the layer of photoresist has been removed from above the surface of the substrate thereby leaving the layer  24  of spacer material essentially in place. The methods that can be used for the process of removing the photoresist have already been highlighted above under FIG. 3; these same methods can be applied for the removal of the layer of photoresist that has been indicated in FIG.  5 . 
     FIG. 6 shows a partial removal of the layer  24  of spacer material. The partial removal of the layer  24  can be achieved by an anisotropic dry etch using Ar/CF 4  as an etchant at a temperature of between about 120 and 160 degrees C. and a pressure of between about 0.30 and 0.40 Torr for a time of between about 33 and 39 seconds. The partial removal of the layer  24  of gate material has completely removed the gate material  24  from above the structure  22  of the gate electrode while the thickness of the gate material that overlies the surface of the substrate  10  has been reduced to a thickness of between about 200 and 400 Angstrom. This thinner layer of TEOS oxide remains in place above the regions of the source and drain areas of the gate structure, that is essentially the surface of the substrate  10  that surrounds the gate structure  22 . This latter point is of importance since this layer of TEOS oxide which overlies the surface of the substrate  10  serves to protect the surface of the substrate  10  during the final etch of the spacer material. 
     FIG. 7 shows a cross section of the gate structure after the completion of the final etch of the spacers of the gate structure. During this etch, the layer of TEOS oxide that was left on the surface of the substrate has been removed. It must also be noted that the spacers  24  on the side of the gate structure  22  have been considerably reduced as part of this etch. The processing steps to remove the spacer material as reflected in FIG. 7 is a wet dip process using a HF solution. A HF wet dip is a one time process performed at atmospheric pressure using a conventional wet bench process with a gas source of H 2 O:HF=100:1 for a duration of about 2 minutes. 
     The processing sequence of the invention can be summarized as follows: 
     FIG. 2, a gate electrode structure is formed 
     FIG. 2, a layer of spacer material is,deposited over the gate electrode and the surrounding surface of the substrate 
     FIG. 2, a thin layer of photoresist is deposited over the layer of spacer material. Prior Art does not use this layer of photoresist, the layer of photoresist serves the purpose of a partial stop layer during the spacer etch 
     FIG. 3, partially strip the layer of photoresist removing the photoresist from above the gate electrode structure but leaving photoresist in place adjacent to the gate electrode so that this photoresist can serve as a partial stop layer when forming the gate spacers 
     FIG. 4, partially etch the layer of spacer material that is overlying the gate electrode structure reducing the thickness of the layer of spacer material that is overlying the gate structure to preferably about 400 Angstrom thick. During this process of partially etching the spacer material, the layer of photoresist has served as stop layer thereby leaving the layer of spacer material essentially in place in the areas surrounding the gate structure while essentially removing the spacer material where the photoresist is not present, that is from above the gate structure 
     FIG. 5, removing the remainder of the photoresist since the function of partial stop layer for the etch of the spacer material has been performed 
     FIG. 6, perform a dry etch of the layer of spacer material leaving a thinner layer of spacer material overlying the source/drain regions of the gate structure 
     FIG. 7, perform the final etch for the gate spacers with a wet etch using a HF solution. 
     Essential in the above indicated processing sequence is the deposition of the layer of photoresist, the partial stripping of the photoresist and the function that the photoresist plays of stop layer during the etch of the spacer material thereby enabling a partial etch of the spacer material. This sets the stage for etching of the spacer material using a two step sequence, a dry etch followed by a wet etch. The dry etch leaves a layer of oxide over the surface of the substrate which acts as a protective layer. The final wet etch does therefore not result in the typical plasma damage to the surface of the substrate. As an added benefit, the two step etch of the spacer material (a partial etch followed by a final etch) extends the overall processing time (enlarges the processing window) which makes it easier to detect the end point for the etch of the spacer material. 
     Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.