Abstract:
Flow process for producing non-volatile memories with differentiated removal of the sacrificial oxide in the NO-DPCC diagram including a series of steps that permit the removal of the oxide in two distinct moments from the matrix area and from the circuitry area. In this manner the active circuitry areas are preserved from the danger of breaking the tunnel oxide, thus avoiding the degradation of the quality of the oxides and increasing, in addition, the level of reliability of the device itself.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process flow for producing non-volatile memories with differentiated removal of the sacrificial oxide, and in particular to an NO-DPCC (double short-circuited poly) process or for any memory device in which a first layer of polysilicon is removed. 
     2. Discussion of the Related Art 
     In the conventional process flow for producing non-volatile memory devices of the flash type, immediately after a step in which a field oxide is generated, after a step in which the layers suitable for forming the active areas (HARD MASK) are removed, after a step in which a thermal oxide about 300-500 A thick is generated and after a step in which implants of boron in the memory cells (EPM) in the matrix area have been made, this thermal oxide, also called sacrificial oxide, is completely removed by a blanket attack and with it also all the impurities and residues left from the previous operations. Therefore the sacrificial oxide is removed both from the area of the memory matrix and from the circuitry area. 
     In an NO-DPCC circuit, the flash memory cells consist of a first layer of polysilicon, called polyI, and a second layer of polysilicon, called polyII, isolated by a dielectric layer, called interpoly dielectric. After the formation of a tunnel oxide, of the polyI and the interpoly dielectric over the whole surface of the device, said layers must be completely removed from the circuitry area before the successive formation of the polyII. 
     A problem that can occur during the interpoly dielectric removal step and in particular of the polyI is that damage can be made, in the circuitry zone, of the area below, due to an overattack and a possible breaking of the tunnel oxide, caused, for example, by machine process imprecision. 
     In view of the state of the art described, an object of the present invention is to prevent this breaking of the tunnel oxide from occurring during the attack of the interpoly dielectric and the polyI. 
     SUMMARY OF THE INVENTION 
     According to the present invention, this and other objects are achieved by a process flow including the steps of: 
     (a) a step of growth of a layer of sacrificial oxide on a substrate of silicon on said matrix area and said circuitry area; 
     (b) a step in which a layer of sacrificial oxide is removed by means of an attack in said matrix area; 
     (c) a step of growth of a layer of tunnel oxide on said matrix area and said circuitry area; 
     (d) a step of deposition a first layer of polysilicon; 
     (e) a step of formation of a layer of interpoly dielectric after said first layer of polysilicon on said circuitry and matrix areas; 
     (f) a step of attack to said layer of dielectric in said circuitry; 
     (g) a step of attack to said first layer of polysilicon in said circuitry area; 
     (h) a step of attack to said layer of sacrificial oxide and on the tunnel oxide by means of a wet attack in said circuitry area. 
     Thanks to this invention such oxide can be removed in the circuitry area while protecting the layers below from any damage caused by attacks previously made without introducing any additional step to the memory cell implementation process. In this manner the active areas of the circuitry are protected from the danger of oxide breaking, thus avoiding the degradation of the quality of the circuitry oxides and, in addition, increasing the level of reliability of the device itself. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and the advantages of the present invention will be evident from the following detailed description of an embodiment thereof, illustrated as non-limiting example in the enclosed drawings, in which: 
     FIG. 1 shows a flash cell according to the known art; 
     FIG. 2 illustrates a flow diagram of an NO-DPCC process according to the known art; 
     FIG. 3 shows a flow diagram of an NO-DPCC process according to the present invention; 
     FIG. 4 illustrates a masking step of the process according to the invention; 
     FIG. 5 shows a growth step of more than one layer of the process according to the invention; 
     FIG. 6 illustrates a further masking step of the process according to the invention. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1 a flash cell according to the known art is shown. 
     An NO-DPCC process is taken into consideration, according to which the cell of a non-volatile memory device, for example a flash cell  1 , includes a p type substrate  40 , in which a floating gate  4  is made in a first layer of polysilicon, polyI, where the charges are stored, and a control gate  5  is made in a second layer of polysilicon, polyII, said gates being isolated from each other by means of a layer of dielectric  6 , called interpoly dielectric. 
     Moreover, we can note the presence of pockets  8  and  11 , doped n+, can be noted suitable for forming the diffusion zones for drain electrodes  8  and source electrodes  11 , and the presence of a line of metalization  9 . 
     In addition, in said cell  1 , there is a tunnel area  7 , through which the electric charges pass, injected into the floating gate  4  from the channel that forms between the n+ doped pockets  8  and  11 , by channel hot electron effect. 
     According to the flow diagram of an NO-DPCC process for producing a flash memory, after the formation of the tunnel area  7 , of the floating gate  4  (polyI) and of the interpoly dielectric  6  over the entire surface of the device  1 , the layers must be completely removed from the circuitry area before the successive formation of the control gate  5  (polyII). This removal is made by a mask, called matrix mask, which completely covers the matrix and leaves the circuitry uncovered and it is in this step that there is the risk of perforating the tunnel oxide, thus degrading the electric performance of the device. 
     In the present disclosure, the term matrix means the area of the chip dedicated to the memory region and the term circuitry means the area of the chip dedicated to the input/output circuits of the memory region. 
     A flow diagram of an NO-DPCC process according to the known art is shown in FIG.  2 . 
     Usually, according to such NO-DPCC diagram, the sacrificial oxide is completely removed from the entire surface of the device. 
     In fact after a first step  12  of growth of a layer of thermal oxide, also called sacrificial oxide, with a thickness of about 40 nm, step  13  is provided in which a mask (resist) is deposited for the formation of active areas on the matrix part. Successively, step  14  is carried out during which the ionic implantation occurs, resorting to an implant of boron atoms, to raise the threshold voltages of the memory cells. 
     At this point a step  15  is provided in which the resist is removed, that is a protective layer is removed, and a step  16  in which the sacrificial oxide is removed by a blanket attack in HF (hydrofluoric acid). 
     Then, a step  17  is provided during which the growth of a tunnel oxide with a thickness of about 10 nm occurs. Successively there is a deposition of a first layer of polysilicon, step  18 . 
     During step  19  the application of a photolitographic mask is provided which is followed by a step  20  of selective attack and removal of said first layer of polysilicon and a step  21  of removal of the protective layer. 
     At this point, after the removal of the mask, deposited during step  19 , a step  22  is provided in which the formation of a layer of dielectric occurs, preferably including a multilayer of ONO (Oxide-Nitride-Oxide), made at a low temperature by CVD deposition of nitride and silicon oxide. Then step  23  is carried out, in which said ONO dielectric is densified to make it less porous. 
     All the previous steps are carried out over the whole surface of the wafer, that is both on the matrix area and on the circuitry area. 
     Through a photolitographic mask, deposited during step  24 , which covers the matrix area, step  25  of attack to said ONO dielectric layer is provided, step  26  of attack to said first layer of polysilicon and step  27  of attack to the tunnel oxide by means of a wet attack, for example by using a BOE (Buffered Oxide Etch) attack, well-known to technicians in the art, for a depth of about 18 nm so that the surface of the substrate of silicon below is not damaged. 
     The process steps  25 ,  26  and  27  are only carried out on the circuitry area. 
     In conclusion, provision is made for step  28  in which the layer of protective resist is removed and the operation flow proceeds until the device is completed, step  29 . 
     One problem that could arise during the removal step, in particular during the removal of said first layer of polysilicon, step  26 , and said layer of interpoly dielectric, step  25 , is the damaging in the circuitry of the active areas below, due to an overattack and possible breaking of the tunnel oxide, leading to considerable degradation of the performances and reliability of the device. 
     FIG. 3 shows a flow diagram of an NO-DPCC process according to the present invention. 
     In this inventive flow diagram the attack of the sacrificial oxide is differentiated, that is the removal of the sacrificial oxide from the circuitry and matrix areas is carried out in two different steps, so that there is an additional protective layer during the removal steps  25  and  26 . 
     The sacrificial oxide is removed from the matrix area after steps  12 ,  13  and  14  of the known process have been carried out, that is immediately after step  14  which provided the implants EMP 1 -EMP 2  using the photolitgraphic mask, deposited during step  13 . This implies that the mask for ionic implantation of boron atoms is removed only after the attack on the sacrificial oxide has been made, so that said oxide is eliminated only from the matrix area. 
     During step  30 , suitably calibrating the attack, the same modifications of the oxide are obtained as those that the attack according to the known diagram would have produced. 
     Therefore, it can be derived that the sacrificial oxide remains integral in the circuitry zone and when the tunnel oxidation operation is carried out, successively, said tunnel oxide overlaps said sacrificial oxide making it thicker. 
     The inventive diagram proceeds following the same steps as those of the known diagram until it reaches the point in which after the attack on said first layer of polysilicon, step  26 , there is a step  32  of attack to said sacrificial oxide and to said tunnel oxide through an attack preferably of BOE type for a depth of about 50-60 nm, thus eliminating the sacrificial oxide only from the circuitry area. 
     Therefore the sacrificial oxide is removed from the circuitry area after the attack of the first layer of polysilicon, step  26 , during the tunnel oxide removal operation, step  32 . 
     In this manner the sacrificial oxide, which has a thickness of about 40 nm, and the tunnel oxide, which has a thickness of about 10-20 nm, act as protection of the active area against overattack phenomena during the removal of said first layer of polysilicon, step  26 . The fact of having a thicker oxide during said step  26  guarantees a factor of solidity which is definitely higher compared to the process of the known art. This means that also in the presence of an attack to said layer of polysilicon, step  26 , or also of an attack to the ONO dielectric layer, step  25 , whose thickness is extended beyond normal, the active circuitry areas and the quality of the circuitry oxides are conserved. 
     It is therefore evident that in the process diagram according to the invention neither additional masks, nor modifications to the existing masks, nor additional process steps are necessary. 
     The process according to the invention therefore can be used indifferently for any memory device that provides for the removal of the first layer of polysilicon in the circuitry. 
     In addition, the step sequence of the innovative process can be detected, as compared to the traditional process, because in the overlapping area of the two masks, deposited during steps  13  and  24 , that is in the matrix border zone and typically on the field oxide, because of the intrinsic misalignment between the said two masks and as the attacks are carried out in two distinct steps, the presence of a rim can be observed, as the attack does not occur where the masks overlap, or the presence of an etching can be observed, as there is a double attack where the masks detached. 
     FIG. 4 shows a masking step of the process according to the invention. 
     In such figure it can be noted that the photolithographic mask  39  of the device  36  is located on the circuitry area  33 , in which an alternating of p type pockets  34  and n type pockets  35  and the presence of oxides  41  can be noted. 
     Said photolithographic mask  39  is the last mask before the ionic implantation of the boron atoms and therefore acts as a protective layer only for the circuitry part. 
     The attack to the sacrificial oxide during the presence of this mask and the removal of the layer of protective resist only from the matrix area is provided for by the flow according to the present invention. 
     FIG. 5 illustrates a growth step of several layers of the process according to the invention. 
     In such figure it can be noted that over the entire surface of the circuitry device  36 , layers  37  of polysilicon and of dielectric are formed, through step  19  which provides for a masking process, and through step  17 . 
     FIG. 6 shows a further masking step of the process according to the invention. 
     According to the illustration in such figure it can be noted that the part of the matrix area of the device  36  is covered by the “matrix” mask  38 , which is positioned above the polysilicon and dielectric layers  37 , leaving, therefore, uncovered the circuitry part  33 . 
     In this situation, that is, when the mask protects the matrix area, the attack to the layer of interpoly dielectric, step, the attack to the first layer of polysilicon, step, and the removal of the sacrificial oxide and the tunnel oxide are made. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.