Method of Manufacturing a Solid-State Image Pickup Device

1. Field of the Invention

This invention relates to a method of manufacturing a solid-state image pickup device for use in a photoelectric converter employing a photoconductive film, and more particularly to a method of manufacturing such a device having a low dark current and low lag characteristic.

2. Description of the Prior Art

A "two-level" solid-state image pickup device is a device having a signal charge reading-out portion constituted by a charge-coupled device (CCD) formed on a silicon monocrystal substrate, and a photoelectric conversion portion contituted by a photoconductive film formed on the signal charge reading-out portion. The "two-level" solid state image pick-up device has many advantages, such as increased sensitivity, due to its being able to present a large aperture to incident light, and low blooming. However, there are problems with the characteristics of such a construction as will be explained below.

FIG. 1 is a cross-sectional view of a conventional two-level solid-state image pickup device. Reference numeral 11 designates, for example, a p-type Si substrate, on the surface of which are provided a first n.sup.+ layer 12a providing the CCD channel for read-out of signal charge, a second n.sup.+ layer 12b constituting a signal charge accumulation region, and n.sup.++ layer 14 provided to establish an ohmic connection between this accumulation region 12b and a first metallic electrode 17a formed for example of aluminum (Al), and a channel stopper p.sup.+ layer 13. On top of a gate oxide film, which is itself provided on top of the CCD channel 12a, there are formed CCD transfer electrodes 15a, 15b, which are made, for example, of two layers of polycrystalline silicon. A first insulating film 16a is deposited on top of these two transfer electrode layers 15a and 15b. The first metallic electrode 17a, which is connected to the n.sup.++ layer 14 through contact holes formed in the insulating film 16a, is provided so as to extend over the CCD channel. A second insulating film 16b is then deposited on this, and second metallic electrodes 17b, formed of aluminum (Al), which are pixel electrodes, are provided for each pixel region. These second metallic electrodes 17b are connected to the first metallic electrodes 17a through contact holes 20 provided in the second insulating film 16b. The contact holes 20 are formed by a reactive ion etching (RIE) of the second insulating film 16b using a patterned photoresist as a mask. On top of the second metallic electrodes 17b, a photoconductive film 18 consisting of, for example, amorphous Si and a transparent conductive layer provided by an ITO film 19 are formed.

The n.sup.+ layer 12b constituting the signal charge accumulation region, n.sup.++ layer 14, first metallic electrode 17a and second metallic electrode 17b are separately provided for each pixel. That is, the second metallic electrodes 17b define image pickup pixel regions and the potential variations obtained by the second metallic electrodes 17b as a result of the image pickup performed by the photoelectric film 18 are transmitted through the first metallic electrode 17a to the n.sup.+ layer 12b constituting the accumulation region, and are accumulated in this region as signal charges. In the view shown in FIG. 1, the channel stopper p.sup.+ layer 13 can be seen, but transfer regions which transfer the signal charges of the n.sup.+ layer 12a are of course provided for each pixel.

In such a solid-state image pickup device, the substrate surface before the photoconductive film 18 is formed must be as flat as possible. If it is not flat, this results in appreciable dark current and lag of the solid-state image pickup device, with adverse effects on the image pickup characteristics.

The reason for this is that the film quality of the photoconductive film is poorer over substrate surface irregularities as compared with what it is where the substrate surface is smooth, and this increases the number of generation and recombination centers, which cause dark current and lag. It is therefore necessary to ensure that the surface of the second insulating film 16b is as flat as possible, and that the contact holds 20 formed in the second insulating film 16b provide as few irregularities as possible.

However, in the previous method, wherein the contact holes 20 were formed by RIE using a photoresist mask, the process was very complicated because many process steps such as forming a photoresist film, patterning thereof and etching of the insulating layer by RIE were necessary. Furthermore, in the previous method in which RIE was used, the sides of the contact holes were very steep so if the second metallic electrodes 17b were made thin, breaks in the metallic film became a considerable problem. Therefore, the minimum thickness of the metallic electrodes 17b was limited to approximately 4000 .ANG.-5000 .ANG., and irregularities of the substrate surface could not be minimized.

Accordingly, the objects of the present invention are to provide a method of manufacturing a two-level solid-state image pickup device wherein the aforementioned problems are solved and which has little dark current or lag.

These and other objects are achieved according to the present invention by providing a novel method wherein part of the first metallic electrode is made to project to the highest position above the substrate on which it is formed, a coating of an organic insulating film is then applied to produce a flat surface, and the entire surface of the organic insulating film is then etched to expose the projections of the first metallic electrode. One advantage of this is that the contact holes for connecting the second metallic electrodes to the first metallic electrodes are formed automatically without recourse to a photolithographic process.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a conventional solid-state image pickup device constructed of various layers and employing a photoconductive film as a photoelectric converter;

FIGS. 2a to 2f are cross-sectional showing the manufacturing stages of an embodiment of the present invention;

FIG. 3 is a cross-sectional view of another embodiment of the present invention;

FIGS. 4a and 4b are cross-sectional views of yet another embodiment of the present invention; and

FIG. 5 is a cross-sectional view of another embodiment of the present invention.

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIGS. 2a to 2f thereof, there is shown the manufacturing stages of one embodiment according to the present invention.

FIG. 2a shows the condition in which p.sup.+ layers 22 are formed as channel stopper regions of the p type silicon substrate 21, field oxide films 23 projecting upwards are formed by selective oxidation, the CCD channel n.sup.+ layers 24a is formed, a signal charge accumulation n.sup.+ layers 24b are formed, and an n.sup.++ layer 25 is formed constituting a contact region for the n.sup.+ layers 24b and metallic electrodes.

Next, as shown in FIG. 2b, on top of the gate oxide film, CCD transfer electrodes 26a and 26b are formed of polysilicon extending over the field oxide film 23, and the entire surface of the device is covered with a silicon oxide film 27, which is a first insulating film, by CVD.

The contact holes 33 are then formed, and evaporation of Al and patterning thereof are carried out, so that first metallic electrodes 28a are formed extending over the CCD channel and contacting the n.sup.++ layer 25, as shown in FIG. 2c. As can be seen in FIG. 2c, the first metallic electrodes 28a are formed with portions A that project upwards.

As shown in FIG. 2d, a polyimide film 29, which is a second insulating film, constituting an organic insulating film is then applied onto the substrate by, for example, a spin coating process. The use of this polyimide film 29 produces a very flat surface. The polyimide film 29 is then fixed by heating for about 30 minutes at a temperature of 400.degree. C.-450.degree. C.

Next, as shown in FIG. 2e, the entire surface of the polimide film 29 is etched by reactive ion etching (RIE) using for example oxygen plasma, and the etching is stopped when the aforesaid projections A of the first metallic electrodes 28a are exposed. Stopping of the etching can be easily established by determining the etching time or by detecting reflected light from the first metallic electrode 28a. Contact portions for the first metallic electrodes 28a are therefore formed automatically and without requiring a masking step and in a manner giving a flat surface.

As shown in FIG. 2f, second metallic electrodes 28b formed of Al are then formed in the same manner as in the conventional device and the photoconductive film 30 and ITO film 31 are deposited over the entire surface to complete the device.

With this embodiment, the first metallic electrode 28a and second metallic electrode 28b can be electrically connected with self-alignment without patterning by photolithography, and the shape of the projection A of the first metallic electrode 28a, which constitutes the contact portion, reflects the surface shape of the first insulating film 27.

Therefore, according to the present invention, device manufacture is simplified and easy, and thin second metallic electrodes, the thickness of which is less than 1000 .ANG., can be used without any risk of disconnection, so the dark current of the two-level solid-state image pickup device is reduced and the lag characteristics is much improved.

As shown in FIG. 3, the portion constituted by the oxide film 27 can be replaced by a combination of an oxide film 27a having formed thereon a phosphor silicate glass (PSG) 27b. The PSG layer 27b is then heat-treated at for example 1000.degree. C.-1100.degree. C. to melt the surface thereof. As a result, the projections A subsequently formed become even smoother convexities, and therefore, a continuous smooth connection can be achieved between the projection A and the surface of the polyimide film 29. This enables the second metallic electrode 28b to be made thinner and the overall surface to be made flat, so that the photoconductive film 30 that is formed on this substrate can likewise be deposited in a uniform manner. This results in much improvements in respect of the dark current and lag characteristics. Deterioration in sensitivity can also be prevented because the photoconductive film is much more uniform over the surface of the substrate and has few trap levels.

With the embodiment of FIGS. 2a to 2f and FIG. 3, a flat surface is achieved by application of a thick polyimide film 29, but, when this method is performed using RIE, the inside wall of the reaction vessel can become very dirty. This does not directly affect the characteristics of the device, but cleaning of the inner wall of the reaction vessel is very laborious, while if it is left dirty problems arise when RIE is next carried out.

Accordingly, in the embodiment shown in FIGS. 4a and 4b, a two-stage process is performed, in which the flatness shown in FIG. 2d is achieved by first of all depositing a relatively thin polyimide film 29a sufficient to cover the projections A of the second metallic electrode 28a, then applying a photoresist film 29b onto the surface of film 29a, as shown in FIG. 4a.

The whole surface is then etched by RIE as in the preceding embodiment, to expose the projections A of the second metallic electrode 28a, as shown in FIG. 4b.

This procedure involving RIE of a photoresist has been found effective in practically eliminating soiling of the inside wall of the reactive vessel.

FIG. 5 shows yet a further embodiment of the present invention. It differs from FIG. 2 in that a buried electrode 32 consisting of a third polysilicon layer is formed in the oxide film 27 between the transfer electrodes 26a and 26b and first metallic electrode 28a on the channel stopper p.sup.+ layer 22. This buried electrode 32 is grounded or has a DC voltage applied thereto and provides electrostatic shielding between the transfer electrodes 26a and 26b and the first metallic electrode 28a.

In the FIG. 5 embodiment, since the third polysilicon layer constituting the buried electrode 32 is further provided underneath the projections A, these projections A project further above the rest of the surface layers, and this facilitates the process of exposing only these projections A.

This electrostatic shielding provided by electrode 32 also has the effect of preventing deterioration of the lag characteristics or unstable operation. Specifically, the storage diode constituted by the n.sup.++ layer 25 and p types substrate 21, that is connected to the first metallic electrode 28a is in an electrically floating condition during the period for which the signal charge is stored, so application of a clock pulse to the transfer electrodes 26a and 26b causes capacitative variation of the potential of the n.sup.++ layer 25. This results in some of the signal charge that has reached this n.sup.++ layer 25 being returned to the photoconductive film, causing deterioration of the lag characteristics and operational instability, such as flickering. The electrostatic shielding effect of the buried electrode 32 of this embodiment is effective in preventing such adverse effects on the characteristics of the finished device.

Polysilicon was used as the material of the CCD transfer electrodes and buried electrode in the above description but other conductive materials such as molybdenum (Mo), molybdenum silicide (Mo-Si) or tungsten (W) could be used.

The etching of the polyimide film 29 was performed by reactive ion etching using oxygen plasma, but other methods such as chemical etching using hydrazine solution could be used provided they produce uniform etching of the entire surface. Other organic insulating materials instead of the polyimide film 29 could be used, such as for example rubbery or acrylic macromolecular films.

The above description concerns mainly the effective sensitive region of the solid-state image pickup device. If the metallic electrodes in the region of the bonding pad that is provided around the edges of the element for external connections are left exposed at the same time as the first metallic electrodes 28a are exposed, it is not necessary to carry out patterning by photolithography on the bonding pad.

The above description assumes the use of a CCD substrate, but substrates having other means of enabling reading out of the signal charge generated by photoconductive film could be used.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.