1. Field of the Invention
This invention relates to a process for producing at a low cost an amorphous silicon electrophotographic photosensitive member which may reduce image defects, has high charging performance, can provide high density and can maintain good image formation over a long period of time. This invention also relates to such an electrophotographic photosensitive member, and an electrophotographic apparatus having the same.
2. Related Background Art
Materials that form photoconductive layers in solid-state image pick-up devices or in electrophotographic light-receiving members in the field of image formation or in character readers are required to have properties as follows: They are highly sensitive, have a high SN ratio [photocurrent (Ip)/(Id)], have absorption spectra suited to spectral characteristics of electromagnetic waves to be radiated, have a high response to light, have the desired dark resistance and are harmless to human bodies when used; and also, in the solid-state image pick-up devices, the materials are required to have properties that enable remaining images to be easily processed in a prescribed time. In particular, in the case of electrophotographic photosensitive members of electrophotographic apparatus used as business machines in offices, the harmlessness in their use is an important point.
Materials that attract notice from such viewpoints include amorphous silicon (hereinafter “a-Si”) whose dangling bonds have been modified with monovalent elements such as hydrogen or halogen atoms, and its application to electrophotographic photosensitive members is disclosed in, e.g., Japanese Patent Application Laid-Open No. 54-86341 (corresponding to U.S. Pat. No. 4,265,991).
As processes by which electrophotographic photosensitive members comprised of a-Si are formed on conductive supports, many processes are known in the art, as exemplified by sputtering, a process in which source gases are decomposed by heat (thermal CVD), a process in which source gases are decomposed by light (photo-assisted CVD) and a process in which source gases are decomposed by plasma (plasma-assisted CVD). In particular, one having been put into practical use in a very advanced state at present is plasma-assisted CVD (chemical vapor deposition), i.e., a process in which source gases are decomposed by direct-current or high-frequency or microwave glow discharge to form deposited films on the conductive support.
As layer structures of such deposited films, proposed are those constructed to have a “surface layer” or an “upper-part blocking layer,” having blocking power, which is further provided on the surface side, in addition to electrophotographic photosensitive members composed chiefly of a-Si and modification elements added appropriately, as conventionally done.
For example, Japanese Patent Application Laid-Open No. 08-15882 discloses an electrophotographic photosensitive member provided between a photoconductive layer and a surface layer with an intermediate layer (upper-part blocking layer) having carbon atoms in a smaller content than the surface layer and incorporated with atoms capable of controlling conductivity.
After a copy has been taken in an electrophotographic apparatus, toner remains partly on the periphery of the photosensitive member, and hence such residual toner must be removed. Such residual toner is commonly removed by means of a cleaning step making use of a cleaning blade, a fur brush or a magnet brush.
Meanwhile, conventional processes for forming electrophotographic photosensitive members have made it possible to obtain electrophotographic photosensitive members having characteristics and uniformity which are practical to a certain extent. Strict cleaning of the interiors of vacuum reactors also makes it possible to obtain electrophotographic photosensitive members having less defects to a certain extent. However, such conventional processes for producing electrophotographic photosensitive members have had such a problem that, regarding products in which large-area and relatively thick deposited films are required as in electrophotographic photosensitive members, it is difficult to obtain in a high yield deposited films that have uniform film quality, can meet requirements on various optical and electrical properties and also may lessen image defects when images are formed by an electrophotographic process.
In particular, a-Si films have a disposition that, where any dust in the order of micrometers have adhered to the substrate surface, the films may undergo abnormal growth on the dust serving as nuclei during film formation, i.e., the growth of “spherical protuberances.” Such spherical protuberances have the shape of a reversed cone whose vertex starts from the dust, and have a disposition that they lower electrical resistance because there are a great many localized levels at the boundaries between a normal deposited portion and spherical protuberant portions, and make the acquired electric charges pass through the boundaries toward the substrate side. Hence, some part of the spherical protuberances appears in the form of white dots in solid black images on images formed (in the case of reverse development, appears in the form of black dots in solid white images). This image defect called “dots” is subjected to severer standards year by year, and images are treated as being poor in some cases even when only few dots are present in an A3-size sheet, depending on their size. Moreover, where electrophotographic photosensitive members are set in color copying machines, the standards come much severer, and images are treated as being poor in some cases even when only one dot is present in an A3-size sheet.
Such spherical protuberances start from the dust, and hence substrates to be used are strictly cleaned before films are formed thereon, where the steps of setting the substrates in a film formation apparatus are all operated in a clean room or in vacuum. In this way, efforts have been made so as to lessen as far as possible the dust which may adhere to the substrate surface before the film formation is started, and the desired effects have been obtained. However, the cause of the occurrence of spherical protuberances is not only the dust having adhered to the substrate surface. That is, where a-Si electrophotographic photosensitive members are produced, the layer thickness required is as large as several micrometers to tens of micrometers, and hence the film formation time reaches several hours to tens of hours. During such film formation, the a-Si becomes deposited not only on the substrates, but also on walls of the film-forming chamber and on structures inside the film-forming chamber. These chamber walls and structures do not have any surfaces that have been controlled like the substrates. Hence, they may have weakly adhered to cause film come-off (or film peel-off) in some cases during film formation carried out over a long time. Once even any slight film has come off during film formation, it causes dust, and the dust adheres to the surfaces of photosensitive members under deposition, so that, starting from the dust, the abnormal growth of spherical protuberances takes place inevitably. Accordingly, in order to maintain a high yield, careful management is required not only on the management of substrates before film formation but also on the prevention of film come-off in the film-forming chamber during the film formation. This has made it difficult to produce the a-Si photosensitive members.
It is known, as disclosed in Japanese Patent Applications Laid-Open No. 11-133640 and No. 11-133641 (corresponding to U.S. Pat. No. 6,001,521), that it is effective to use an amorphous carbon layer containing hydrogen (hereinafter referred to as “a-C:H films”). This a-C:H film, as it is also called diamond-like carbon (DLC), has a very high hardness. Hence, it can prevent scratches and wear and at the same time has a peculiar solid lubricity.
In practice, it has been ascertained that the use of the a-C:H film at the outermost surface of the photosensitive member enables filming to be effectively prevented in various environments.
However, in the process of producing electrophotographic photosensitive members making use of this a-C:H film as a surface layer, there has been a problem in production steps. Usually, in forming deposited films by using high-frequency plasma, by-products (polysilane) generated during deposited-film formation are removed by dry etching after the deposited-film formation has been completed, and the inside of the reactor is cleaned. However, the time for etching treatment after continuous formation of light-sensitive layers up to surface layers (a-C:H) is longer than the time for continuous formation of light-sensitive layers up to conventional surface layers (a-SiC). This is due to the fact that the a-C:H can be etched with great difficulty, and has been one of factors in bringing about a rise in production costs.
In addition, there is a case where residues of a-C:H films remains thin after the etching treatment, and this may cause image defects in the next deposited film formation.
Meanwhile, in electrophotographic apparatus, depending on the surface state of an a-Si photosensitive member, any damage of a cleaning blade that are caused by surface roughness, the above spherical protuberances or the like or too good slipperiness between the photosensitive member and the cleaning blade at the initial stage of service may cause faulty cleaning such as slip-through of developer (toner) to cause black lines on images.
To cope with such a difficulty, blade materials, contact pressure, developer composition and so forth may carefully be selected corresponding to the surface state of the photosensitive member. For example, the contact pressure of the cleaning blade at the initial stage is set a little high, and is thereafter made lower little by little. Such measure can lessen the difficulty to a certain extent. However, during the use of the electrophotographic apparatus over a long period of time, maintenance must be made in a large number of times in order to improve the quality of images, and further the maintenance may come complicated. Hence, the efficiency of operating the electrographic apparatus can not sufficiently be improved, bringing about an additional difficulty such as enlargement in the number of component parts in some cases.
Depending on the surface state of the photosensitive member and the state thereof with respect to the cleaning blade, during the use of the electrophotographic apparatus over a long period of time, the cleaning blade may gradually be turned up as the photosensitive member is rotated, to become unable to remove the toner sufficiently by cleaning.
With regard to processes for producing a-Si photosensitive members, plasma-assisted CVD carried out at VHF band frequency makes it possible to improve the film deposition rate more vastly than any cases making use of RF bands. However, with regard to surface properties, depending on production conditions, the surface state may come coarser on the level of a microscopic visual field (in the order of submicrons) than the surfaces of photosensitive members produced using RF bands. Hence, the photosensitive members produced using VHF bands may tend to cause damage of the cleaning blade or cause faulty cleaning such as slip-through of toner, bringing about a narrow latitude for coping with difficulties in some cases.
Especially in recent years, the advancement of digitization of electrophotographic apparatus has raised the level of a demand for image quality, and has reached a situation such that image defects in the extent that has been tolerated in conventional analogue type apparatus must be questioned.
Accordingly, any effective measures to remove the factors of image defects are desired.