A plasma display panel (hereinafter, PDP) includes a front panel and a back panel oppositely disposed to each other and sealed at their periphery with a sealing member.  The front panel and the back panel have a discharge space therebetween filled with a discharge gas including neon (Ne) and xenon (Xe). 
The front panel includes a glass substrate; display electrode pairs; a dielectric layer; and a protective layer.  The display electrode pairs each consist of a scan electrode and a sustain electrode formed in a stripe pattern on the glass substrate.  The dielectric layer coats the display electrode pairs, and the protective layer coats the dielectric layer.  Each of the display electrode pair consists of a transparent electrode and a metal bus electrode formed thereon. 
On the other hand, the back panel includes a glass substrate; address electrodes; an base dielectric layer; barrier ribs; and phosphor layers of red, green, and blue.  The address electrodes are formed in a stripe pattern on the glass substrate.  The base dielectric layer coats the address electrodes.  The barrier ribs are formed in a strip pattern on the base dielectric layer so as to partition the discharge space in correspondence with the address electrodes.  The phosphor layers are formed on the base dielectric layer between the barrier ribs and also on side surfaces of the barrier ribs. 
The front panel and the back panel are oppositely disposed to each other so that the display electrode pairs and the address electrodes can be at right angles to each other and have discharge cells at their intersections.  The discharge cells are arranged in a matrix where three adjacent discharge cells having red, green, and blue phosphor layers arranged in the direction of the display electrode pairs form a pixel for color display.  In a PDP, a predetermined voltage is applied between the scan electrodes and the address electrodes and between the scan electrodes and the sustain electrodes so as to generate a gas discharge.  The gas discharge generates ultraviolet light which excites the phosphor layers, allowing them to emit light so as to display color images. 
In a PDP thus structured, the protective layer is required to have a high resistance to sputtering and a large secondary electron emission coefficient.  For this reason, a protective layer of magnesium oxide (MgO) is widely used.  The sputtering resistance and secondary emission characteristics can protect the dielectric layer from sputtering and reduce the discharge voltage. 
The protective layer, which can be formed by electron beam deposition or using a plasma gun, can have very different film properties depending on the method and conditions of its formation.  Japanese Patent Unexamined Publication No.  2005-50804 shows an example of stably manufacturing a protective layer having excellent film properties in the following manner.  When the protective layer is formed by electron beam deposition of magnesium oxide (MgO), the partial pressures of various gases including oxygen present in the evaporation chamber are controlled in a certain range. 
The protective layer of magnesium oxide (MgO) can significantly change film properties due to oxygen deficiency or impurity incorporation during its formation.  In a widely used film-forming apparatus of substrate transfer type, a glass substrate finished up to the dielectric layer is placed on a tray and applied with the protective layer in a film-forming chamber.  When the glass substrate passes through the film-forming chamber, the magnesium oxide (MgO) film pieces can adhere to the tray and the mask.  When the glass substrate is taken out into the atmosphere from the film-forming chamber, the magnesium oxide (MgO) adhering to the tray and the mask absorbs moisture in the atmosphere.  When another magnesium oxide (MgO) film is formed on the next glass substrate that is being transferred into the film-forming chamber using the same tray and mask, the moisture absorbed in the tray and the mask is released into the film-forming chamber.  Part of the moisture is dissociated into hydrogen and oxygen in the film-forming chamber.  These gases cause a change in the partial pressures in the film-forming chamber, thereby causing variations in the film properties of the magnesium oxide (MgO) film. 
As a way to stabilize the partial pressures so as to reduce the amount of water brought into the film-forming chamber, in a film-forming apparatus of substrate transfer type, different trays and different masks are used inside and outside the film-forming chamber.  However, as the substrate size becomes larger and more diverse, the transfer mechanism becomes more complicated.  As a result, the apparatus has a lower reliability and a higher cost. 
As another way to stabilize the partial pressures so as to reduce the amount of water brought into the film-forming chamber, the atmosphere during the transfer of the substrate is made low in dew point and water content.  This method requires a plurality of pumps such as cryopumps or turbomolecular pumps having a high exhausting capacity.  In addition, it is necessary to vary the exhaust velocity by changing the opening of a conductance valve disposed between the film-forming chamber and the pumps or the number of the pumps.  The problem is that this approach causes variation in the exhaust velocity not only of the water but also of the other gases, making it difficult to keep the partial pressures in the film-forming chamber constant.  On the other hand, in the case where the exhaust velocity is varied by changing the speed of the turbomolecular pumps, a change in the compression ratio causes a change in the component ratio of the exhaust gas, making it impossible to control the partial pressure of water independently.