1.  Field of the Invention
The present invention relates to a liquid discharge head used with an ink jet recording apparatus. 
2.  Description of the Related Art
A recording apparatus that is configured with an ink jet method, namely, an ink jet recording apparatus, is adapted to discharge and blow a liquid (recording liquid) from the discharge ports of nozzles of a liquid discharge head onto a recording medium thereby to effect recording. 
The configuration of the aforesaid type of liquid discharge head has a silicon substrate provided, on the front surface thereof, with electric wiring and a plurality of energy generating elements, which generate energy for discharging a liquid, and an orifice substrate, which is made of a resin material and which is deposited on the silicon substrate.  The orifice substrate has nozzles provided at positions corresponding to the energy generating elements.  Each of the nozzles has a bubble forming chamber in which air bubbles are generated by an energy generating element and a fine discharge port through which a liquid is discharged.  Further, a groove provided in the orifice substrate and the silicon substrate together form a passage for supplying the liquid from a liquid supply port to a nozzle, which will be discussed later.  A liquid supply port for supplying the liquid from a liquid tank or the like is provided in the silicon substrate such that the liquid supply port penetrates the silicon substrate.  The liquid is supplied from the liquid tank or the like to the nozzle through the liquid supply port and the passage. 
In the liquid discharge head constructed as described above, the liquid supplied from the back surface of the silicon substrate is passed through the liquid supply port and the passage and charged into the bubble forming chamber of each nozzle.  The liquid filled in the bubble forming chamber is pushed out in a direction substantially orthogonal to the silicon substrate by an air bubble generated from film boiling by the energy generating element.  Thus, the liquid is discharged from the discharge port. 
To satisfy the need for a higher recording quality and recording at higher speed, an ink jet recording apparatus is required to have a higher density of the nozzles thereof and to achieve smaller droplets of a liquid to be discharged.  The higher density of the nozzles has been achieved by, for example, forming the orifice substrate by using a photosensitive resin material and carrying the patterning by a photolithographic technique.  The smaller droplets have been achieved by reducing the diameter of each discharge port and the size of each bubble forming chamber.  To reduce the size of the bubble forming chamber, the distance between the front surface of the orifice substrate and the surface of the energy generating element, i.e., the heater (hereinafter referred to as “the OH distance”), has been designed to be shorter. 
As one method for shortening the OH distance, the orifice substrate is made thinner.  According to, for example, Japanese Patent Application Laid-Open No.  S61-037439, only the area of an orifice substrate in the vicinity of a discharge port involved in the discharge characteristics of a liquid is made thinner, while the remaining area is made thick to enhance the strength of the orifice substrate, thus achieving a reduced OH distance.  Hence, the front surface of the orifice substrate, i.e., the surface opposite from the silicon substrate, is shaped to have a recess around each discharge port.  However, there is limitation on reducing the thickness of the orifice substrate due to the required strength of the substrate. 
Another method for reducing the OH distance is to reduce the height of a passage.  In this case, the OH distance can be shortened by reducing the interval between the front surface of the silicon substrate, which is the surface with the energy generating elements disposed thereon, and the back surface of the orifice substrate.  However, reducing the height of the passage inconveniently increases the passage resistance, resulting in longer time required for nozzles to be refilled with a liquid.  This is disadvantageous for achieving higher-speed recording.  As a solution, Japanese Patent Application Laid-Open No.  2003-025595, for example, discloses a method in which the height of a part of a passage is increased by providing a portion constituting the passage in the back surface of an orifice substrate with a recess (hereinafter referred to as “the back surface groove”) so as to allow nozzles to be refilled with a liquid at high speed and also to reduce the OH distance at the same time.  According to this method, the height of the passage is increased while reducing the OH distance, so that a reduced passage resistance and high-speed refilling of a liquid can be achieved. 
However, the method disclosed in Japanese Patent Application Laid-Open No.  2003-025595 poses the problems described below. 
(1) The resin material constituting the orifice substrate shrinks during the production process of a liquid discharge head. 
(2) The amount of shrinkage of the orifice substrate in the direction parallel to a plane differs between an area near the front surface and an area near the back surface of the orifice substrate. 
Because of the two problems mentioned above, deformation in which the orifice substrate decreases the height of the passage at the position of the back surface groove takes place, meaning that the orifice substrate warps toward the silicon substrate.  The deformation gives rise to a problem in that the passage resistance increases, adversely affecting the refilling of the liquid. 
More specifically, in the manufacturing process of the liquid discharge head having the orifice substrate made of a resin material, curing is generally carried out in a final process to provide resistance to liquid and adhesion between substrates.  The curing process causes the orifice substrate made of a standard resin material to shrink by about a few percent to about ten-odd percent.  In the orifice substrate having a back surface groove, the side portions of the back surface groove project toward the silicon substrate, so that the orifice substrate will have unfixed free ends in the direction parallel to the plane.  This inconveniently leads to an increased amount of shrinkage of the side portions of the back surface groove of the orifice substrate.  As a result, a tensile stress occurs in the vicinity of the back surface groove.  Consequently, after the curing process, the orifice substrate in the vicinity of the back surface groove develops the deformation that reduces the height of the passage, i.e., the orifice substrate warps toward the silicon substrate.  Thus, the passage becomes lower than a desired height and the effect provided by the back surface groove is undesirably impaired.  As a possible solution, the depth of the back surface groove could be increased to compensate for the reduction in the height of the passage caused by the curing process.  This, however, causes the orifice substrate to become excessively thin at the back surface groove, presenting another problem, namely, inadequate strength.  The orifice substrate could be made of a material that does not shrink during the curing process.  However, the material constituting the liquid discharge head is required to have characteristics suited for a liquid to be used, such as the elution to the liquid and adhesion, thus it would leave an unsolved problem because of the limited choice of available materials.